// Copyright 2022 The Goscript Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//
//
// This code is adapted from the offical Go code written in Go
// with license as follows:
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

use crate::SourceRead;

use super::super::constant::Value;
use super::super::lookup;
use super::super::objects::TypeKey;
use super::super::operand::{Operand, OperandMode};
use super::super::typ::{self, BasicType, Type};
use super::super::universe::ExprKind;
use super::check::{Checker, ExprInfo, FilesContext};
use super::stmt::BodyContainer;
use go_parser::ast::{Expr, Node};
use go_parser::{Map, Pos, Token};
use std::collections::HashSet;

///Basic algorithm:
///
///Expressions are checked recursively, top down. Expression checker functions
///are generally of the form:
///
///  fn f(x &mut operand, e &Expr, ...)
///
///where e is the expression to be checked, and x is the result of the check.
///The check performed by f may fail in which case x.mode == OperandMode::invalid,
///and related error messages will have been issued by f.
///
///If a hint argument is present, it is the composite literal element type
///of an outer composite literal; it is used to type-check composite literal
///elements that have no explicit type specification in the source
///(e.g.: []T{{...}, {...}}, the hint is the type T in this case).
///
///All expressions are checked via raw_expr, which dispatches according
///to expression kind. Upon returning, raw_expr is recording the types and
///constant values for all expressions that have an untyped type (those types
///may change on the way up in the expression tree). Usually these are constants,
///but the results of comparisons or non-constant shifts of untyped constants
///may also be untyped, but not constant.
///
///Untyped expressions may eventually become fully typed (i.e., not untyped),
///typically when the value is assigned to a variable, or is used otherwise.
///The update_expr_type method is used to record this final type and update
///the recorded types: the type-checked expression tree is again traversed down,
///and the new type is propagated as needed. Untyped constant expression values
///that become fully typed must now be representable by the full type (constant
///sub-expression trees are left alone except for their roots). This mechanism
///ensures that a client sees the actual (run-time) type an untyped value would
///have. It also permits type-checking of lhs shift operands "as if the shift
///were not present": when update_expr_type visits an untyped lhs shift operand
///and assigns it it's final type, that type must be an integer type, and a
///constant lhs must be representable as an integer.
///
///When an expression gets its final type, either on the way out from raw_expr,
///on the way down in update_expr_type, or at the end of the type checker run,
///the type (and constant value, if any) is recorded via Info.Types, if present.

impl<'a, S: SourceRead> Checker<'a, S> {
    fn op_token(&self, x: &mut Operand, token: &Token, binary: bool) -> bool {
        let pred = |t: &Token, ty: TypeKey| -> Option<bool> {
            if binary {
                match t {
                    Token::ADD => {
                        Some(typ::is_numeric(ty, self.tc_objs) || typ::is_string(ty, self.tc_objs))
                    }
                    Token::SUB => Some(typ::is_numeric(ty, self.tc_objs)),
                    Token::MUL => Some(typ::is_numeric(ty, self.tc_objs)),
                    Token::QUO => Some(typ::is_numeric(ty, self.tc_objs)),
                    Token::REM => Some(typ::is_integer(ty, self.tc_objs)),
                    Token::AND => Some(typ::is_integer(ty, self.tc_objs)),
                    Token::OR => Some(typ::is_integer(ty, self.tc_objs)),
                    Token::XOR => Some(typ::is_integer(ty, self.tc_objs)),
                    Token::AND_NOT => Some(typ::is_integer(ty, self.tc_objs)),
                    Token::LAND => Some(typ::is_boolean(ty, self.tc_objs)),
                    Token::LOR => Some(typ::is_boolean(ty, self.tc_objs)),
                    _ => None,
                }
            } else {
                match t {
                    Token::ADD => Some(typ::is_numeric(ty, self.tc_objs)),
                    Token::SUB => Some(typ::is_numeric(ty, self.tc_objs)),
                    Token::XOR => Some(typ::is_integer(ty, self.tc_objs)),
                    Token::NOT => Some(typ::is_boolean(ty, self.tc_objs)),
                    _ => None,
                }
            }
        };

        if let Some(ok) = pred(token, x.typ.unwrap()) {
            if !ok {
                let xd = self.new_dis(x);
                self.invalid_op(
                    xd.pos(),
                    &format!("operator {} not defined for {}", token, xd),
                );
            }
            ok
        } else {
            self.invalid_ast(x.pos(self.ast_objs), &format!("unknown operator {}", token));
            false
        }
    }

    /// The unary expression e may be None. It's passed in for better error messages only.
    fn unary(&mut self, x: &mut Operand, e: Option<Expr>, op: &Token) {
        match op {
            Token::AND => {
                // spec: "As an exception to the addressability requirement
                // x may also be a composite literal."
                match Checker::<S>::unparen(x.expr.as_ref().unwrap()) {
                    Expr::CompositeLit(_) => {}
                    _ => {
                        if x.mode != OperandMode::Variable {
                            let xd = self.new_dis(x);
                            self.invalid_op(xd.pos(), &format!("cannot take address of {}", xd));
                            x.mode = OperandMode::Invalid;
                            return;
                        }
                    }
                }
                x.mode = OperandMode::Value;
                x.typ = Some(self.tc_objs.new_t_pointer(x.typ.unwrap()));
            }
            Token::ARROW => {
                if let Some(chan) = self
                    .otype(x.typ.unwrap())
                    .underlying_val(self.tc_objs)
                    .try_as_chan()
                {
                    if chan.dir() == typ::ChanDir::SendOnly {
                        let xd = self.new_dis(x);
                        self.invalid_op(
                            xd.pos(),
                            &format!("cannot receive from send-only channel {}", xd),
                        );
                        return;
                    }
                    x.mode = OperandMode::CommaOk;
                    x.typ = Some(chan.elem());
                    self.octx.has_call_or_recv = true;
                    return;
                } else {
                    let xd = self.new_dis(x);
                    self.invalid_op(xd.pos(), &format!("cannot receive from non-channel {}", xd));
                }
            }
            _ => {
                if !self.op_token(x, op, false) {
                    x.mode = OperandMode::Invalid;
                    return;
                }
                if let OperandMode::Constant(v) = &mut x.mode {
                    let ty = typ::underlying_type(x.typ.unwrap(), self.tc_objs);
                    let tval = self.otype(ty);
                    let prec = if tval.is_unsigned(self.tc_objs) {
                        tval.try_as_basic().unwrap().size_of()
                    } else {
                        0
                    };
                    *v = Value::unary_op(op, v, prec);
                    // Typed constants must be representable in
                    // their type after each constant operation.
                    if tval.is_typed(self.tc_objs) {
                        if e.is_some() {
                            x.expr = e // for better error message
                        }
                        self.representable(x, ty);
                    }
                    return;
                }
                x.mode = OperandMode::Value;
                // x.typ remains unchanged
            }
        }
    }

    fn is_shift(op: &Token) -> bool {
        match op {
            Token::SHL | Token::SHR => true,
            _ => false,
        }
    }

    fn is_comparison(op: &Token) -> bool {
        match op {
            Token::EQL | Token::NEQ | Token::LSS | Token::LEQ | Token::GTR | Token::GEQ => true,
            _ => false,
        }
    }

    /// representable checks that a constant operand is representable in the given basic type.
    pub fn representable(&mut self, x: &mut Operand, t: TypeKey) {
        let tval = self.otype(t);
        let tbasic = tval.try_as_basic().unwrap();
        if let OperandMode::Constant(v) = &mut x.mode {
            let clone = v.clone();
            if !clone.representable(tbasic, Some(v)) {
                let o = &self.tc_objs;
                let xtval = self.otype(x.typ.unwrap());
                let tval = self.otype(t);
                let xd = self.new_dis(x);
                let td = self.new_dis(&t);
                // numeric conversion : error msg
                //
                // integer -> integer : overflows
                // integer -> float   : overflows (actually not possible)
                // float   -> integer : truncated
                // float   -> float   : overflows
                let msg = if xtval.is_numeric(o) && tval.is_numeric(o) {
                    if !xtval.is_integer(o) && tval.is_integer(o) {
                        format!("{} truncated to {}", xd, td)
                    } else {
                        format!("{} overflows {}", xd, td)
                    }
                } else {
                    format!("cannot convert {} to {}", xd, td)
                };
                self.error(xd.pos(), msg);
                x.mode = OperandMode::Invalid;
            }
        } else {
            unreachable!()
        }
    }

    /// update_expr_type updates the type of x to typ and invokes itself
    /// recursively for the operands of x, depending on expression kind.
    /// If typ is still an untyped and not the final type, update_expr_type
    /// only updates the recorded untyped type for x and possibly its
    /// operands. Otherwise (i.e., typ is not an untyped type anymore,
    /// or it is the final type for x), the type and value are recorded.
    /// Also, if x is a constant, it must be representable as a value of typ,
    /// and if x is the (formerly untyped) lhs operand of a non-constant
    /// shift, it must be an integer value.
    pub fn update_expr_type(
        &mut self,
        e: &Expr,
        t: TypeKey,
        final_: bool,
        fctx: &mut FilesContext<S>,
    ) {
        let old_opt = fctx.untyped.get(&e.id());
        if old_opt.is_none() {
            return; // nothing to do
        }
        let old = old_opt.unwrap();

        // update operands of x if necessary
        match e {
            Expr::Bad(_)
            | Expr::FuncLit(_)
            | Expr::CompositeLit(_)
            | Expr::Index(_)
            | Expr::Slice(_)
            | Expr::TypeAssert(_)
            | Expr::Star(_)
            | Expr::KeyValue(_)
            | Expr::Array(_)
            | Expr::Struct(_)
            | Expr::Func(_)
            | Expr::Interface(_)
            | Expr::Map(_)
            | Expr::Chan(_) => {
                if cfg!(debug_assertions) {
                    let ed = self.new_dis(e);
                    let (otd, td) = (self.new_td_o(&old.typ), self.new_dis(&t));
                    self.dump(
                        Some(ed.pos()),
                        &format!("found old type({}): {} (new: {})", ed, otd, td),
                    );
                    unreachable!()
                }
                return;
            }
            Expr::Call(_) => {
                // Resulting in an untyped constant (e.g., built-in complex).
                // The respective calls take care of calling update_expr_type
                // for the arguments if necessary.
            }
            Expr::Ident(_) | Expr::BasicLit(_) | Expr::Selector(_) => {
                // An identifier denoting a constant, a constant literal,
                // or a qualified identifier (imported untyped constant).
                // No operands to take care of.
            }
            Expr::Paren(p) => {
                self.update_expr_type(&p.expr, t, final_, fctx);
            }
            Expr::Unary(u) => {
                // If x is a constant, the operands were constants.
                // The operands don't need to be updated since they
                // never get "materialized" into a typed value. If
                // left in the untyped map, they will be processed
                // at the end of the type check.
                if old.mode.constant_val().is_none() {
                    self.update_expr_type(&u.expr, t, final_, fctx);
                }
            }
            Expr::Binary(b) => {
                if old.mode.constant_val().is_none() {
                    if Checker::<S>::is_comparison(&b.op) {
                        // The result type is independent of operand types
                        // and the operand types must have final types.
                    } else if Checker::<S>::is_shift(&b.op) {
                        // The result type depends only on lhs operand.
                        // The rhs type was updated when checking the shift.
                        self.update_expr_type(&b.expr_a, t, final_, fctx);
                    } else {
                        // The operand types match the result type.
                        self.update_expr_type(&b.expr_a, t, final_, fctx);
                        self.update_expr_type(&b.expr_b, t, final_, fctx);
                    }
                }
            }
            _ => unreachable!(),
        }

        // If the new type is not final and still untyped, just
        // update the recorded type.
        let o = &self.tc_objs;
        if !final_ && typ::is_untyped(t, o) {
            let old = fctx.untyped.get_mut(&e.id()).unwrap();
            old.typ = Some(typ::underlying_type(t, o));
            return;
        }

        // Otherwise we have the final (typed or untyped type).
        // Remove it from the map of yet untyped expressions.
        let removed = fctx.untyped.remove(&e.id());
        let old = removed.as_ref().unwrap();

        if old.is_lhs {
            // If x is the lhs of a shift, its final type must be integer.
            // We already know from the shift check that it is representable
            // as an integer if it is a constant.
            if !typ::is_integer(t, o) {
                let ed = self.new_dis(e);
                let td = self.new_dis(&t);
                self.invalid_op(
                    ed.pos(),
                    &format!("shifted operand {} (type {}) must be integer", ed, td),
                );
                return;
            }
            // Even if we have an integer, if the value is a constant we
            // still must check that it is representable as the specific
            // int type requested
        }
        if old.mode.constant_val().is_some() {
            // If x is a constant, it must be representable as a value of typ.
            let mut c = Operand::with(old.mode.clone(), Some(e.clone()), old.typ);
            self.convert_untyped(&mut c, t, fctx);
            if c.invalid() {
                return;
            }
        }

        // Everything's fine, record final type and value for x.
        self.result.record_type_and_value(e, old.mode.clone(), t);
    }

    /// update_expr_val updates the value of x to val.
    fn update_expr_val(e: &Expr, val: Value, fctx: &mut FilesContext<S>) {
        if let Some(info) = fctx.untyped.get_mut(&e.id()) {
            if let OperandMode::Constant(v) = &mut info.mode {
                *v = val;
            } else {
                unreachable!()
            }
        }
    }

    /// convert_untyped attempts to set the type of an untyped value to the target type.
    pub fn convert_untyped(
        &mut self,
        x: &mut Operand,
        target: TypeKey,
        fctx: &mut FilesContext<S>,
    ) {
        let o = &self.tc_objs;
        if x.invalid() || typ::is_typed(x.typ.unwrap(), o) || target == self.invalid_type() {
            return;
        }

        let on_err = |c: &mut Checker<S>, x: &mut Operand| {
            let xd = c.new_dis(x);
            let td = c.new_dis(&target);
            c.error(xd.pos(), format!("cannot convert {} to {}", xd, td));
            x.mode = OperandMode::Invalid;
        };

        if typ::is_untyped(target, o) {
            // both x and target are untyped
            let order = |bt: BasicType| -> usize {
                match bt {
                    BasicType::UntypedInt => 1,
                    BasicType::UntypedRune => 2,
                    BasicType::UntypedFloat => 3,
                    BasicType::UntypedComplex => 4,
                    _ => unreachable!(),
                }
            };
            let xtval = self.otype(x.typ.unwrap());
            let ttval = self.otype(target);
            let xbasic = xtval.try_as_basic().unwrap().typ();
            let tbasic = ttval.try_as_basic().unwrap().typ();
            if xbasic != tbasic {
                if xtval.is_numeric(o) && ttval.is_numeric(o) {
                    if order(xbasic) < order(tbasic) {
                        x.typ = Some(target);
                        self.update_expr_type(x.expr.as_ref().unwrap(), target, false, fctx);
                    }
                } else {
                    return on_err(self, x);
                }
            }
            return;
        }

        let t = typ::underlying_type(target, o);
        let xtype = x.typ.unwrap();
        let tval = self.otype(t);
        let final_target = match tval {
            Type::Basic(_) => {
                if let OperandMode::Constant(v) = &x.mode {
                    let v_clone = v.clone();
                    self.representable(x, t);
                    if x.invalid() {
                        return;
                    }
                    // expression value may have been rounded - update if needed
                    Checker::update_expr_val(x.expr.as_ref().unwrap(), v_clone, fctx);
                    Some(target)
                } else {
                    // Non-constant untyped values may appear as the
                    // result of comparisons (untyped bool), intermediate
                    // (delayed-checked) rhs operands of shifts, and as
                    // the value nil.
                    let ok = match self.otype(x.typ.unwrap()).try_as_basic().unwrap().typ() {
                        BasicType::UntypedBool => tval.is_boolean(o),
                        BasicType::UntypedInt
                        | BasicType::UntypedRune
                        | BasicType::UntypedFloat
                        | BasicType::UntypedComplex => tval.is_numeric(o),
                        BasicType::UntypedString => unreachable!(),
                        BasicType::UntypedNil => tval.has_nil(o),
                        _ => false,
                    };
                    if ok {
                        Some(target)
                    } else {
                        None
                    }
                }
            }
            Type::Interface(detail) => {
                // Update operand types to the default type rather then
                // the target (interface) type: values must have concrete
                // dynamic types. If the value is nil, keep it untyped
                if x.is_nil(self.tc_objs.universe()) {
                    Some(self.basic_type(BasicType::UntypedNil))
                } else {
                    if detail.is_empty() {
                        Some(typ::untyped_default_type(xtype, o))
                    } else {
                        // cannot assign untyped values to non-empty interfaces
                        None
                    }
                }
            }
            Type::Pointer(_)
            | Type::Signature(_)
            | Type::Slice(_)
            | Type::Map(_)
            | Type::Chan(_) => {
                if x.is_nil(self.tc_objs.universe()) {
                    Some(self.basic_type(BasicType::UntypedNil))
                } else {
                    None
                }
            }
            _ => None,
        };

        if let Some(t) = final_target {
            x.typ = final_target;
            self.update_expr_type(x.expr.as_ref().unwrap(), t, true, fctx);
        } else {
            on_err(self, x);
        }
    }

    pub fn comparison(
        &mut self,
        x: &mut Operand,
        y: &Operand,
        op: &Token,
        fctx: &mut FilesContext<S>,
    ) {
        // spec: "In any comparison, the first operand must be assignable
        // to the type of the second operand, or vice versa."
        let (xtype, ytype) = (x.typ.unwrap(), y.typ.unwrap());
        let assignable =
            x.assignable_to(ytype, None, self, fctx) || y.assignable_to(xtype, None, self, fctx);
        let o = &self.tc_objs;
        let u = o.universe();
        let emsg = if assignable {
            let (xtval, ytval) = (self.otype(xtype), self.otype(ytype));
            let defined = match op {
                Token::EQL | Token::NEQ => {
                    (xtval.comparable(o) && ytval.comparable(o))
                        || (x.is_nil(u) && ytval.has_nil(o))
                        || (y.is_nil(u) && xtval.has_nil(o))
                }
                Token::LSS | Token::LEQ | Token::GTR | Token::GEQ => {
                    xtval.is_ordered(o) && ytval.is_ordered(o)
                }
                _ => unreachable!(),
            };
            if !defined {
                let t = if x.is_nil(u) { ytype } else { xtype };
                let td = self.new_dis(&t);
                Some(format!("operator {} not defined for {}", op, td))
            } else {
                None
            }
        } else {
            let (xd, yd) = (self.new_dis(&xtype), self.new_dis(&ytype));
            Some(format!("mismatched types {} and {}", xd, yd))
        };
        if let Some(m) = emsg {
            let pos = x.pos(self.ast_objs);
            let xd = self.new_dis(x.expr.as_ref().unwrap());
            let yd = self.new_dis(y.expr.as_ref().unwrap());
            self.error(pos, format!("cannot compare {} {} {} ({})", xd, op, yd, m));
            x.mode = OperandMode::Invalid;
            return;
        }

        match (&mut x.mode, &y.mode) {
            (OperandMode::Constant(vx), OperandMode::Constant(vy)) => {
                *vx = Value::with_bool(Value::compare(vx, op, vy));
                // The operands are never materialized; no need to update
                // their types.
            }
            _ => {
                x.mode = OperandMode::Value;
                // The operands have now their final types, which at run-
                // time will be materialized. Update the expression trees.
                // If the current types are untyped, the materialized type
                // is the respective default type.
                self.update_expr_type(
                    x.expr.as_ref().unwrap(),
                    typ::untyped_default_type(xtype, self.tc_objs),
                    true,
                    fctx,
                );
                self.update_expr_type(
                    y.expr.as_ref().unwrap(),
                    typ::untyped_default_type(ytype, self.tc_objs),
                    true,
                    fctx,
                );
            }
        }
        // spec: "Comparison operators compare two operands and yield
        //        an untyped boolean value."
        x.typ = Some(self.basic_type(BasicType::UntypedBool));
    }

    fn shift(
        &mut self,
        x: &mut Operand,
        y: &mut Operand,
        op: &Token,
        e: Option<&Expr>,
        fctx: &mut FilesContext<S>,
    ) {
        let o = &self.tc_objs;
        let xtval = self.otype(x.typ.unwrap());
        let xt_untyped = xtval.is_untyped(o);
        let xt_integer = xtval.is_integer(o);
        let x_const = x.mode.constant_val().map(|x| x.to_int().into_owned());

        // The lhs is of integer type or an untyped constant representable
        // as an integer
        let lhs_ok =
            xt_integer || (xt_untyped && x_const.is_some() && x_const.as_ref().unwrap().is_int());
        if !lhs_ok {
            let xd = self.new_dis(x);
            self.invalid_op(xd.pos(), &format!("shifted operand {} must be integer", xd));
            x.mode = OperandMode::Invalid;
            return;
        }

        // spec: "The right operand in a shift expression must have unsigned
        // integer type or be an untyped constant representable by a value of
        // type uint."
        let ytval = self.otype(y.typ.unwrap());
        if ytval.is_unsigned(o) {
            //ok
        } else if ytval.is_untyped(o) {
            self.convert_untyped(y, self.basic_type(BasicType::Uint), fctx);
            if y.invalid() {
                x.mode = OperandMode::Invalid;
                return;
            }
        } else {
            let yd = self.new_dis(y);
            self.error(
                yd.pos(),
                format!("shift count {} must be unsigned integer", yd),
            );
            x.mode = OperandMode::Invalid;
            return;
        }

        if let OperandMode::Constant(xv) = &mut x.mode {
            if let OperandMode::Constant(yv) = &y.mode {
                // rhs must be an integer value
                let yval = yv.to_int();
                if !yval.is_int() {
                    let yd = self.new_dis(y);
                    self.invalid_op(
                        yd.pos(),
                        &format!("shift count {} must be unsigned integer", yd),
                    );
                    x.mode = OperandMode::Invalid;
                    return;
                }
                // rhs must be within reasonable bounds
                let shift_bound = 1023 - 1 + 52; // so we can express smallestFloat64
                let (s, ok) = yval.int_as_u64();
                if !ok || s > shift_bound {
                    let yd = self.new_dis(y);
                    self.invalid_op(yd.pos(), &format!("invalid shift count {}", yd));
                    x.mode = OperandMode::Invalid;
                    return;
                }
                // The lhs is representable as an integer but may not be an integer
                // (e.g., 2.0, an untyped float) - this can only happen for untyped
                // non-integer numeric constants. Correct the type so that the shift
                // result is of integer type.
                if !xt_integer {
                    x.typ = Some(self.basic_type(BasicType::UntypedInt));
                }
                // x is a constant so xval != nil and it must be of Int kind.
                *xv = Value::shift(xv.to_int().as_ref(), op, s as usize);
                // Typed constants must be representable in
                // their type after each constant operation.
                if typ::is_typed(x.typ.unwrap(), self.tc_objs) {
                    if e.is_some() {
                        x.expr = e.map(|x| x.clone()); // for better error message
                    }
                    self.representable(x, x.typ.unwrap());
                }
                return;
            }

            if xt_untyped {
                // spec: "If the left operand of a non-constant shift
                // expression is an untyped constant, the type of the
                // constant is what it would be if the shift expression
                // were replaced by its left operand alone.".
                //
                // Delay operand checking until we know the final type
                // by marking the lhs expression as lhs shift operand.
                //
                // Usually (in correct programs), the lhs expression
                // is in the untyped map. However, it is possible to
                // create incorrect programs where the same expression
                // is evaluated twice (via a declaration cycle) such
                // that the lhs expression type is determined in the
                // first round and thus deleted from the map, and then
                // not found in the second round (double insertion of
                // the same expr node still just leads to one entry for
                // that node, and it can only be deleted once).
                // Be cautious and check for presence of entry.
                // Example: var e, f = int(1<<""[f])
                if let Some(info) = fctx.untyped.get_mut(&x.expr.as_ref().unwrap().id()) {
                    info.is_lhs = true;
                }
                // keep x's type
                x.mode = OperandMode::Value;
                return;
            }
        }

        // constant rhs must be >= 0
        if let OperandMode::Constant(v) = &y.mode {
            if v.sign() < 0 {
                let yd = self.new_dis(y);
                self.invalid_op(
                    yd.pos(),
                    &format!("shift count {} must not be negative", yd),
                );
            }
        }

        if !typ::is_integer(x.typ.unwrap(), self.tc_objs) {
            let xd = self.new_dis(x);
            self.invalid_op(xd.pos(), &format!("shifted operand {} must be integer", xd));
            x.mode = OperandMode::Value;
            return;
        }
        x.mode = OperandMode::Value;
    }

    /// The binary expression e may be None. It's passed in for better error messages only.
    pub fn binary(
        &mut self,
        x: &mut Operand,
        e: Option<&Expr>,
        lhs: &Expr,
        rhs: &Expr,
        op: &Token,
        fctx: &mut FilesContext<S>,
    ) {
        let mut y = Operand::new();
        self.expr(x, &lhs, fctx);
        self.expr(&mut y, &rhs, fctx);

        if x.invalid() {
            return;
        }
        if y.invalid() {
            x.mode = OperandMode::Invalid;
            x.expr = y.expr.clone();
            return;
        }

        if Checker::<S>::is_shift(op) {
            self.shift(x, &mut y, op, e, fctx);
            return;
        }

        self.convert_untyped(x, y.typ.unwrap(), fctx);
        if x.invalid() {
            return;
        }

        self.convert_untyped(&mut y, x.typ.unwrap(), fctx);
        if y.invalid() {
            x.mode = OperandMode::Invalid;
            return;
        }

        if Checker::<S>::is_comparison(op) {
            self.comparison(x, &y, op, fctx);
            return;
        }

        if !typ::identical_o(x.typ, y.typ, self.tc_objs) {
            // only report an error if we have valid types
            // (otherwise we had an error reported elsewhere already)
            let invalid = Some(self.invalid_type());
            if x.typ != invalid && y.typ != invalid {
                let xd = self.new_td_o(&x.typ);
                let yd = self.new_td_o(&y.typ);
                self.invalid_op(
                    x.pos(self.ast_objs),
                    &format!("mismatched types {} and {}", xd, yd),
                );
            }
            x.mode = OperandMode::Invalid;
            return;
        }

        if !self.op_token(x, op, true) {
            x.mode = OperandMode::Invalid;
            return;
        }

        let o = &self.tc_objs;
        if *op == Token::QUO || *op == Token::REM {
            // check for zero divisor
            if x.mode.constant_val().is_some() || typ::is_integer(x.typ.unwrap(), o) {
                if let Some(v) = y.mode.constant_val() {
                    if v.sign() == 0 {
                        self.invalid_op(y.pos(self.ast_objs), "division by zero");
                        x.mode = OperandMode::Invalid;
                        return;
                    }
                }
            }
            // check for divisor underflow in complex division
            if x.mode.constant_val().is_some() && typ::is_complex(x.typ.unwrap(), o) {
                if let Some(v) = y.mode.constant_val() {
                    let (re, im) = (v.real(), v.imag());
                    let re2 = Value::binary_op(&re, &Token::MUL, &re);
                    let im2 = Value::binary_op(&im, &Token::MUL, &im);
                    if re2.sign() == 0 && im2.sign() == 0 {
                        self.invalid_op(y.pos(self.ast_objs), "division by zero");
                        x.mode = OperandMode::Invalid;
                        return;
                    }
                }
            }
        }

        match (&mut x.mode, &y.mode) {
            (OperandMode::Constant(vx), OperandMode::Constant(vy)) => {
                let ty = typ::underlying_type(x.typ.unwrap(), o);
                // force integer division of integer operands
                // (not real QUO_ASSIGN, just borrowing it)
                let op2 = if *op == Token::QUO && typ::is_integer(ty, o) {
                    &Token::QUO_ASSIGN
                } else {
                    op
                };
                *vx = Value::binary_op(vx, op2, vy);
                // Typed constants must be representable in
                // their type after each constant operation.
                if typ::is_typed(ty, o) {
                    x.expr = e.map(|x| x.clone()); // for better error message
                    self.representable(x, ty)
                }
            }
            _ => {
                x.mode = OperandMode::Value;
                // x.typ is unchanged
            }
        }
    }

    /// index checks an index expression for validity.
    /// max is the upper bound for index.
    /// returns the value of the index when it's a constant, returns None if it's not
    pub fn index(
        &mut self,
        index: &Expr,
        max: Option<u64>,
        fctx: &mut FilesContext<S>,
    ) -> Result<Option<u64>, ()> {
        let x = &mut Operand::new();
        self.expr(x, index, fctx);
        if x.invalid() {
            return Err(());
        }

        // an untyped constant must be representable as Int
        self.convert_untyped(x, self.basic_type(BasicType::Int), fctx);
        if x.invalid() {
            return Err(());
        }

        // the index must be of integer type
        if !typ::is_integer(x.typ.unwrap(), self.tc_objs) {
            let xd = self.new_dis(x);
            self.invalid_arg(xd.pos(), &format!("index {} must be integer", xd));
            return Err(());
        }

        // a constant index i must be in bounds
        if let OperandMode::Constant(v) = &x.mode {
            if v.sign() < 0 {
                let xd = self.new_dis(x);
                self.invalid_arg(xd.pos(), &format!("index {} must not be negative", xd));
                return Err(());
            }
            let (i, valid) = v.to_int().int_as_u64();
            if !valid || max.map_or(false, |x| i >= x) {
                let xd = self.new_dis(x);
                self.invalid_arg(xd.pos(), &format!("index {} out of bounds", xd));
                return Err(());
            }
            return Ok(Some(i));
        }

        Ok(None)
    }

    /// indexed_elems checks the elements of an array or slice composite literal
    /// against the literal's element type, and the element indices against
    /// the literal length if known . It returns the length of the literal
    /// (maximum index value + 1).
    fn indexed_elems(
        &mut self,
        elems: &Vec<Expr>,
        t: TypeKey,
        length: Option<u64>,
        fctx: &mut FilesContext<S>,
    ) -> u64 {
        let mut visited = HashSet::new();
        let (_, max) = elems.iter().fold((0, 0), |(index, max), e| {
            let (valid_index, eval) = if let Expr::KeyValue(kv) = e {
                let i = self.index(&kv.key, length, fctx);
                let kv_index = if i.is_ok() {
                    if let Some(index) = i.unwrap() {
                        Some(index)
                    } else {
                        let pos = e.pos(self.ast_objs);
                        let kd = self.new_dis(&kv.key);
                        self.error(pos, format!("index {} must be integer constant", kd));
                        None
                    }
                } else {
                    None
                };
                (kv_index, &kv.val)
            } else if length.is_some() && index >= length.unwrap() {
                self.error(
                    e.pos(self.ast_objs),
                    format!("index {} is out of bounds (>= {})", index, length.unwrap()),
                );
                (None, e)
            } else {
                (Some(index), e)
            };

            let (mut new_index, mut new_max) = (index, max);
            if let Some(i) = valid_index {
                if visited.contains(&i) {
                    self.error(
                        e.pos(self.ast_objs),
                        format!("duplicate index {} in array or slice literal", i),
                    );
                }
                visited.insert(i);

                new_index = i + 1;
                if new_index > new_max {
                    new_max = new_index;
                }
            }

            // check element against composite literal element type
            let x = &mut Operand::new();
            self.expr_with_hint(x, eval, t, fctx);
            self.assignment(x, Some(t), "array or slice literal", fctx);

            (new_index, new_max)
        });
        max
    }

    /// raw_expr typechecks expression e and initializes x with the expression
    /// value or type. If an error occurred, x.mode is set to invalid.
    /// If hint is_some(), it is the type of a composite literal element.
    pub fn raw_expr(
        &mut self,
        x: &mut Operand,
        e: &Expr,
        hint: Option<TypeKey>,
        fctx: &mut FilesContext<S>,
    ) -> ExprKind {
        if self.trace() {
            let ed = self.new_dis(e);
            self.trace_begin(ed.pos(), &format!("{}", ed));
        }

        let kind = self.raw_internal(x, e, hint, fctx);

        let ty = match &x.mode {
            OperandMode::Invalid => self.invalid_type(),
            OperandMode::NoValue => *self.tc_objs.universe().no_value_tuple(),
            _ => x.typ.unwrap(),
        };

        if typ::is_untyped(ty, self.tc_objs) {
            // delay type and value recording until we know the type
            // or until the end of type checking
            fctx.remember_untyped(
                x.expr.as_ref().unwrap(),
                ExprInfo {
                    is_lhs: false,
                    mode: x.mode.clone(),
                    typ: Some(ty),
                },
            )
        } else {
            self.result.record_type_and_value(e, x.mode.clone(), ty);
        }

        if self.trace() {
            let pos = e.pos(self.ast_objs);
            self.trace_end(pos, &format!("=> {}", self.new_dis(x)));
        }

        kind
    }

    /// raw_internal contains the core of type checking of expressions.
    /// Must only be called by raw_expr.
    fn raw_internal(
        &mut self,
        x: &mut Operand,
        e: &Expr,
        hint: Option<TypeKey>,
        fctx: &mut FilesContext<S>,
    ) -> ExprKind {
        // make sure x has a valid state in case of bailout
        x.mode = OperandMode::Invalid;
        x.typ = Some(self.invalid_type());
        let on_err = |x: &mut Operand| {
            x.mode = OperandMode::Invalid;
            x.expr = Some(e.clone());
            ExprKind::Statement // avoid follow-up errors
        };

        let epos = e.pos(self.ast_objs);
        match e {
            Expr::Bad(_) => return on_err(x),
            Expr::Ident(i) => self.ident(x, *i, None, false, fctx),
            Expr::Ellipsis(_) => {
                // ellipses are handled explicitly where they are legal
                // (array composite literals and parameter lists)
                self.error_str(epos, "invalid use of '...'");
                return on_err(x);
            }
            Expr::BasicLit(bl) => {
                x.set_const(&bl.token, self.tc_objs.universe());
                if x.invalid() {
                    let lit = bl.token.get_literal();
                    self.invalid_ast(epos, &format!("invalid literal {}", lit));
                    return on_err(x);
                }
            }
            Expr::FuncLit(fl) => {
                let t = self.type_expr(&Expr::Func(fl.typ), fctx);
                if let Some(_) = self.otype(t).try_as_signature() {
                    let decl = self.octx.decl;
                    let body = BodyContainer::FuncLitExpr(e.clone());
                    let iota = self.octx.iota.clone();
                    let f = move |checker: &mut Checker<S>, fctx: &mut FilesContext<S>| {
                        checker.func_body(decl, "<function literal>", t, body, iota, fctx);
                    };
                    fctx.later(Box::new(f));
                    x.mode = OperandMode::Value;
                    x.typ = Some(t);
                } else {
                    let ed = self.new_dis(e);
                    self.invalid_ast(epos, &format!("invalid function literal {}", ed));
                    return on_err(x);
                }
            }
            Expr::CompositeLit(cl) => {
                let (ty, base) = if let Some(etype) = &cl.typ {
                    // composite literal type present - use it
                    // [...]T array types may only appear with composite literals.
                    // Check for them here so we don't have to handle ... in general.
                    let mut elem = None;
                    if let Expr::Array(arr) = etype {
                        if let Some(len_expr) = &arr.len {
                            if let Expr::Ellipsis(ell) = len_expr {
                                if ell.elt.is_none() {
                                    elem = Some(&arr.elt);
                                }
                            }
                        }
                    }
                    let t = if let Some(el) = elem {
                        let elem_ty = self.type_expr(el, fctx);
                        self.tc_objs.new_t_array(elem_ty, None)
                    } else {
                        self.type_expr(&etype, fctx)
                    };
                    (t, t)
                } else if let Some(h) = hint {
                    // no composite literal type present - use hint (element type of enclosing type)
                    let (base, _) =
                        lookup::try_deref(typ::underlying_type(h, self.tc_objs), self.tc_objs);
                    (h, base)
                } else {
                    self.error_str(epos, "missing type in composite literal");
                    return on_err(x);
                };

                let utype_key = typ::underlying_type(base, self.tc_objs);
                let utype = &self.tc_objs.types[utype_key];
                match utype {
                    Type::Struct(detail) => {
                        if cl.elts.len() > 0 {
                            let fields = detail.fields().clone();
                            if let Expr::KeyValue(_) = &cl.elts[0] {
                                let mut visited: HashSet<usize> = HashSet::new();
                                for e in cl.elts.iter() {
                                    let kv = if let Expr::KeyValue(kv) = e {
                                        kv
                                    } else {
                                        let msg = "mixture of field:value and value elements in struct literal";
                                        self.error_str(e.pos(self.ast_objs), msg);
                                        continue;
                                    };
                                    // do all possible checks early (before exiting due to errors)
                                    // so we don't drop information on the floor
                                    self.expr(x, &kv.val, fctx);
                                    let keykey = if let Expr::Ident(ikey) = kv.key {
                                        ikey
                                    } else {
                                        let ed = self.new_dis(&kv.key);
                                        self.error(
                                            e.pos(self.ast_objs),
                                            format!("invalid field name {} in struct literal", ed),
                                        );
                                        continue;
                                    };
                                    let key = &self.ast_objs.idents[keykey];
                                    let i = if let Some(i) = lookup::field_index(
                                        &fields,
                                        Some(self.pkg),
                                        &key.name,
                                        self.tc_objs,
                                    ) {
                                        i
                                    } else {
                                        self.error(
                                            e.pos(self.ast_objs),
                                            format!(
                                                "unknown field {} in struct literal",
                                                &key.name
                                            ),
                                        );
                                        continue;
                                    };
                                    let fld = fields[i];
                                    self.result.record_use(keykey, fld);
                                    let etype = self.lobj(fld).typ().unwrap();
                                    self.assignment(x, Some(etype), "struct literal", fctx);
                                    if visited.contains(&i) {
                                        self.error(
                                            e.pos(self.ast_objs),
                                            format!(
                                                "duplicate field name {} in struct literal",
                                                &self.ast_objs.idents[keykey].name
                                            ),
                                        );
                                        continue;
                                    } else {
                                        visited.insert(i);
                                    }
                                }
                            } else {
                                for (i, e) in cl.elts.iter().enumerate() {
                                    if let Expr::KeyValue(_) = e {
                                        let msg = "mixture of field:value and value elements in struct literal";
                                        self.error_str(e.pos(self.ast_objs), msg);
                                        continue;
                                    }
                                    self.expr(x, e, fctx);
                                    if i >= fields.len() {
                                        let pos = x.pos(self.ast_objs);
                                        self.error_str(pos, "too many values in struct literal");
                                        break; // cannot continue
                                    }
                                    let fld = self.lobj(fields[i]);
                                    if !fld.exported() && fld.pkg() != Some(self.pkg) {
                                        let pos = x.pos(self.ast_objs);
                                        let (n, td) = (fld.name(), self.new_dis(&ty));
                                        let msg = format!(
                                            "implicit assignment to unexported field {} in {} literal", n, td);
                                        self.error(pos, msg);
                                        continue;
                                    }
                                    let field_type = fld.typ();
                                    self.assignment(x, field_type, "struct literal", fctx);
                                }
                                if cl.elts.len() < fields.len() {
                                    self.error_str(cl.r_brace, "too few values in struct literal");
                                    // ok to continue
                                }
                            }
                        }
                    }
                    Type::Array(detail) => {
                        // todo: the go code checks if detail.elem is nil, do we need that?
                        // see the original go code for details
                        let arr_len = detail.len();
                        let elem = detail.elem();
                        let len = detail.len();
                        let n = self.indexed_elems(&cl.elts, elem, len, fctx);
                        // If we have an array of unknown length (usually [...]T arrays, but also
                        // arrays [n]T where n is invalid) set the length now that we know it and
                        // record the type for the array (usually done by check.typ which is not
                        // called for [...]T). We handle [...]T arrays and arrays with invalid
                        // length the same here because it makes sense to "guess" the length for
                        // the latter if we have a composite literal; e.g. for [n]int{1, 2, 3}
                        // where n is invalid for some reason, it seems fair to assume it should
                        // be 3
                        if arr_len.is_none() {
                            self.otype_mut(utype_key)
                                .try_as_array_mut()
                                .unwrap()
                                .set_len(n as u64);
                            // cl.Type is missing if we have a composite literal element
                            // that is itself a composite literal with omitted type. In
                            // that case there is nothing to record (there is no type in
                            // the source at that point).
                            if let Some(te) = &cl.typ {
                                self.result.record_type_and_value(
                                    te,
                                    OperandMode::TypeExpr,
                                    utype_key,
                                );
                            }
                        }
                    }
                    Type::Slice(detail) => {
                        // todo: the go code checks if detail.elem is nil, do we need that?
                        // see the original go code for details
                        let elem_t = detail.elem();
                        self.indexed_elems(&cl.elts, elem_t, None, fctx);
                    }
                    Type::Map(detail) => {
                        // todo: the go code checks if detail.key/elem is nil, do we need that?
                        // see the original go code for details
                        let iface_key = self
                            .otype(detail.key())
                            .underlying_val(self.tc_objs)
                            .try_as_interface()
                            .is_some();
                        let (t_key, t_elem) = (detail.key(), detail.elem());
                        let mut visited = Map::new();
                        for e in cl.elts.iter() {
                            let kv = match e {
                                Expr::KeyValue(kv) => kv,
                                _ => {
                                    let pos = e.pos(self.ast_objs);
                                    self.error_str(pos, "missing key in map literal");
                                    continue;
                                }
                            };
                            self.expr_with_hint(x, &kv.key, t_key, fctx);
                            self.assignment(x, Some(t_key), "map literal", fctx);
                            if x.invalid() {
                                continue;
                            }
                            if let OperandMode::Constant(v) = &x.mode {
                                // if the key is of interface type, the type is also significant
                                // when checking for duplicates
                                let duplicate = if iface_key {
                                    let o = &self.tc_objs;
                                    let xtype = x.typ.unwrap();
                                    if !visited.contains_key(v) {
                                        visited.insert(v.clone(), Some(vec![]));
                                    }
                                    let types = visited.get_mut(v).unwrap().as_mut().unwrap();
                                    let dup = types
                                        .iter()
                                        .find(|&&ty| typ::identical(ty, xtype, o))
                                        .is_some();
                                    types.push(xtype);
                                    dup
                                } else {
                                    let dup = visited.contains_key(v);
                                    if !dup {
                                        visited.insert(v.clone(), None);
                                    }
                                    dup
                                };
                                if duplicate {
                                    self.error(
                                        x.pos(self.ast_objs),
                                        format!("duplicate key {} in map literal", v),
                                    );
                                    continue;
                                }
                            }
                            self.expr_with_hint(x, &kv.val, t_elem, fctx);
                            self.assignment(x, Some(t_elem), "map literal", fctx);
                        }
                    }
                    _ => {
                        // when "using" all elements unpack KeyValueExpr
                        // explicitly because check.use doesn't accept them
                        for e in cl.elts.iter() {
                            let unpack = match e {
                                // Ideally, we should also "use" kv.Key but we can't know
                                // if it's an externally defined struct key or not. Going
                                // forward anyway can lead to other errors. Give up instead.
                                Expr::KeyValue(kv) => &kv.key,
                                _ => e,
                            };
                            self.use_exprs(&vec![unpack.clone()], fctx);
                        }
                        // if utype is invalid, an error was reported before
                        if utype_key != self.invalid_type() {
                            let td = self.new_dis(&ty);
                            self.error(epos, format!("invalid composite literal type {}", td));
                            return on_err(x);
                        }
                    }
                }

                x.mode = OperandMode::Value;
                x.typ = Some(ty);
            }
            Expr::Paren(p) => {
                let kind = self.raw_expr(x, &p.expr, None, fctx);
                x.expr = Some(e.clone());
                return kind;
            }
            Expr::Selector(s) => {
                self.selector(x, s, fctx);
            }
            Expr::Index(ie) => {
                self.expr(x, &ie.expr, fctx);
                if x.invalid() {
                    self.use_exprs(&vec![ie.index.clone()], fctx);
                    return on_err(x);
                }

                let typ_val = self.otype(x.typ.unwrap()).underlying_val(self.tc_objs);
                let (valid, length) = match typ_val {
                    Type::Basic(detail) => {
                        if detail.info() == typ::BasicInfo::IsString {
                            let len = if let OperandMode::Constant(v) = &x.mode {
                                Some(v.str_as_string().len() as u64)
                            } else {
                                None
                            };
                            // an indexed string always yields a byte value
                            // (not a constant) even if the string and the
                            // index are constant
                            x.mode = OperandMode::Value;
                            x.typ = Some(*self.tc_objs.universe().byte());
                            (true, len)
                        } else {
                            (false, None)
                        }
                    }
                    Type::Array(detail) => {
                        if x.mode != OperandMode::Variable {
                            x.mode = OperandMode::Value;
                        }
                        x.typ = Some(detail.elem());
                        (true, detail.len())
                    }
                    Type::Pointer(detail) => {
                        if let Some(arr) = self
                            .otype(detail.base())
                            .underlying_val(self.tc_objs)
                            .try_as_array()
                        {
                            x.mode = OperandMode::Variable;
                            x.typ = Some(arr.elem());
                            (true, arr.len())
                        } else {
                            (false, None)
                        }
                    }
                    Type::Slice(detail) => {
                        x.mode = OperandMode::Variable;
                        x.typ = Some(detail.elem());
                        (true, None)
                    }
                    Type::Map(detail) => {
                        let (key, elem) = (detail.key(), detail.elem());
                        let xkey = &mut Operand::new();
                        self.expr(xkey, &ie.index, fctx);
                        self.assignment(xkey, Some(key), "map index", fctx);
                        if x.invalid() {
                            return on_err(x);
                        }
                        x.mode = OperandMode::MapIndex;
                        x.typ = Some(elem);
                        x.expr = Some(e.clone());
                        return ExprKind::Expression;
                    }
                    _ => (false, None),
                };

                if !valid {
                    let xd = self.new_dis(x);
                    self.invalid_op(xd.pos(), &format!("cannot index {}", xd));
                    return on_err(x);
                }
                let _ = self.index(&ie.index, length, fctx);
                // ok to continue
            }
            Expr::Slice(se) => {
                self.expr(x, &se.expr, fctx);
                if x.invalid() {
                    let exprs = [se.low.as_ref(), se.high.as_ref(), se.max.as_ref()]
                        .iter()
                        .filter_map(|x| x.map(|ex| ex.clone()))
                        .collect();
                    self.use_exprs(&exprs, fctx);
                    return on_err(x);
                }

                let typ_val = self.otype(x.typ.unwrap()).underlying_val(self.tc_objs);
                let (valid, length) = match typ_val {
                    Type::Basic(detail) => {
                        if detail.info() == typ::BasicInfo::IsString {
                            if se.slice3 {
                                self.error_str(epos, "3-index slice of string");
                                return on_err(x);
                            }
                            let len = if let OperandMode::Constant(v) = &x.mode {
                                Some(v.str_as_string().len() as u64)
                            } else {
                                None
                            };
                            // spec: "For untyped string operands the result
                            // is a non-constant value of type string."
                            if detail.typ() == typ::BasicType::UntypedString {
                                x.typ = Some(self.basic_type(BasicType::Str));
                            }
                            (true, len)
                        } else {
                            (false, None)
                        }
                    }
                    Type::Array(detail) => {
                        if x.mode != OperandMode::Variable {
                            let xd = self.new_dis(x);
                            self.invalid_op(
                                xd.pos(),
                                &format!("cannot slice {} (value not addressable)", xd),
                            );
                            return on_err(x);
                        }
                        let (elem, len) = (detail.elem(), detail.len());
                        x.typ = Some(self.tc_objs.new_t_slice(elem));
                        (true, len)
                    }
                    Type::Pointer(detail) => {
                        if let Some(arr) = self
                            .otype(detail.base())
                            .underlying_val(self.tc_objs)
                            .try_as_array()
                        {
                            x.mode = OperandMode::Variable;
                            let (elem, len) = (arr.elem(), arr.len());
                            x.typ = Some(self.tc_objs.new_t_slice(elem));
                            (true, len)
                        } else {
                            (false, None)
                        }
                    }
                    Type::Slice(_) => (true, None),
                    _ => (false, None),
                };

                if !valid {
                    let xd = self.new_dis(x);
                    self.invalid_op(xd.pos(), &format!("cannot slice {}", xd));
                    return on_err(x);
                }
                x.mode = OperandMode::Value;

                // spec: "Only the first index may be omitted; it defaults to 0."
                if se.slice3 && (se.high.is_none() || se.max.is_none()) {
                    self.error_str(se.r_brack, "2nd and 3rd index required in 3-index slice");
                    return on_err(x);
                }

                // check indices
                let ind: Vec<Option<u64>> = [se.low.as_ref(), se.high.as_ref(), se.max.as_ref()]
                    .iter()
                    .enumerate()
                    .map(|(i, x)| {
                        if let Some(e) = x {
                            // The "capacity" is only known statically for strings, arrays,
                            // and pointers to arrays, and it is the same as the length for
                            // those types.
                            let max = length.map(|x| x + 1);
                            self.index(e, max, fctx).unwrap_or(None)
                        } else if i == 0 {
                            Some(0)
                        } else {
                            length
                        }
                    })
                    .collect();
                // constant indices must be in range
                // (check.index already checks that existing indices >= 0)
                let pairs = [[ind[2], ind[1]], [ind[2], ind[0]], [ind[1], ind[0]]];
                for p in pairs.iter() {
                    if let (Some(a), Some(b)) = (p[0], p[1]) {
                        if a < b {
                            self.error(se.r_brack, format!("invalid slice indices: {} > {}", b, a));
                            break; // only report one error
                        }
                    }
                }
            }
            Expr::TypeAssert(ta) => {
                self.expr(x, &ta.expr, fctx);
                if x.invalid() {
                    return on_err(x);
                }
                let xtype = typ::underlying_type(x.typ.unwrap(), self.tc_objs);
                if self.otype(xtype).try_as_interface().is_none() {
                    let dx = self.new_dis(x);
                    self.invalid_op(dx.pos(), &format!("{} is not an interface", dx));
                    return on_err(x);
                }
                // x.(type) expressions are handled explicitly in type switches
                if ta.typ.is_none() {
                    self.invalid_ast(epos, "use of .(type) outside type switch");
                    return on_err(x);
                }
                let t = self.type_expr(ta.typ.as_ref().unwrap(), fctx);
                if t == self.invalid_type() {
                    return on_err(x);
                }
                self.type_assertion(None, x, xtype, t, fctx);
                x.mode = OperandMode::CommaOk;
                x.typ = Some(t)
            }
            Expr::Call(c) => return self.call(x, c, fctx),
            Expr::Star(se) => {
                self.expr_or_type(x, &se.expr, fctx);
                match &x.mode {
                    OperandMode::Invalid => return on_err(x),
                    OperandMode::TypeExpr => {
                        x.typ = Some(self.tc_objs.new_t_pointer(x.typ.unwrap()))
                    }
                    _ => {
                        if let Some(ptype) = self
                            .otype(x.typ.unwrap())
                            .underlying_val(self.tc_objs)
                            .try_as_pointer()
                        {
                            x.mode = OperandMode::Variable;
                            x.typ = Some(ptype.base());
                        } else {
                            let xd = self.new_dis(x);
                            self.invalid_op(xd.pos(), &format!("cannot indirect {}", xd));
                            return on_err(x);
                        }
                    }
                }
            }
            Expr::Unary(ue) => {
                self.expr(x, &ue.expr, fctx);
                if x.invalid() {
                    return on_err(x);
                }
                self.unary(x, Some(e.clone()), &ue.op);
                if x.invalid() {
                    return on_err(x);
                }
                if ue.op == Token::ARROW {
                    x.expr = Some(e.clone());
                    // receive operations may appear in statement context
                    return ExprKind::Statement;
                }
            }
            Expr::Binary(be) => {
                self.binary(x, Some(e), &be.expr_a, &be.expr_b, &be.op, fctx);
                if x.invalid() {
                    return on_err(x);
                }
            }
            Expr::KeyValue(_) => {
                // key:value expressions are handled in composite literals
                self.invalid_ast(epos, "no key:value expected");
                return on_err(x);
            }
            Expr::Array(_)
            | Expr::Struct(_)
            | Expr::Func(_)
            | Expr::Interface(_)
            | Expr::Map(_)
            | Expr::Chan(_) => {
                x.mode = OperandMode::TypeExpr;
                x.typ = Some(self.type_expr(e, fctx));
            }
        }

        x.expr = Some(e.clone());
        ExprKind::Expression
    }

    /// type_assertion checks that x.(T) is legal; xtyp must be the type of x.
    pub fn type_assertion(
        &mut self,
        pos: Option<Pos>,
        x: &mut Operand,
        xtype: TypeKey,
        t: TypeKey,
        fctx: &mut FilesContext<S>,
    ) {
        if let Some((method, wrong_type)) = lookup::assertable_to(xtype, t, self, fctx) {
            let dx = self.new_dis(x);
            self.error(
                pos.unwrap_or_else(|| dx.pos()),
                format!(
                    "{} cannot have dynamic type {} ({} {})",
                    dx,
                    self.new_dis(&t),
                    if wrong_type {
                        "wrong type for method"
                    } else {
                        "missing method"
                    },
                    self.lobj(method).name()
                ),
            );
        }
    }

    fn expr_value_err(&self, x: &mut Operand) {
        let msg = match &x.mode {
            OperandMode::NoValue => Some("used as value"),
            OperandMode::Builtin(_) => Some("must be called"),
            OperandMode::TypeExpr => Some("is not an expression"),
            _ => None,
        };
        if let Some(m) = msg {
            let xd = self.new_dis(x);
            self.error(xd.pos(), format!("{} {}", xd, m));
            x.mode = OperandMode::Invalid;
        }
    }

    pub fn single_value(&self, x: &mut Operand) {
        if x.mode == OperandMode::Value {
            // tuple types are never named - no need for underlying type below
            if let Some(tuple) = self.otype(x.typ.unwrap()).try_as_tuple() {
                let len = tuple.vars().len();
                assert_ne!(len, 1);
                let xd = self.new_dis(x);
                self.error(
                    xd.pos(),
                    format!("{}-valued {} where single value is expected", len, xd),
                );
                x.mode = OperandMode::Invalid;
            }
        }
    }

    /// expr typechecks expression e and initializes x with the expression value.
    /// The result must be a single value.
    /// If an error occurred, x.mode is set to invalid.
    pub fn expr(&mut self, x: &mut Operand, e: &Expr, fctx: &mut FilesContext<S>) {
        self.multi_expr(x, e, fctx);
        self.single_value(x);
    }

    /// multi_expr is like expr but the result may be a multi-value.
    pub fn multi_expr(&mut self, x: &mut Operand, e: &Expr, fctx: &mut FilesContext<S>) {
        self.raw_expr(x, e, None, fctx);
        self.expr_value_err(x);
    }

    /// expr_or_type typechecks expression or type e and initializes x with the expression
    /// value or type. If an error occurred, x.mode is set to invalid.
    pub fn expr_or_type(&mut self, x: &mut Operand, e: &Expr, fctx: &mut FilesContext<S>) {
        self.raw_expr(x, e, None, fctx);
        self.single_value(x);
        if x.mode == OperandMode::NoValue {
            let xd = self.new_dis(x);
            self.error(xd.pos(), format!("{} used as value or type", xd));
            x.mode = OperandMode::Invalid;
        }
    }

    /// expr_with_hint typechecks expression e and initializes x with the expression value;
    /// hint is the type of a composite literal element.
    /// If an error occurred, x.mode is set to invalid.
    pub fn expr_with_hint(
        &mut self,
        x: &mut Operand,
        e: &Expr,
        hint: TypeKey,
        fctx: &mut FilesContext<S>,
    ) {
        self.raw_expr(x, e, Some(hint), fctx);
        self.single_value(x);
        self.expr_value_err(x);
    }
}