mir-analyzer 0.59.2

/// Generic type inference — infer template bindings from argument types and
/// substitute them into return types.
use rustc_hash::{FxHashMap, FxHashSet};

use mir_codebase::definitions::{DeclaredParam, TemplateParam};
use mir_types::{atomic::ArrayKey, union::empty_type_params, Atomic, Name, Type};

use crate::db::MirDatabase;
use crate::subtype::is_subtype;

// ---------------------------------------------------------------------------
// Public API
// ---------------------------------------------------------------------------

/// `arg_names[i]` is the name the call used for `arg_types[i]` (`None` for a
/// positional argument), so an argument doesn't have to sit at its declared
/// parameter's index to bind that parameter's template correctly. Pass `&[]`
/// when the call site has no named arguments to track.
pub fn infer_template_bindings(
    db: &dyn MirDatabase,
    template_params: &[TemplateParam],
    params: &[DeclaredParam],
    arg_types: &[Type],
    arg_names: &[Option<String>],
) -> (FxHashMap<Name, Type>, FxHashSet<Name>) {
    let (mut bindings, unchecked) =
        infer_arg_template_bindings(db, template_params, params, arg_types, arg_names);

    // For any template not bound through arguments, fall back to its declared
    // default (`@template T = Default`), then its bound, then mixed.
    for tp in template_params {
        bindings
            .entry(Name::from(tp.name.as_ref()))
            .or_insert_with(|| {
                tp.default
                    .as_deref()
                    .or(tp.bound.as_deref())
                    .cloned()
                    .unwrap_or_else(Type::mixed)
            });
    }

    (bindings, unchecked)
}

/// Same named-argument handling as [`infer_template_bindings`], but skips its
/// bound/mixed fallback for templates no argument bound.
pub fn infer_arg_template_bindings(
    db: &dyn MirDatabase,
    template_params: &[TemplateParam],
    params: &[DeclaredParam],
    arg_types: &[Type],
    arg_names: &[Option<String>],
) -> (FxHashMap<Name, Type>, FxHashSet<Name>) {
    let mut bindings: FxHashMap<Name, Type> = FxHashMap::default();
    let mut unchecked: FxHashSet<Name> = FxHashSet::default();
    let template_names: FxHashSet<Name> = template_params
        .iter()
        .map(|tp| Name::from(tp.name.as_ref()))
        .collect();

    for (param, arg_ty) in bind_args_to_params(params, arg_types, arg_names) {
        if let Some(param_ty) = &param.ty {
            if param.is_variadic {
                // Variadic docblock types are written aggregate-style
                // (`@param array<X> $args`); each individual argument is an
                // `X`, so unwrap one array layer before matching.
                let elem = variadic_element_type(param_ty);
                infer_from_pair(
                    db,
                    elem,
                    arg_ty,
                    &template_names,
                    &mut bindings,
                    &mut unchecked,
                );
            } else {
                infer_from_pair(
                    db,
                    param_ty,
                    arg_ty,
                    &template_names,
                    &mut bindings,
                    &mut unchecked,
                );
            }
        }
    }

    (bindings, unchecked)
}

/// Pair each argument with the parameter it actually binds to, honoring
/// named-argument reordering the same way `call::args::counts::check_counts`
/// does (a diagnostics-emitting superset of this same resolution) — a named
/// argument can appear at any textual position, so template inference can't
/// assume `arg_types[i]` feeds `params[i]`. `arg_names` shorter than
/// `arg_types` (or empty) is treated as if every remaining argument were
/// positional, so callers that never track argument names can pass `&[]`.
fn bind_args_to_params<'p, 'a>(
    params: &'p [DeclaredParam],
    arg_types: &'a [Type],
    arg_names: &[Option<String>],
) -> Vec<(&'p DeclaredParam, &'a Type)> {
    let variadic_index = params.iter().position(|p| p.is_variadic);
    let max_positional = variadic_index.unwrap_or(params.len());
    let mut used = vec![false; params.len()];
    let mut out = Vec::with_capacity(arg_types.len());
    let mut positional = 0usize;

    for (i, arg_ty) in arg_types.iter().enumerate() {
        if let Some(Some(name)) = arg_names.get(i) {
            if let Some(pi) = params.iter().position(|p| p.name.as_ref() == name.as_str()) {
                if !used[pi] {
                    used[pi] = true;
                    out.push((&params[pi], arg_ty));
                }
            } else if let Some(vi) = variadic_index {
                out.push((&params[vi], arg_ty));
            }
            continue;
        }

        while positional < max_positional && used[positional] {
            positional += 1;
        }
        let pi = if positional < max_positional {
            Some(positional)
        } else {
            variadic_index
        };
        if let Some(pi) = pi {
            if pi < max_positional {
                used[pi] = true;
                positional += 1;
            }
            out.push((&params[pi], arg_ty));
        }
    }

    out
}

/// For a variadic parameter declared aggregate-style (`@param array<X> $args`
/// or `list<X>`), return the element type `X` that each individual argument
/// must match. Types that aren't a single array/list atomic are returned
/// unchanged (e.g. `@param string ...$args` stores the bare element type).
fn variadic_element_type(ty: &Type) -> &Type {
    if ty.types.len() == 1 {
        match &ty.types[0] {
            Atomic::TArray { value, .. } | Atomic::TNonEmptyArray { value, .. } => value,
            Atomic::TList { value } | Atomic::TNonEmptyList { value } => value,
            _ => ty,
        }
    } else {
        ty
    }
}

/// Check that each binding satisfies the template's declared bound, using
/// the codebase to resolve class inheritance chains. This is inheritance-aware
/// and will accept subclasses that satisfy their parent's bound.
/// Returns a list of `(template_name, inferred_type, resolved_bound)` for
/// violations. `resolved_bound` has any other already-bound template params
/// (e.g. `@template U of T`) substituted in, so it names the concrete type
/// the violation was actually checked against rather than the bound's raw,
/// possibly-still-templated docblock spelling.
///
/// `receiver_fqcn` is the call site's late-static-bound class (the actual
/// receiver for an instance/static method call; `None` when there is no
/// receiver, e.g. a free function call or a class-declaration-time check
/// like `@implements`'s own type-arg bound check). A `@template T of static`
/// bound was collected once, at the class's own declaration site, as
/// `static(DeclaringClass)` — the only class known then — so without this it
/// always checks against the declaring class instead of the real receiver,
/// missing violations where a call through a subclass narrows what `static`
/// must mean. `@template T of self` is intentionally left untouched: unlike
/// `static`, `self` always means the exact declaring class regardless of
/// the receiver.
pub fn check_template_bounds_with_inheritance<'a>(
    db: &dyn MirDatabase,
    bindings: &'a FxHashMap<Name, Type>,
    template_params: &'a [TemplateParam],
    unchecked: &FxHashSet<Name>,
    receiver_fqcn: Option<&str>,
) -> Vec<(&'a Name, &'a Type, Type)> {
    // An inferred type that still contains unresolved template placeholders or
    // self/static cannot be meaningfully checked against the bound here — it
    // resolves only at a concrete call site (e.g. Eloquent's TRelatedModel
    // bound by `self`/`static` inside the defining class). An intersection
    // (e.g. `TChild&Countable`) is unresolved as a whole if any part is, since
    // `is_subtype` would otherwise compare the still-templated part literally.
    fn is_unresolved(ty: &Type, template_params: &[TemplateParam]) -> bool {
        ty.types.iter().any(|a| match a {
            Atomic::TTemplateParam { .. }
            | Atomic::TSelf { .. }
            | Atomic::TStaticObject { .. }
            | Atomic::TParent { .. } => true,
            Atomic::TNamedObject { fqcn, type_params } => {
                (type_params.is_empty() && !fqcn.contains('\\') && {
                    let name = fqcn.as_str();
                    name.eq_ignore_ascii_case("self")
                        || name.eq_ignore_ascii_case("static")
                        || name.eq_ignore_ascii_case("parent")
                        || template_params.iter().any(|tp| tp.name.as_ref() == name)
                }) || type_params
                    .iter()
                    .any(|t| is_unresolved_shallow(t, template_params))
            }
            Atomic::TIntersection { parts } => {
                parts.iter().any(|p| is_unresolved(p, template_params))
            }
            _ => false,
        })
    }

    let mut violations = Vec::new();
    // A template name declared twice (e.g. a bare `@template T` alongside a
    // bounded `@template T of X`, or two conflicting bounds) collapses to one
    // binding in `bindings` — checking that single binding against EVERY
    // duplicate's own bound treats each extra declaration as an unrelated,
    // additional constraint the caller never actually agreed to satisfy. Only
    // check the first declaration for a given name, matching the "first
    // wins" convention `infer_template_bindings`'s unbound-fallback already
    // uses for the same malformed-docblock case.
    let mut seen_names: FxHashSet<&Name> = FxHashSet::default();
    for tp in template_params {
        if !seen_names.insert(&tp.name) {
            continue;
        }
        if unchecked.contains(&tp.name) {
            continue;
        }
        if let Some(bound) = &tp.bound {
            if let Some(inferred) = bindings.get(&tp.name) {
                // Substitute already-bound template params into the bound before
                // comparing — handles `@template B of A` where A itself is a
                // template that was just bound from another argument.
                let resolved_bound =
                    resolve_static_in_bound(bound.substitute_templates(bindings), receiver_fqcn);
                if !resolved_bound.is_mixed()
                    && !inferred.is_mixed()
                    && !is_unresolved(inferred, template_params)
                    && !is_subtype(db, inferred, &resolved_bound)
                {
                    violations.push((&tp.name, inferred, resolved_bound));
                }
            }
        }
    }
    violations
}

/// Replace a `static` atom in a (already template-substituted) bound with
/// the call site's actual receiver class, when known. `static` was filled in
/// at collection time with the class that DECLARES the template — the only
/// class known at that point — so without this, `@template T of static` can
/// only ever be checked against the declaring class, missing violations
/// where a call through a subclass narrows what `static` actually means at
/// this call site (`self` is left alone; it never late-binds).
fn resolve_static_in_bound(bound: Type, receiver_fqcn: Option<&str>) -> Type {
    let Some(fqcn) = receiver_fqcn else {
        return bound;
    };
    let from_docblock = bound.from_docblock;
    let possibly_undefined = bound.possibly_undefined;
    let types: Vec<Atomic> = bound
        .types
        .into_iter()
        .map(|a| match a {
            Atomic::TStaticObject { .. } => Atomic::TNamedObject {
                fqcn: Name::from(fqcn),
                type_params: empty_type_params(),
            },
            other => other,
        })
        .collect();
    let mut result = Type::from_vec(types);
    result.from_docblock = from_docblock;
    result.possibly_undefined = possibly_undefined;
    result
}

/// Shallow variant of the unresolved-placeholder check for nested type params
/// (one level is enough: `Collection<TKey, ...>` with a bare `TKey` inside).
/// Must check the name against the actual declared `template_params` — an
/// ordinary concrete class name is also a bare, namespace-less `TNamedObject`
/// and must not be mistaken for an unbound placeholder.
fn is_unresolved_shallow(ty: &Type, template_params: &[TemplateParam]) -> bool {
    ty.types.iter().any(|a| match a {
        Atomic::TTemplateParam { .. }
        | Atomic::TSelf { .. }
        | Atomic::TStaticObject { .. }
        | Atomic::TParent { .. } => true,
        Atomic::TNamedObject { fqcn, type_params } => {
            type_params.is_empty() && !fqcn.contains('\\') && {
                let name = fqcn.as_str();
                name.eq_ignore_ascii_case("self")
                    || name.eq_ignore_ascii_case("static")
                    || name.eq_ignore_ascii_case("parent")
                    || template_params.iter().any(|tp| tp.name.as_ref() == name)
            }
        }
        _ => false,
    })
}

/// Build template bindings from a receiver's concrete type params.
///
/// Zips `class_template_params` (e.g. `[T]` declared on the class) with
/// `receiver_type_params` (e.g. `[User]` from `Collection<User>`) to produce
/// `{ T → User }`. If the receiver supplies fewer type params than the class
/// declares, the trailing template params are left unbound. If the receiver
/// supplies more, the extras are ignored.
pub fn build_class_bindings(
    class_template_params: &[TemplateParam],
    receiver_type_params: &[Type],
) -> FxHashMap<Name, Type> {
    class_template_params
        .iter()
        .zip(receiver_type_params.iter())
        .map(|(tp, ty)| (Name::from(tp.name.as_ref()), ty.clone()))
        .collect()
}

// ---------------------------------------------------------------------------
// Internal helpers
// ---------------------------------------------------------------------------

/// Outcome of matching a param union's template/concrete alternatives against
/// an arg for a single call site.
enum TemplateResidual {
    /// No filtering applies (no concrete alternatives in the union, or none of
    /// them matched anything in the arg): bind the template directly to the
    /// full `arg_ty`.
    UseArg,
    /// A concrete alternative absorbed part of the arg; bind the template to
    /// what's left after subtracting it.
    Filtered(Type),
    /// Every arg atomic was already explained by a concrete alternative in the
    /// union (e.g. the `null` in `T|null` matching a bare `null` argument).
    /// `T` itself was never actually exercised by this call — the wrapped
    /// value is still `arg_ty`, so a caller that substitutes the binding into
    /// a return type keeps propagating it (e.g. `T|null` called with `null`
    /// still returns `null`, not `mixed|null`) — but the binding must be
    /// excluded from bound-checking: it proves nothing about what `T` is.
    FullyExplainedByAlternative(Type),
}

impl TemplateResidual {
    fn bind_value<'a>(&'a self, arg_ty: &'a Type) -> &'a Type {
        match self {
            TemplateResidual::UseArg => arg_ty,
            TemplateResidual::Filtered(t) | TemplateResidual::FullyExplainedByAlternative(t) => t,
        }
    }

    fn is_risky(&self) -> bool {
        matches!(self, TemplateResidual::FullyExplainedByAlternative(_))
    }
}

/// If `param_ty` is a union mixing template placeholders with concrete atomics,
/// compute what the template should actually bind to after subtracting the
/// concrete atomics matched in `arg_ty` — see [`TemplateResidual`].
fn compute_template_residual(
    param_ty: &Type,
    arg_ty: &Type,
    template_names: &FxHashSet<Name>,
) -> TemplateResidual {
    let mut has_template = false;
    let mut has_template_class_string = false;
    let mut concrete: Vec<&Atomic> = Vec::new();
    for a in &param_ty.types {
        if is_template_atomic(a, template_names) {
            has_template = true;
        } else if matches!(a, Atomic::TClassString(Some(n)) | Atomic::TInterfaceString(Some(n)) if template_names.contains(n))
        {
            // `class-string<T>`/`interface-string<T>` alongside a bare `T`
            // (Mockery's `class-string<TMock>|TMock` pattern): the
            // class-string/interface-string alternative binds those args
            // itself, so the bare template must not also absorb them.
            has_template_class_string = true;
        } else {
            concrete.push(a);
        }
    }
    if !has_template || (concrete.is_empty() && !has_template_class_string) {
        return TemplateResidual::UseArg;
    }
    let mut residual = Type::empty();
    residual.from_docblock = arg_ty.from_docblock;
    residual.possibly_undefined = arg_ty.possibly_undefined;
    let mut class_string_consumed = false;
    for a in &arg_ty.types {
        let consumed_by_class_string = has_template_class_string
            && matches!(a, Atomic::TClassString(_) | Atomic::TInterfaceString(_))
            || matches!(a, Atomic::TLiteralString(s) if has_template_class_string && literal_is_class_like(s));
        if consumed_by_class_string {
            class_string_consumed = true;
            continue;
        }
        if !concrete.iter().any(|c| atomics_match_for_filter(c, a)) {
            residual.add_type(a.clone());
        }
    }
    if residual.types.is_empty() {
        // An EMPTY residual is meaningful when a `class-string<T>` alternative
        // consumed the args: the bare template binds nothing at all.
        if class_string_consumed {
            return TemplateResidual::Filtered(residual);
        }
        // Otherwise every arg atomic matched a plain concrete alternative
        // (e.g. `null` against `T|null`) — bind the raw arg for substitution
        // purposes, but flag it as unfit for bound-checking.
        return TemplateResidual::FullyExplainedByAlternative(arg_ty.clone());
    }
    if residual.types.len() == arg_ty.types.len() && !class_string_consumed {
        return TemplateResidual::UseArg;
    }
    TemplateResidual::Filtered(residual)
}

/// Whether a string literal is shaped like a class reference: backslash-
/// separated identifier segments, with at least one backslash or an
/// uppercase first letter. Filters out Mockery's `'alias:Foo'` /
/// `'overload:Foo'` prefixes and plain lowercase words.
fn literal_is_class_like(s: &str) -> bool {
    let t = s.trim_start_matches('\\');
    if t.is_empty() {
        return false;
    }
    let shape_ok = t.split('\\').all(|seg| {
        !seg.is_empty()
            && seg
                .chars()
                .all(|c| c.is_ascii_alphanumeric() || c == '_' || !c.is_ascii())
            && !seg.chars().next().is_some_and(|c| c.is_ascii_digit())
    });
    shape_ok && (s.contains('\\') || t.chars().next().is_some_and(|c| c.is_ascii_uppercase()))
}

fn is_template_atomic(a: &Atomic, template_names: &FxHashSet<Name>) -> bool {
    match a {
        Atomic::TTemplateParam { .. } => true,
        Atomic::TNamedObject { fqcn, type_params } => {
            type_params.is_empty() && !fqcn.contains('\\') && template_names.contains(fqcn)
        }
        _ => false,
    }
}

/// Conservative atomic-kind match for filtering arg atomics out of a residual.
/// Returns true when an arg atomic is "covered" by a concrete param atomic so
/// the template need not absorb it. Only matches the simple kinds we expect to
/// see paired with templates in unions (null, bool, int, string, etc.).
fn atomics_match_for_filter(concrete: &Atomic, arg: &Atomic) -> bool {
    matches!(
        (concrete, arg),
        (Atomic::TNull, Atomic::TNull)
            | (Atomic::TBool, Atomic::TBool)
            | (Atomic::TBool, Atomic::TTrue)
            | (Atomic::TBool, Atomic::TFalse)
            | (Atomic::TTrue, Atomic::TTrue)
            | (Atomic::TFalse, Atomic::TFalse)
            | (Atomic::TInt, Atomic::TInt)
            | (Atomic::TFloat, Atomic::TFloat)
            | (Atomic::TIntegralFloat, Atomic::TIntegralFloat)
            | (Atomic::TFloat, Atomic::TIntegralFloat)
            | (Atomic::TIntegralFloat, Atomic::TFloat)
            | (Atomic::TString, Atomic::TString)
    )
}

fn infer_from_pair(
    db: &dyn MirDatabase,
    param_ty: &Type,
    arg_ty: &Type,
    template_names: &FxHashSet<Name>,
    bindings: &mut FxHashMap<Name, Type>,
    risky: &mut FxHashSet<Name>,
) {
    // When the parameter is a union mixing template placeholders with concrete
    // atomics (e.g. `T|null` against `Bar|null`), the template should bind to
    // the residual after subtracting matching concrete atomics — otherwise
    // `T` ends up as `Bar|null` instead of `Bar`.
    let template_residual = compute_template_residual(param_ty, arg_ty, template_names);

    for p_atomic in &param_ty.types {
        match p_atomic {
            // Direct template placeholder: T → bind T = residual(arg_ty).
            // Only when `name` belongs to the template set actually being
            // inferred right now (`template_names`) — a nested signature
            // (e.g. a `callable(T): R` parameter, where T is the class's own
            // template and R is this method's) can carry an ALREADY-BOUND
            // template from a different declaring entity as a bare
            // `TTemplateParam` atom too. Rebinding that from the argument's
            // structure here would silently overwrite the correct binding
            // computed elsewhere (e.g. from the receiver) once the two
            // binding maps are merged. Mirrors the `template_names.contains`
            // gate the `TNamedObject` arm below already has.
            Atomic::TTemplateParam { name, .. } if template_names.contains(name) => {
                let bind = template_residual.bind_value(arg_ty);
                if bind.types.is_empty() {
                    // Empty residual: every arg atomic was consumed by another
                    // union alternative (e.g. `class-string<T>`); nothing left
                    // for the bare template to bind.
                    continue;
                }
                let entry = bindings.entry(*name).or_insert_with(Type::empty);
                entry.merge_with(bind);
                if template_residual.is_risky() {
                    risky.insert(*name);
                }
            }

            // non-empty-array<K, V> matched against array<k_ty, v_ty>, array{...}
            // or a list<t_ty> (a list is a subtype of array<int, t_ty>).
            // Same inference logic as TArray below — delegates to the TArray handler.
            Atomic::TNonEmptyArray { key: pk, value: pv } => {
                for a_atomic in &arg_ty.types {
                    match a_atomic {
                        Atomic::TArray { key: ak, value: av }
                        | Atomic::TNonEmptyArray { key: ak, value: av } => {
                            infer_from_pair(db, pk, ak, template_names, bindings, risky);
                            infer_from_pair(db, pv, av, template_names, bindings, risky);
                        }
                        Atomic::TList { value: av } | Atomic::TNonEmptyList { value: av } => {
                            infer_from_pair(
                                db,
                                pk,
                                &Type::single(Atomic::TInt),
                                template_names,
                                bindings,
                                risky,
                            );
                            infer_from_pair(db, pv, av, template_names, bindings, risky);
                        }
                        Atomic::TKeyedArray { properties, .. } => {
                            let mut key_union = Type::empty();
                            let mut val_union = Type::empty();
                            for (k, prop) in properties.iter() {
                                let key_atomic = match k {
                                    ArrayKey::String(_) => Atomic::TString,
                                    ArrayKey::Int(_) => Atomic::TInt,
                                };
                                key_union.add_type(key_atomic);
                                val_union.merge_with(&prop.ty);
                            }
                            if !key_union.types.is_empty() {
                                infer_from_pair(
                                    db,
                                    pk,
                                    &key_union,
                                    template_names,
                                    bindings,
                                    risky,
                                );
                                infer_from_pair(
                                    db,
                                    pv,
                                    &val_union,
                                    template_names,
                                    bindings,
                                    risky,
                                );
                            }
                        }
                        _ => {}
                    }
                }
            }

            // array<K, V> matched against array<k_ty, v_ty>, array{...} or
            // list<t_ty> (a list is a subtype of array<int, t_ty>).
            Atomic::TArray { key: pk, value: pv } => {
                for a_atomic in &arg_ty.types {
                    match a_atomic {
                        Atomic::TArray { key: ak, value: av }
                        | Atomic::TNonEmptyArray { key: ak, value: av } => {
                            infer_from_pair(db, pk, ak, template_names, bindings, risky);
                            infer_from_pair(db, pv, av, template_names, bindings, risky);
                        }
                        Atomic::TList { value: av } | Atomic::TNonEmptyList { value: av } => {
                            infer_from_pair(
                                db,
                                pk,
                                &Type::single(Atomic::TInt),
                                template_names,
                                bindings,
                                risky,
                            );
                            infer_from_pair(db, pv, av, template_names, bindings, risky);
                        }
                        Atomic::TKeyedArray { properties, .. } => {
                            let mut key_union = Type::empty();
                            let mut val_union = Type::empty();
                            for (k, prop) in properties.iter() {
                                let key_atomic = match k {
                                    ArrayKey::String(_) => Atomic::TString,
                                    ArrayKey::Int(_) => Atomic::TInt,
                                };
                                key_union.add_type(key_atomic);
                                val_union.merge_with(&prop.ty);
                            }
                            if !key_union.types.is_empty() {
                                infer_from_pair(
                                    db,
                                    pk,
                                    &key_union,
                                    template_names,
                                    bindings,
                                    risky,
                                );
                                infer_from_pair(
                                    db,
                                    pv,
                                    &val_union,
                                    template_names,
                                    bindings,
                                    risky,
                                );
                            }
                        }
                        _ => {}
                    }
                }
            }

            // list<T> matched against list<t_ty> or a literal/keyed array whose
            // shape is a list (`array_is_list()`-true `TKeyedArray`, e.g. array
            // literals like `['a', 'b']`, which never construct `TList` directly).
            Atomic::TList { value: pv } | Atomic::TNonEmptyList { value: pv } => {
                for a_atomic in &arg_ty.types {
                    match a_atomic {
                        Atomic::TList { value: av } | Atomic::TNonEmptyList { value: av } => {
                            infer_from_pair(db, pv, av, template_names, bindings, risky);
                        }
                        Atomic::TKeyedArray {
                            properties,
                            is_list: true,
                            ..
                        } => {
                            let mut val_union = Type::empty();
                            for prop in properties.values() {
                                val_union.merge_with(&prop.ty);
                            }
                            if !val_union.types.is_empty() {
                                infer_from_pair(
                                    db,
                                    pv,
                                    &val_union,
                                    template_names,
                                    bindings,
                                    risky,
                                );
                            }
                        }
                        _ => {}
                    }
                }
            }

            // ClassName<T> matched against ClassName<t_ty> — or, if the bare
            // name is itself a declared template, bind it to arg_ty.
            Atomic::TNamedObject {
                fqcn: pfqcn,
                type_params: pp,
            } => {
                if pp.is_empty() && !pfqcn.contains('\\') && template_names.contains(pfqcn) {
                    let bind = template_residual.bind_value(arg_ty);
                    if bind.types.is_empty() {
                        continue; // see TTemplateParam arm above
                    }
                    let entry = bindings.entry(*pfqcn).or_insert_with(Type::empty);
                    entry.merge_with(bind);
                    if template_residual.is_risky() {
                        risky.insert(*pfqcn);
                    }
                    continue;
                }
                for a_atomic in &arg_ty.types {
                    if let Atomic::TNamedObject {
                        fqcn: afqcn,
                        type_params: ap,
                    } = a_atomic
                    {
                        if pfqcn == afqcn {
                            for (p_param, a_param) in pp.iter().zip(ap.iter()) {
                                infer_from_pair(
                                    db,
                                    p_param,
                                    a_param,
                                    template_names,
                                    bindings,
                                    risky,
                                );
                            }
                        } else if !pp.is_empty() {
                            // `afqcn` may be a DIFFERENT, more specific class than
                            // `pfqcn` (e.g. a param typed `Collection<T>` matched
                            // against a `TypedList<Dog>` argument where `TypedList
                            // implements Collection<T>`) — resolve what `afqcn`
                            // supplies for `pfqcn`'s own template params through
                            // its `@extends`/`@implements` chain before giving up.
                            infer_from_generic_ancestor(
                                db,
                                pfqcn.as_ref(),
                                pp,
                                afqcn.as_ref(),
                                ap,
                                template_names,
                                bindings,
                                risky,
                            );
                        }
                    }
                }
            }

            // Closure(T1, T2): R matched against Closure(t1, t2): r
            Atomic::TClosure { data: p_data } => {
                let (p_params, p_ret) = (&p_data.params, &p_data.return_type);
                for a_atomic in &arg_ty.types {
                    match a_atomic {
                        Atomic::TClosure { data: a_data } => {
                            let (a_params, a_ret) = (&a_data.params, &a_data.return_type);
                            for (pp, ap) in p_params.iter().zip(a_params.iter()) {
                                if let (Some(pt), Some(at)) = (pp.ty.as_ref(), ap.ty.as_ref()) {
                                    infer_from_pair(
                                        db,
                                        &pt.to_union(),
                                        &at.to_union(),
                                        template_names,
                                        bindings,
                                        risky,
                                    );
                                }
                            }
                            infer_from_pair(db, p_ret, a_ret, template_names, bindings, risky);
                        }
                        Atomic::TCallable {
                            params: Some(a_params),
                            return_type: Some(a_ret),
                        } => {
                            for (pp, ap) in p_params.iter().zip(a_params.iter()) {
                                if let (Some(pt), Some(at)) = (pp.ty.as_ref(), ap.ty.as_ref()) {
                                    infer_from_pair(
                                        db,
                                        &pt.to_union(),
                                        &at.to_union(),
                                        template_names,
                                        bindings,
                                        risky,
                                    );
                                }
                            }
                            infer_from_pair(db, p_ret, a_ret, template_names, bindings, risky);
                        }
                        _ => {}
                    }
                }
            }

            // callable(T1, T2): R matched against callable(t1, t2): r or Closure(...)
            Atomic::TCallable {
                params: Some(p_params),
                return_type: Some(p_ret),
            } => {
                for a_atomic in &arg_ty.types {
                    match a_atomic {
                        Atomic::TCallable {
                            params: Some(a_params),
                            return_type: Some(a_ret),
                        } => {
                            for (pp, ap) in p_params.iter().zip(a_params.iter()) {
                                if let (Some(pt), Some(at)) = (pp.ty.as_ref(), ap.ty.as_ref()) {
                                    infer_from_pair(
                                        db,
                                        &pt.to_union(),
                                        &at.to_union(),
                                        template_names,
                                        bindings,
                                        risky,
                                    );
                                }
                            }
                            infer_from_pair(db, p_ret, a_ret, template_names, bindings, risky);
                        }
                        Atomic::TClosure { data: a_data } => {
                            let (a_params, a_ret) = (&a_data.params, &a_data.return_type);
                            for (pp, ap) in p_params.iter().zip(a_params.iter()) {
                                if let (Some(pt), Some(at)) = (pp.ty.as_ref(), ap.ty.as_ref()) {
                                    infer_from_pair(
                                        db,
                                        &pt.to_union(),
                                        &at.to_union(),
                                        template_names,
                                        bindings,
                                        risky,
                                    );
                                }
                            }
                            infer_from_pair(db, p_ret, a_ret, template_names, bindings, risky);
                        }
                        _ => {}
                    }
                }
            }

            // A&B intersection — recurse each part against the arg. Use the
            // residual-filtered arg when the surrounding union computed one:
            // atomics consumed by sibling alternatives (e.g. `class-string<T>`)
            // must not leak into bare-template parts of the intersection.
            Atomic::TIntersection { parts } => {
                let arg = template_residual.bind_value(arg_ty);
                if arg.types.is_empty() {
                    continue;
                }
                for part in parts.iter() {
                    infer_from_pair(db, part, arg, template_names, bindings, risky);
                }
            }

            // class-string<T> matched against class-string<SomeClass>
            Atomic::TClassString(Some(param_name)) if template_names.contains(param_name) => {
                for a_atomic in &arg_ty.types {
                    let cls_ty = match a_atomic {
                        Atomic::TClassString(Some(arg_cls)) => {
                            Some(Type::single(Atomic::TNamedObject {
                                fqcn: *arg_cls,
                                type_params: empty_type_params(),
                            }))
                        }
                        Atomic::TClassString(None) => Some(Type::single(Atomic::TObject)),
                        // A class-name-shaped string literal coerces to
                        // class-string (Psalm-style): `m::mock('Foo\Bar')`.
                        Atomic::TLiteralString(s) if literal_is_class_like(s) => {
                            Some(Type::single(Atomic::TNamedObject {
                                fqcn: Name::new(s.trim_start_matches('\\')),
                                type_params: empty_type_params(),
                            }))
                        }
                        _ => None,
                    };
                    if let Some(cls_ty) = cls_ty {
                        let entry = bindings.entry(*param_name).or_insert_with(Type::empty);
                        entry.merge_with(&cls_ty);
                    }
                }
            }

            // interface-string<T> matched against interface-string<SomeIface> or
            // class-string<SomeIface> (e.g. `SomeIface::class` types as class-string).
            Atomic::TInterfaceString(Some(param_name)) if template_names.contains(param_name) => {
                for a_atomic in &arg_ty.types {
                    let cls_ty = match a_atomic {
                        Atomic::TInterfaceString(Some(arg_cls))
                        | Atomic::TClassString(Some(arg_cls)) => {
                            Some(Type::single(Atomic::TNamedObject {
                                fqcn: *arg_cls,
                                type_params: empty_type_params(),
                            }))
                        }
                        Atomic::TInterfaceString(None) | Atomic::TClassString(None) => {
                            Some(Type::single(Atomic::TObject))
                        }
                        Atomic::TLiteralString(s) if literal_is_class_like(s) => {
                            Some(Type::single(Atomic::TNamedObject {
                                fqcn: Name::new(s.trim_start_matches('\\')),
                                type_params: empty_type_params(),
                            }))
                        }
                        _ => None,
                    };
                    if let Some(cls_ty) = cls_ty {
                        let entry = bindings.entry(*param_name).or_insert_with(Type::empty);
                        entry.merge_with(&cls_ty);
                    }
                }
            }

            // TConditional in param position is intentionally unsupported —
            // binding a template from a conditional type requires a constraint
            // solver that doesn't exist here.
            _ => {}
        }
    }
}

/// When a param typed `PFqcn<...pp>` is matched against an arg of a
/// DIFFERENT, more specific class `AFqcn<...ap>`, resolve what `AFqcn`
/// supplies for `PFqcn`'s own template params through its
/// `@extends`/`@implements` chain (the same chain
/// `variance_compatible_across_hierarchy` walks for subtype checks — this is
/// the analogous walk for the inference direction) and recurse `infer_from_pair`
/// on each resolved pair. A no-op if `AFqcn` isn't actually an ancestor of
/// `PFqcn`, or if `PFqcn` declares no template params of its own.
#[allow(clippy::too_many_arguments)]
fn infer_from_generic_ancestor(
    db: &dyn MirDatabase,
    pfqcn: &str,
    pp: &[Type],
    afqcn: &str,
    ap: &[Type],
    template_names: &FxHashSet<Name>,
    bindings: &mut FxHashMap<Name, Type>,
    risky: &mut FxHashSet<Name>,
) {
    let Some(pfqcn_tps) = crate::db::class_template_params(db, pfqcn) else {
        return;
    };
    if pfqcn_tps.is_empty() || !crate::db::extends_or_implements(db, afqcn, pfqcn) {
        return;
    }
    let Some(afqcn_tps) = crate::db::class_template_params(db, afqcn) else {
        return;
    };
    let own_bindings: FxHashMap<Name, Type> = afqcn_tps
        .iter()
        .zip(ap)
        .map(|(tp, ty)| (tp.name, ty.clone()))
        .collect();
    let ancestor_bindings = crate::db::inherited_template_bindings(db, afqcn, &own_bindings);
    for (p_param, tp) in pp.iter().zip(pfqcn_tps.iter()) {
        let Some(resolved) = ancestor_bindings.get(&tp.name) else {
            continue;
        };
        infer_from_pair(db, p_param, resolved, template_names, bindings, risky);
    }
}