brink_runtime/

error.rs

//! Runtime error types.

use alloc::string::String;

use brink_format::{DecodeError, DefinitionId};

/// Why execution ran out of content — the call-stack shape C#'s
/// `Story.Continue()` inspects to pick one of four messages (`Story.cs`,
/// the `AddError` calls guarding the "ran out of content" branch: it checks
/// `callStack.CanPop(PushPopType.Tunnel)`, then `.CanPop(PushPopType.Function)`,
/// then `!callStack.canPop`, with a final backstop for none of the above).
///
/// Classified the moment a frame's content is discovered exhausted
/// (`vm::handle_frame_exhaustion`, the same instant C# reads
/// `callStack.CanPop`), but only *stashed* on `Flow` — for the *next*
/// `continue_single` call to raise as the deferred fault (issue #1574 ruled
/// the deferred timing stays — this only changes which cause is attached,
/// not *when* the fault fires) — on the paths where this exhaustion is
/// itself the terminal one (`vm::Stepped::Done`). A frame whose exhaustion
/// instead resumes execution (a completed thread with a parent to fall back
/// to, a popped frame with content still below it) never writes its cause:
/// otherwise a transient exhaustion elsewhere on the same flow (e.g. a
/// `Story::call_function` boundary evaluating a function that calls a void
/// helper) would clobber a cause an earlier, still-pending exhaustion had
/// already recorded, and a later, unrelated `Done` would read it stale. It
/// has to be classified that early rather than read fresh at fault time:
/// unlike C#, this runtime's own exhaustion recovery always pops the
/// exhausted frame (even a Tunnel with nothing pending), so by the time the
/// deferred fault fires the frame that triggered it is usually long gone
/// from the call stack.
///
/// In practice, only [`Plain`](Self::Plain) is reachable through any story
/// today: a Tunnel or Function frame's exhaustion classifies correctly at
/// the instant it happens, but this runtime's frame-popping (unlike C#'s)
/// keeps unwinding past it instead of stopping there — cascading all the
/// way to the root frame's own exhaustion, which is the one that actually
/// produces the terminal `Done` and gets its `Plain` cause stashed, before
/// the fault ever surfaces. Making the other three arms reachable needs a
/// separate, deliberate fix to that popping behavior — tracked in #2005.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, thiserror::Error)]
pub enum RanOutOfContentCause {
    /// The top call-stack frame is a tunnel (`->t->`) — content ran out
    /// mid-tunnel with no `->->` to return. Mirrors
    /// `callStack.CanPop(PushPopType.Tunnel)`.
    #[error("unexpectedly reached end of content. Do you need a '->->' to return from a tunnel?")]
    Tunnel,
    /// The top call-stack frame is a function call — content ran out
    /// mid-function with no `~ return`. Mirrors
    /// `callStack.CanPop(PushPopType.Function)`.
    #[error("unexpectedly reached end of content. Do you need a '~ return'?")]
    Function,
    /// The call stack can't pop at all (only the root frame remains) — the
    /// plain "story fell off the end" case, and the default when nothing
    /// more specific was ever recorded. Mirrors `!callStack.canPop`.
    #[default]
    #[error("ran out of content. Do you need a '-> DONE' or '-> END'?")]
    Plain,
    /// The call stack can still pop, but the exhausted frame that produced
    /// the terminal `Done` is neither a tunnel nor a function — e.g. a
    /// `Thread` boundary with no parent thread left to fall back to, or a
    /// `FunctionEvalFromGame` boundary that is itself the last frame
    /// standing. C#'s backstop for a call-stack shape well-formed compiler
    /// output should never produce; an *ordinary* `<- thread` completing
    /// (a `Thread` frame exhausting with a parent thread still waiting)
    /// never reaches this arm — that path resumes via `ThreadCompleted` and
    /// records no cause at all.
    #[error("unexpectedly reached end of content for unknown reason. Please debug compiler!")]
    Unknown,
}

/// Errors that can occur during story linking or execution.
#[derive(Debug, Clone, PartialEq, thiserror::Error)]
pub enum RuntimeError {
    #[error("bytecode decode error: {0}")]
    Decode(#[from] DecodeError),

    #[error("unresolved definition: {0}")]
    UnresolvedDefinition(DefinitionId),

    #[error("no root container found")]
    NoRootContainer,

    #[error("value stack underflow")]
    StackUnderflow,

    #[error("call stack underflow")]
    CallStackUnderflow,

    #[error("container stack underflow")]
    ContainerStackUnderflow,

    #[error("invalid choice index: {index} (available: {available})")]
    InvalidChoiceIndex { index: usize, available: usize },

    #[error("not waiting for choice")]
    NotWaitingForChoice,

    #[error("story has ended")]
    StoryEnded,

    #[error("unresolved global: {0}")]
    UnresolvedGlobal(DefinitionId),

    #[error("type error: {0}")]
    TypeError(String),

    #[error("division by zero")]
    DivisionByZero,

    #[error("unimplemented opcode: {0}")]
    Unimplemented(String),

    #[error("unresolved external function call: {0}")]
    UnresolvedExternalCall(DefinitionId),

    #[error("output capture underflow (no checkpoint)")]
    CaptureUnderflow,

    #[error("unknown flow: {0}")]
    UnknownFlow(String),

    #[error("flow already exists: {0}")]
    FlowAlreadyExists(String),

    #[error("{0}")]
    RanOutOfContent(RanOutOfContentCause),

    #[error("step limit exceeded ({0} steps)")]
    StepLimitExceeded(u64),

    #[error("line limit exceeded ({0} lines in a single turn)")]
    LineLimitExceeded(usize),

    #[error("locale checksum mismatch: expected {expected:#010x}, got {actual:#010x}")]
    LocaleChecksumMismatch { expected: u32, actual: u32 },

    #[error("locale scope not in base program: {0}")]
    LocaleScopeNotInBase(DefinitionId),

    #[error("locale missing scope required by strict mode: {0}")]
    LocaleScopeMissing(DefinitionId),

    #[error(
        "function evaluation yielded (a function called from the engine cannot present choices or end the story)"
    )]
    FunctionYielded,

    #[error("no function evaluation in progress")]
    NotEvaluatingFunction,

    #[error("a function evaluation is already in progress on this flow")]
    AlreadyEvaluatingFunction,

    /// `call_function` was given a name that resolves to no function/knot.
    #[error("function not found: {0}")]
    FunctionNotFound(String),

    /// A function evaluated via the synchronous `call_function` path called an
    /// external whose handler deferred (`Pending`) — it can't be resolved in a
    /// one-shot synchronous call.
    #[error("external '{0}' is async; cannot resolve during a synchronous call_function")]
    AsyncExternalInCall(String),

    /// `choose_path_string` was given a path that resolves to no knot,
    /// stitch, or label.
    #[error("no knot or stitch found at path '{0}'")]
    UnknownPath(String),

    /// `choose_path_string` was called while the flow is parked on an
    /// unresolved external call. A pending host call cannot be silently
    /// abandoned — resolve it (or reset the story) before jumping.
    #[error(
        "cannot jump to '{path}': the flow is parked on unresolved external '{external}' — \
         resolve it before jumping"
    )]
    JumpWhileAwaitingExternal { path: String, external: String },

    /// A host-directed entry (`choose_path_string_with_args`) — or a
    /// `call_function` — was given the wrong number of arguments for the
    /// target's declared parameters.
    #[error("'{target}' expects {expected} argument(s), got {got}")]
    ArgCountMismatch {
        /// The knot/stitch/function path or name.
        target: String,
        /// Declared parameter count.
        expected: u8,
        /// Arguments the host supplied.
        got: usize,
    },

    /// A host **semantic** access (variable get/set, entry lookup, function
    /// eval) targeted a `#@private` definition while visibility enforcement
    /// was on (M-2b, `docs/modules-spec.md` §4 boundary rule 2). The host is
    /// outside every module. Dev tooling (play-from-here) opts out via
    /// [`Story::set_visibility_enforcement`](crate::Story::set_visibility_enforcement).
    /// Persistence (save/load/journal/replay) is unaffected — it never routes
    /// through the enforced surface.
    #[error(
        "'{name}' is #@private and cannot be accessed by the host \
         (dev tooling may override visibility enforcement)"
    )]
    PrivateAccess {
        /// The private definition's name or path, as the host supplied it.
        name: String,
    },

    // ── T1b collections (docs/value-model-spec.md §11c) ──────────────
    //
    // Out-of-bounds/missing-key reads and writes are turn-terminating
    // runtime faults — total operations with no silent growth on
    // write-past-end (`docs/t1b-surface-spec.md` §4). Propagating as
    // `RuntimeError` (rather than a special in-band value) is exactly what
    // "turn-terminating" already means in this VM: it unwinds `step()`,
    // ending the current turn, the same mechanism `DivisionByZero` uses.
    /// Array index read/write out of bounds (`0 <= index < len` required).
    #[error("array index {index} out of bounds (len {len})")]
    IndexOutOfBounds { index: i32, len: usize },
    /// Map key *read* (`m[k]`, `MapGet`) on a key that isn't present, or a
    /// path-projection *write* through a `ref` whose final segment key
    /// isn't present (`docs/t1e-spec.md` §4). Indexed *assignment*
    /// (`m[k] = v` via the `IndexSet` opcode) no longer raises this fault on
    /// a missing key — it inserts instead (JS/Python semantics, issue #856,
    /// ruled 2026-07-15).
    #[error("map has no key {key}")]
    MapKeyNotFound { key: String },
    /// `a[i]`/`a[i] = v`/`m[k]`/`m[k] = v` where `a`/`m` isn't an
    /// `Array`/`Map`.
    #[error("cannot index into a {0} value")]
    NotIndexable(&'static str),
    /// Array index expression didn't evaluate to an `Int`.
    #[error("array index must be an int, got {0}")]
    InvalidArrayIndex(&'static str),
    /// Map key expression evaluated to a type outside the ratified key
    /// domain (int/string/bool — value-model-spec §4).
    #[error("map key must be int, string, or bool, got {0}")]
    InvalidMapKeyType(&'static str),
    /// `PushLiteral(idx)` referenced an index outside the literal pool —
    /// malformed bytecode, not an author-triggerable condition.
    #[error("literal pool index {0} out of range")]
    InvalidLiteralIndex(u32),
    /// A `NameId` (container/address-path name) referenced an index outside
    /// `StoryData::name_table` — malformed bytecode, not an
    /// author-triggerable condition. Caught at link time, before any of the
    /// name is used to build path lookup tables.
    #[error("name id {0} out of range")]
    InvalidNameId(u16),

    // ── TM-4 records (docs/typed-mode-spec.md §6 / value-model-spec §11c) ──
    /// `RecordNew(shape_id)` referenced a shape id outside the compiled
    /// `StructShapes` table — malformed bytecode.
    #[error("struct shape id {0} out of range")]
    InvalidShapeId(u32),
    /// `RecordGetDyn`/`RecordSetDyn` on a value that isn't a `Record`.
    #[error("cannot access a field on a {0} value")]
    NotARecord(&'static str),
    /// `RecordGetDyn`/`RecordSetDyn` named a field the record's shape
    /// doesn't declare — a compile-time typo under strict mode (surfaced as
    /// a diagnostic there) or a genuine dynamic mismatch under gradual mode,
    /// both turn-terminating at runtime (spec §11c pattern).
    #[error("struct has no field {0:?}")]
    RecordFieldNotFound(String),
    /// `RecordGet(offset)`/`RecordSet(offset)` (TM-4c static-offset field
    /// ops) with an offset outside the popped record's own field vector.
    /// These ops never re-check the record's shape (that's the payoff over
    /// `RecordGetDyn`/`RecordSetDyn`) — only the field count is verified, so
    /// this is the sole fault this pair can produce, malformed bytecode or
    /// otherwise.
    #[error("struct field offset {offset} out of range (record has {len} fields)")]
    RecordFieldOffsetOutOfRange { offset: u16, len: usize },

    // ── TM-3 completion: conversion intrinsics (docs/typed-mode-spec.md
    // §4, maintainer ruling 2026-07-13, issue #659) ──────────────────────
    /// `int(x)`/`float(x)` where `x` is a `String` that fails to parse as
    /// the target numeric type. Turn-terminating fault — no
    /// zero-defaulting, no silent garbage (ruling 1: "Parse failure is a
    /// turn-terminating fault... like a missing map key"). Unlike this,
    /// the classic uppercase `INT()`/`FLOAT()` builtins keep their
    /// pre-existing silent-0-on-string-parse-failure legacy behavior
    /// (`value_ops::cast_to_int`/`cast_to_float`) untouched within their own
    /// `Int`/`Float`/`Bool`/`String` domain — oracle-byte-identical, a
    /// distinct code path. Outside that domain (divert targets, pointers,
    /// collections, records, function/handle/projection values), the
    /// uppercase builtins now raise [`InvalidConversionDomain`](Self::InvalidConversionDomain)
    /// too (issue #955) instead of the wildcard-fold-to-zero they used to —
    /// those variants were never oracle-reachable through `INT()`/`FLOAT()`.
    #[error("cannot parse {input:?} as {target}")]
    ConversionParseFailure { target: &'static str, input: String },
    /// `int(x)`/`float(x)` where `x` is outside the permissive
    /// numeric+bool domain (divert targets, LIST values, arrays, maps,
    /// records) — compile error under `types = strict` (`brink-analyzer`'s
    /// intrinsic typing/domain check), turn-terminating fault under
    /// `types = gradual` (ruling 2). Also raised by the classic uppercase
    /// `INT()`/`FLOAT()` builtins (`value_ops::cast_to_int`/`cast_to_float`)
    /// for the same reason, with an uppercase `target` label (issue #955) —
    /// no spec (`value-model-spec.md`, `t1c`/`t1d`/`t1e-spec.md`) rules a
    /// conversion for those variants, so faulting is the conservative
    /// default rather than the old silent zero.
    #[error("cannot convert a {got} value to {target}")]
    InvalidConversionDomain {
        target: &'static str,
        got: &'static str,
    },

    // ── T1c function values (docs/t1c-spec.md §3/§6, issue #700) ──────────
    /// `call(f, …)` / a direct `f(…)` where the callee value is not a
    /// function value (nor a divert target). Gradual-mode dispatch fault —
    /// "no silent garbage" (spec §3, value-model-spec §11c).
    #[error("cannot call a {0} value as a function")]
    NotCallable(&'static str),
    /// Calling a function value with the wrong number of arguments: the bound
    /// prefix plus the supplied args must exactly equal the target's declared
    /// arity (spec §3). Turn-terminating in gradual mode; strict mode catches
    /// it at compile time (spec §4).
    #[error(
        "function value expects {expected} argument(s), got {got} (bound {bound} + supplied {supplied})"
    )]
    FunctionValueArity {
        expected: usize,
        got: usize,
        bound: usize,
        supplied: usize,
    },
    /// A rehydrated function value's bound env no longer matches the current
    /// signature — a param was renamed, reordered, or re-moded across a
    /// recompile (spec §6). A defined fault, never a silent misbinding.
    #[error("function value no longer matches its target's signature: {0}")]
    FunctionValueRehydrationMismatch(String),
    /// Invoking a function value that `ref`-binds a flow-private (`#@local`)
    /// cell (spec §3). T1c ships this fault instead of creating-flow identity
    /// (#597): a `#@local`-`ref` binding can only be dereferenced safely from
    /// its creating flow, and no creating-flow identity is tracked yet, so the
    /// invocation faults rather than risk a silent cross-flow misbinding. The
    /// payload is the bound cell's name.
    #[error(
        "function value ref-binds flow-private cell `{0}`; cross-flow invocation is a fault in T1c (see #597)"
    )]
    FunctionValueCrossFlowLocal(String),

    // ── T1e path projections (docs/t1e-spec.md §1(2)/§3) ──────────────────
    /// A live path projection's snapshot segments no longer resolve against
    /// the root cell's *current* value at read or write time: a shrunk
    /// array, a removed map key, or a struct field dropped by recompile.
    /// The single ratified turn-terminating fault for every path-invalidation
    /// cause (spec §1(2): "a defined turn-terminating runtime fault — not a
    /// clamp, not UB"). The payload carries the underlying cause (an
    /// `IndexOutOfBounds`/`MapKeyNotFound`/`RecordFieldNotFound`-shaped
    /// message, or a root-resolution failure).
    #[error("projection invalidated: {0}")]
    ProjectionInvalidated(String),

    // ── Stdlib slice 1 completion: `char_at` (`docs/t1b-surface-spec.md`
    // §5, issue #857) ──────────────────────────────────────────────────────
    /// `char_at(s, i)`'s index expression didn't evaluate to an `Int`.
    #[error("char_at index must be an int, got {0}")]
    CharAtIndexNotInt(&'static str),
    /// `char_at(s, i)` where `i` is outside `[0, char_count)` — chars
    /// (Unicode scalar values), not UTF-8 bytes (the issue's "author
    /// sanity" ruling), so `len` is `s.chars().count()`, never
    /// `s.len()`. Turn-terminating fault, no silent empty/clamped result
    /// (value-model-spec §11c) — matches `IndexOutOfBounds`'s posture for
    /// arrays.
    #[error("char_at index {index} out of bounds ({len} chars)")]
    CharAtOutOfBounds { index: i32, len: usize },

    // ── NS-A1 Option[T] + the ruled stdlib flips (`docs/stdlib-spec.md`
    // §§3-5) ──────────────────────────────────────────────────────────────
    /// A stdlib verb was handed a container/argument of the wrong runtime
    /// type — `find` on a non-string, `min`/`first`/`pop` on a non-array,
    /// `get`/`contains_value`/`clear` on a non-map. A malformed *question*
    /// is a bug (the ruled fault-vs-absence doctrine), so this is a
    /// turn-terminating fault, never a `none`.
    #[error("`{verb}` expects {expected}, got {found}")]
    StdlibWrongType {
        verb: &'static str,
        expected: &'static str,
        found: &'static str,
    },
    /// `min`/`max` reached an element outside the currently-orderable set
    /// (int/float/bool/string, homogeneous per the §4b roster), or a
    /// cross-type pair (int vs string). Turn-terminating fault — an
    /// unorderable extremum question is malformed, not absent.
    #[error("`{verb}` cannot order element of type {found}")]
    NotOrderable {
        verb: &'static str,
        found: &'static str,
    },

    // ── NS-A4: the ordering doctrine (`docs/stdlib-spec.md` §4b, issue
    // #1110) ──────────────────────────────────────────────────────────────
    /// DEV mode only: an ordering verb (`sort`/`sorted`/`min`/`max`; A7
    /// adds `heap_push`) reached a float NaN comparand. NaN flows freely
    /// through arithmetic — ordering contexts are where it stops: in dev
    /// mode the upstream bug surfaces at its first ordering consumption as
    /// this turn-terminating fault. PROD mode instead places NaN by the
    /// pinned non-fabricating total order (`-0 == +0` ties, NaN greatest,
    /// NaN-vs-NaN ties) and keeps moving — the mode changes WHERE execution
    /// stops, never WHAT values are fabricated. `sort_by`/`sorted_by`
    /// deliberately do NOT raise this (F14: the comparator owns the order).
    #[error(
        "`{verb}` reached a NaN comparand — NaN cannot be ordered (dev-mode fault; prod mode \
         places NaN by the pinned total order)"
    )]
    UnorderedComparand { verb: &'static str },
    /// `sort_by`/`sorted_by` was handed a comparator that is not a function
    /// value (`FnRef`/`Closure`). Malformed question — turn-terminating.
    #[error("`{verb}` comparator must be a function value `fn(T, T): int`, got {found}")]
    ComparatorNotAFunction {
        verb: &'static str,
        found: &'static str,
    },
    /// A `sort_by`/`sorted_by` comparator returned something other than an
    /// int (F0's ruled shape: negative = less, zero = tie, positive =
    /// greater). Turn-terminating — a silent coercion here would scramble
    /// the order.
    #[error(
        "`{verb}` comparator must return an int (negative = less, zero = tie, positive = \
         greater), got {found}"
    )]
    ComparatorReturnType {
        verb: &'static str,
        found: &'static str,
    },
    /// A fn-value verb (`map`/`filter`/`fold`/`filter_map`/`each`/
    /// `map_each` — `docs/stdlib-spec.md` §4, issue #1679) was handed a
    /// callback that is not a function value (`FnRef`/`Closure`). Malformed
    /// question — turn-terminating, pure or effectful alike. Distinct from
    /// [`ComparatorNotAFunction`](Self::ComparatorNotAFunction) because each
    /// verb names its own expected shape.
    #[error("`{verb}` callback must be a function value {expected}, got {found}")]
    CallbackNotAFunction {
        verb: &'static str,
        /// The callback's declared shape, already back-quoted for the
        /// message (e.g. `` `fn(T): bool` ``).
        expected: &'static str,
        found: &'static str,
    },
    /// A fn-value verb's callback returned a value of the wrong shape —
    /// `filter`, whose predicate must return a bool, and `filter_map`,
    /// whose Option-mapper must return an `Option`. Coercing truthiness or
    /// unwrapping a non-Option here would silently change which elements
    /// survive, so this is turn-terminating.
    #[error("`{verb}` callback must return {expected}, got {found}")]
    CallbackReturnType {
        verb: &'static str,
        expected: &'static str,
        found: &'static str,
    },
    /// A `sort_by`/`sorted_by` comparator, or a fn-value verb's callback
    /// (issue #1679, pure quartet and effectful pair alike), broke a
    /// contract the VM can observe: it presented a choice, reached
    /// `-> DONE`/`-> END`, called an external function, exceeded the
    /// nested-evaluation step budget, or recursed past the nesting depth
    /// limit. These four are architectural — no handler exists mid-opcode —
    /// so they fire for `each`/`map_each` exactly as for the pure quartet;
    /// being effectful widens what a callback may *do*, not what the VM can
    /// honor mid-op. The checker enforces the pure·silent half of the
    /// contract statically where the callee's origin is provable (E119,
    /// pure quartet only); this fault is the gradual-mode runtime residual
    /// either way. `role` names the shape the *author* wrote —
    /// `"comparator"` for `sort_by`/`sorted_by`, `"callback"` for every
    /// fn-value verb (`callback_role`) — so a `map`/`each`/… author is
    /// never told they wrote a bad comparator.
    #[error("`{verb}` {role} {what} — {role}s must be pure, silent functions")]
    ComparatorEscaped {
        verb: &'static str,
        role: &'static str,
        what: &'static str,
    },
    /// DEV mode only (F34, ruled 2026-07-19): a `sort_by`/`sorted_by`
    /// comparator, or a **pure** fn-value verb's callback
    /// (`map`/`filter`/`fold`/`filter_map`, issue #1679), performed a
    /// world-write mid-evaluation — assigned a global (directly, or through
    /// a `ref`-parameter pointer / path projection) or advanced the RNG
    /// cell (a draw IS a write: a random comparator/callback is exactly the
    /// non-determinism the pure·silent contract bans). PROD mode skips the
    /// check entirely and the write executes — defined and deterministic,
    /// because the stable merge-sort's comparison sequence is fixed and the
    /// fn-value verbs walk their array in iteration order (the mode changes
    /// WHERE execution stops, never WHAT is produced). Visit-count
    /// increments from the callee's own invocation are NOT world-writes —
    /// they are the ruled in-story dispatch semantics and stay exempt.
    /// Reads are not guarded at runtime (E119's static bound owns the read
    /// posture, where E119 gates at all). `role` is the same author-facing
    /// noun as [`ComparatorEscaped`](Self::ComparatorEscaped); like it,
    /// this is the gradual-mode runtime residual of the E119 gate. **Never
    /// fires for the effectful pair** (`each`/`map_each`, issue #1679 slice
    /// 2, in either mode): world-writes are exactly what they exist to
    /// permit — see `vm::guard_comparator_write`.
    #[error(
        "`{verb}` {role} {what} — {role}s must be pure, silent functions (dev-mode fault; prod \
         mode executes the write)"
    )]
    ComparatorWroteState {
        verb: &'static str,
        role: &'static str,
        what: &'static str,
    },

    // ── F27: Option has no truthiness (`docs/stdlib-spec.md` §1.6, ruled
    // 2026-07-19, issue #1120) ────────────────────────────────────────────
    /// A `Value::OptionVal` reached the VM's truthiness evaluation (`GotoIf`,
    /// `JumpIfFalse`, `Not`, a choice condition). Option has **no**
    /// truthiness — truthiness is a quiet coercion of exactly the kind
    /// `Option[T] ≠ T` exists to ban — so this is the gradual-mode
    /// turn-terminating fault; `types = strict` reports the same condition
    /// statically (E116). Authors write `== none` / `== some(x)`, or the
    /// `as`-binding (B1b, issue #1475 — see [`Self::AsBindingNotOption`],
    /// its own fault). Supersedes NS-A1's shipped falsy-none behavior.
    #[error("an Option has no truthiness — test `== none` / `== some(x)` explicitly")]
    OptionTruthiness,

    // ── B1b: the `as` binding (`docs/decision-log.md` 2026-07-26, issue
    // #1475) ─────────────────────────────────────────────────────────────
    /// `Opcode::OptionBind` received a non-`Option` operand — `if EXPR as
    /// name { … }` where `EXPR` does not evaluate to an `Option[T]`. The
    /// binding's whole job is to unwrap `Option[T]` to `T`, so there is
    /// nothing to bind. This is the gradual-mode residual of the checker's
    /// strict-mode `E147` (the same statically/dynamically paired posture
    /// [`Self::OptionTruthiness`] has with `E116`); on the native surface,
    /// which is strict-only, `E147` catches every statically classifiable
    /// case first and this fault is the backstop for the rest.
    #[error("the `as` binding requires an Option, got {found}")]
    AsBindingNotOption {
        /// The offending operand's runtime type name (`vm::value_type_name`).
        found: &'static str,
    },

    // ── NS-A5: the inhabited-range refinement (`docs/stdlib-spec.md` §7,
    // F8 ruled 2026-07-19) ────────────────────────────────────────────────
    /// `int(range)` reached an **empty** range at runtime — the F8 gradual-
    /// mode residual, and THE template for every future value refinement:
    /// under gradual typing the refinement check is inert at compile time
    /// and this turn-terminating fault is what remains; under `types =
    /// strict` the same condition is unrepresentable (the checker demands
    /// `NonEmptyRange` evidence — a provably-inhabited literal or a
    /// `non_empty(r)` unwrap — and reports E117 statically). A draw from
    /// nothing is a malformed question, never an absence, so this is a
    /// fault and not a `none` (the ruled fault-vs-absence doctrine;
    /// contrast `pick(0..0)`, which IS absence and returns `none`).
    #[error("`int` cannot draw from the empty range {range} — validate with `non_empty(r)` first")]
    EmptyRangeDraw {
        /// The written form of the offending range (`0..0`, `5..=2`, …).
        range: String,
    },

    // ── NS-A7: Weighted[T] evidence-by-construction (`docs/stdlib-spec.md`
    // §8, issue #1113) ────────────────────────────────────────────────────
    /// `weighted(…)` reached a **computed** weight that is not a positive
    /// int at construction time — the E078-style split's runtime half: a
    /// weight the checker could classify statically is the E120 compile
    /// error; a computed weight that turns out zero/negative/non-int is
    /// this turn-terminating construction fault. Construction is the
    /// validator (the §7 parse-don't-validate shape), so `roll` over any
    /// table that exists is total.
    #[error(
        "`weighted` requires positive int weights, got {found} — construction refuses empty/zero/negative-weight tables"
    )]
    WeightedBadWeight {
        /// Display form of the offending weight value (`0`, `-3`, `1.5`, a
        /// type name for non-numerics).
        found: String,
    },
    /// The `weighted_new` op received a malformed pair row (empty, or an
    /// odd flattened length). Unreachable through the compiler — the E120
    /// gate refuses empty/odd construction shapes statically — so this
    /// guards hand-crafted or corrupt bytecode only (the malformed-bytecode
    /// robustness discipline, never a panic).
    #[error("`weighted` construction received {detail}")]
    WeightedMalformedTable { detail: &'static str },
}