Fixes the bug RED-pinned in 72f3f65: an unbound identifier inside an
instance method body slipped past `ail check` (false-OK exit 0) and
surfaced only at `ail build` as the degraded internal-error diagnostic
`monomorphise_workspace: unknown identifier: <name>` with no source
location, symbol kind, or "did you mean" candidates.
Root cause: `check_def` in `crates/ailang-check/src/lib.rs:1574-1595`
early-returned `Ok(())` for `Def::Class(_) | Def::Instance(_)` without
ever invoking the body walker. The arm's comment claimed iter 22b.2
landed instance-body typechecking; that wiring was never implemented.
Only the workspace-load coherence checks (Orphan/Duplicate/
MissingMethod) touched instance defs, and they inspect schema only,
not the method-body identifier graph.
Fix: split the combined arm so `Def::Class(_)` stays schema-only at
this layer, while `Def::Instance(inst)` routes through a new helper
`check_instance` that lifts each `InstanceMethod`'s `Term::Lam` body
into a synthetic `FnDef`, applies class-method substitution (class
param -> instance type, via the existing `substitute_rigids` /
`substitute_rigids_in_term` helpers), and hands it to `check_fn`.
The reuse of `check_fn` is what gets the body walked through the
same identifier-resolution path as fn bodies, so an unbound name
fires `[unbound-var]` at `ail check` with exit 1 and the standard
structured diagnostic.
Per "minimal fix, no surrounding cleanup" constraint: no widening
to Def::Class (still schema-only); no changes to the
monomorphise_workspace diagnostic wording (defence-in-depth
internal-error path stays as backstop); no changes to mono.rs,
codegen, or any other crate.
Tests: 557 baseline + 2 new RED -> 559 green. Both RED pins in
unbound_in_instance_method_pin.rs now PASS. fn-body level
unbound-var path stays green. All 8 carve-out fixtures classify
unchanged.
Known debt (out of scope per carrier; queued in journal):
`check_instance` does not yet cross-check the substituted method
signature against the class's `ClassMethodEntry.method_ty` — a
malformed instance declaring wrong types in its Lam would still
typecheck cleanly. The body-walk catches unbound vars and
effect-mismatches against the Lam's own declared types, but not
class-method-signature mismatch.
Brummel
77f584abbb