Closes the main-file carveout left by attempt-1 (4072689): a genuinely-
undeclared type used as a field type inside a MAIN-file GENERIC struct still
fell through the type leaf's empty-struct stub and silently compiled —
`Box :: struct($T: Type) { good: T; bad: MissingType; }` with `b : Box(s64)`
exited 0 and printed a value instead of reporting `unknown type 'MissingType'`.
Root cause: `UnknownTypeChecker` is the main-file diagnostic authority (the
type leaf defers to it for `.undeclared` names there), but
`checkStructFieldTypes` SKIPPED every generic struct outright ("its fields
reference `$T`, resolved at instantiation"), so the undeclared name was never
examined. The sibling `walkBodyTypes` `.struct_decl` arm skipped body-local
generic structs the same way.
Fix (semantic_diagnostics.zig, checker only — no leaf change):
- `checkStructFieldTypes`: stop skipping generic structs; walk the field
types with the struct's OWN type params (`$T`, `$N`, `..$Ts`) passed as the
in-scope set. A param name resolves; any OTHER bare name that is neither
declared nor a generic param is reported. Value-param positions (a `Vector`
lane count, a `$N: u32` arg) are still skipped inside
`checkTypeNodeForUnknown` / `isValueParamPosition`.
- `walkBodyTypes` `.struct_decl`: same close for body-local structs — the
local struct's own type params join the enclosing scope's in-scope params
(so it can name both the outer fn's `$T` and its own), any other bare field
type is still flagged.
The `..$Ts` pack field `(..$Ts)` parses to a `spread_expr` inside the tuple,
which hits `checkTypeNodeForUnknown`'s `else` arm — never walked — so the pack
examples (0538-0543, 0414) stay green. The checker walks only MAIN-file decls,
so library generic structs (List, Map) are untouched.
Regression: examples/0171-types-undeclared-type-in-generic-struct-field — the
reviewer's exact shape; `unknown type 'MissingType'` at the field, exit 1.
Fail-before on 4072689 (prints 7, exit 0), pass-after.
Gate: zig build; zig build test (423/423 + LSP corpus sweep 514); run_examples
498 passed / 0 failed (prior 497 byte-identical); m3te ios-sim build exit 0.
