Three more structural checks from C check.c ported to selfhost,
at the AST level (no resolved tinfo).
is/as validity: e is T / e as T require e's declared type to be a
tagged union and T to name a variant. Mirrors the case-variant
check that just landed.
let init-type and return-type assignability: a new exprtype helper
infers an AST type-node for literal/ident/call/cast/?/as/is
expressions; isassignable approximates C type_assignable on the
shapes we can resolve — exact match, untyped numeric → typed
numeric, untyped nil → ptr/slice/chan/fn, variant inclusion, and
two-primitive-mismatch.
isassignable returns (ok, confident). When confident=false the
check emits no error — better to miss a real bug than fire a
false positive on a binary-op expression we can't infer. This
keeps existing selfhost code clean while still catching the
common typo cases (let x: bool = 42; return "hi" from i32 fn).
Naming: all new helpers follow Plan 9 run-together convention per
CLAUDE.md (`typeeqast`, `isassignable`, `exprtype`, ...). Earlier
work that used snake_case helpers (`case_variant_in`,
`check_match_exhaustive`, ...) got the same treatment — bulk
renamed in this commit.
Five new rows in 950_selfcheck exercise the new checks
(is-not-a-variant, two let mismatches, return mismatch, plus the
case-variant row already there).
`match (u) { case T => ... }` where T isn't a variant of u was
silently accepted by both checkers. The cgen would emit a tag
comparison against an index that never appears, leaving the arm
unreachable — wasted code that's almost always a bug or typo.
C check.c now mirrors the existing is/as rule for match arms:
each `case T` and each alt of multi-pattern `case T1 | T2` is
checked against the scrutinee's variant list via variant_present.
selfhost check.ww gets the same shape with AST-level type_eq_ast
comparison. Both checks land in the same scope-aware pass that
already runs exhaustiveness and ? subset.
New test rows in 300_check (C side) and 950_selfcheck (selfhost
side) exercise both single-pattern and multi-pattern alt typos.
The 950 driver's err_present detector picks up the new
"case: not a variant" prefix.
The selfhost checker did name resolution only — anything tagged-
union-shaped sailed through silently. The C check.c implements
three structural checks; this commit mirrors them at the AST level
in selfhost/cmd/wcc/check.ww:
1. Match exhaustiveness: every variant of the scrutinee's tagged
union must be covered by a case arm (incl. multi-pattern alts)
or a default arm. Operates on the scrutinee's declared type
(N_TTAGGED via N_IDENT's sym.decl.lhs).
2. ? subset propagation: each error variant of the operand's type
must be a variant of the enclosing fn's return type. Enclosing
return must itself be a tagged union when the operand has any
errors.
3. !-flag semantics: in flag-aware mode (any variant marked `!T`),
error subset = flagged variants. Legacy mode (no flags) =
everything-but-first. is_error_variant unifies both rules.
No tinfo / type-inference work: the checks read declared AST type
nodes directly. `resolvealias` chases N_TNAME → typedecl body to
handle aliased tagged unions. `type_eq_ast` does structural
comparison on the subset of type-expression shapes the checks
encounter (TNAME by string, TPTR/TSLICE/TCHAN recursive).
Folded into resolvewalk rather than a separate second pass, so the
checks see the same per-statement scope state as resolve. fnret is
threaded through resolvefnbody so ? can find the enclosing return.
New test/wcc/950_selfcheck.c — five rows exercising each error path
(missing variant, non-tagged enclosing, missing error subset
member, the flag-aware happy path, the flag-aware missing-error
case). Test suite now reports 21 ok.
