# Step: strict implementation-vs-definition signature checks

Tag `v3-sigcheck`. Suite **129/129**; fixpoint **OK**
(`bootstrap/fixpoint.sh`, image **2,102,326 bytes**).

## What changed

An `IMPLEMENTATION` procedure heading must now match its `DEFINITION`
(or exported `FORWARD`) beyond arity:

- **result type**, **formal count**, each **formal type**, and each
  formal's **`VAR`-ness**.

Mismatch → new semantic error **235
`implementation does not match definition`**.

Before, only the *arity* was effectively covered and a differing
result type or parameter type was silently accepted (a real
separate-compilation hazard).

## How it works

- `ResumeProc` (the implementation heading resuming a DEFINITION
  procedure) snapshots the definition signature into the symbol:
  `snapRes` (result), `snapT[]`/`snapV[]` (formal types / VAR-ness),
  `nSnap`, and sets `fsnap`.
- The implementation heading re-enters its own formals; as each
  formal's type becomes known (`FixPending`), the formal is compared
  against the snapshot. `SetProcRes` compares the result type (and is
  the backstop for a parameterless heading).
- Comparisons use `FormalOk` (value formals: same type, both
  integer-family, or string-compatible) and `VarParamOk` (VAR
  formals); a mismatched `VAR`-ness is caught directly. InvalidType is
  treated as "no evidence" so a single root error is not multiplied.
- Signatures are bounded to `MaxSnap = 16` formals (self-hosting uses
  far fewer).

## Deliberate leniency

- `INTEGER`/`CARDINAL` are accepted interchangeably for **value**
  formals and results: V3 models them as one integer family (a per-
  type split would reject the compiler's own valid-looking source).
  For `VAR` formals the types must match exactly.
- `VAR` open arrays vs fixed arrays still follow `VarParamOk` (a fixed
  array is an acceptable actual for an open formal).

## Tests

`tests/{sig.def,sig.mod,sig_prog.mod}` + `expect_fail_files sig`:
`sig.mod` declares `Add(a,b):CHAR` where the definition says
`:INTEGER`, and `Fill(buf:ARRAY OF CHAR;n:CARDINAL)` where the
definition has `VAR buf`. Both report 235. Exact-match sessions still
compile (verified with the full `m5` case).

## Notes

- Error 231 is already "opaque type outside definition", so the
  signature mismatch got its own code, 235.
- `qbe`/`.ssa` output is suppressed when 235 fires (the session is
  rejected before codegen), matching other semantic errors.

## Files

`compiler/src/SymTab.def`/`.mod` (`ResumeProc` snapshot, `FixPending`
and `SetProcRes` returning the check result, `FormalOk`, `MaxSnap`),
`compiler/src/M2.atg` (report 235 at `SetProcRes`/`FixPending`),
`compiler/src/compiler.frm` (message 235),
`compiler/tests/{sig.def,sig.mod,sig_prog.mod}`,
`compiler/run_tests.sh`, `docs/features.md`.