htcc-parser-regression-workflow

## Use this skill when - Editing `src/Htcc/Parser/**` or parser-facing code in `app/Main.hs`. - Review feedback mentions parse acceptance, parse errors, `ATEmpty`, `manyTill`, EOF, `Megaparsec`, scope restoration, declarations, initializers, or implicit calls. - A malformed C input can hang, parse successfully by mistake, or produce the wrong diagnostic. ## Do not use this skill when - The change is pure codegen/output behavior after parsing has already succeeded. ## Investigation steps 1. Read the exact parser function and the nearest combinators it composes with. 2. Search for zero-width success in recursive or repeated contexts: `M.many`, `M.manyTill`, `M.option`, `ATEmpty`, `M.eof`, `M.try`. 3. Identify whether the parser must fail, consume input, or produce `ATEmpty` intentionally. 4. Check existing component tests in `test/Tests/ComponentsTests/Parser/Combinators.hs` before adding new helpers. ## Regression test placement - Put parser unit regressions in `test/Tests/ComponentsTests/Parser/Combinators.hs` when direct `parseProgram`, `parseAssignExpr`, or helper assertions can reproduce the bug. - Put CLI parser regressions in subprocess tests only when stderr/exit-code behavior is the relevant contract. - Prefer adding cases under the existing `Parser.Program.*` group matching the construct, such as `function-call`, `scalar-initializer`, or `function-pointer-arithmetic`. - For call argument lists, trailing commas such as `f(1,)` are malformed. For initializer lists, check existing accepted C-like trailing-comma behavior before turning a trailing comma into a rejection. - For initializer EOF regressions, consider both file-scope and local-scope shapes such as `int g =`, `int g = {1`, and `int main(){ int x =`. ## Nontermination safeguards - Any bug involving EOF, repeated combinators, or `manyTill` must have a timeout-guarded manual QA command, for example: ```bash printf 'int main(){ return f(' | timeout 5s stack exec htcc -- /dev/stdin ``` - The expected outcome for malformed input is non-zero exit before the timeout, not success. - Use `timeout` or `gtimeout`, whichever exists on the host, and report the command used. - Do not fix nontermination by accepting malformed input as `ATEmpty`. ## Verification - `stylish-haskell -i <all changed .hs files, including app/Main.hs when touched> test/Tests/ComponentsTests/Parser/Combinators.hs` - `lsp_diagnostics` on edited Haskell files when the diagnostics tool is available. If unavailable, state that and substitute `stack build` plus the relevant test command. - `stack test --test-arguments components`. - Full `stack test` for parser-wide behavior or when the change affects common expression/statement parsing. - `stack build`. - Manual CLI QA for malformed inputs when user-visible parser behavior or hangs are involved. ## Review focus - Does the fix remove the bad acceptance path without breaking intended empty statements or empty `for` sections? - Do the tests cover both minimal malformed shape and comma/trailing-argument shape where applicable? - Does the parser fail with a parse error rather than looping or producing a misleading successful AST?