[mypyc] Fix cross-group call to inherited __mypyc_defaults_setup

[georgesittas]

With separate=True and cross-module inheritance, when only the subclass module is recompiled incrementally and the parent is loaded from mypy's incremental cache, find_attr_initializers gathers no defaults from the parent. The subclass therefore has no __mypyc_defaults_setup of its own, and ClassIR.get_method walks the MRO and returns the parent's.

Without this fix, emit_attr_defaults_func_call emits a raw CPyDef_<parent>___...(...) call. The parent's header only declares that function as a pointer inside struct export_table_<group>, so the symbol isn't reachable as a free function from the subclass's compilation unit and clang/gcc fail with:

error: call to undeclared function 'CPyDef_<parent_module>___<Parent>_____mypyc_defaults_setup';
ISO C99 and later do not support implicit function declarations

A cold build doesn't hit this because the parent's defs.body is populated (everything is freshly parsed), so the subclass gets its own __mypyc_defaults_setup and the call is intra-group. Likewise, an incremental change that propagates through interface-hash deps to the parent makes it get reparsed too, avoiding the trigger. The bug requires an invalidation pattern that touches the subclass but not the parent.

Fix

emit_attr_defaults_func_call in mypyc/codegen/emitclass.py now applies emitter.get_group_prefix(defaults_fn.decl) when emitting the call, matching the pattern already used by the other cross-group call sites in this file (emit_setup_or_dunder_new_call, generate_constructor_for_class, etc.).

get_group_prefix returns "" for same-group calls (so intra-group behaviour is unchanged) and "exports_<group>." when the target lives in a different group. It also registers the target group in context.group_deps so the right header gets #included.

Tests

Added testIncrementalCrossModuleInheritedAttrDefaults in mypyc/test-data/run-multimodule.test, a two-step test that reproduces the bug under TestRunSeparate: parent in other_b.py with attribute defaults, empty subclass in other_a.py, step 2 modifies other_a.py to trigger a recompile without touching the parent. Verified to fail under TestRunSeparate (with the implicit-declaration error) on the unpatched tree and to pass under all three modes (TestRun, TestRunMultiFile, TestRunSeparate) with the fix.

Local checks:

• pre-commit run --all-files — pass
• python runtests.py self — pass
• mypyc unit tests (1077 + 1 skipped) — pass
• TestRunSeparate + TestRunMultiFile run tests (110) — pass

Real-world impact

Surfaced first against sqlglot-mypy==1.20.0.post6 (a downstream of mypy used to compile sqlglot) in CI when build/ and .mypy_cache/ were preserved across GitHub Actions runs and a PR happened to edit only subclass modules. The incremental compile produced malformed C and failed with the implicit-declaration error.

Stripped of sqlglot specifics, the bug requires only separate=True + cross-module inheritance + parent with attribute defaults + subclass without its own defaults + an invalidation pattern where the subclass is rechecked but the parent is not. These conditions are common in any non-trivial codebase using mypyc with separate=True, so this should be considered a latent issue affecting incremental builds of such codebases.

[p-sawicki]

thanks!