The header-graph parse picks C mode for C++ headers without a recognised C++ construct

Index: Known gaps

Status (2026-10-12): CLOSED. The header graph's parse now follows the language mode the primary pass resolved. service_header_graph_attach._attach_header_graph reads it from the snapshot (model.header_lang_mode.recorded_lang_mode: ast_toolchain["resolved_lang_mode"], or a hybrid merge's castxml_/clang_ legs when they agree) and, with no explicit --lang, passes it to acquire_header_graph_ast -> clang_header_dump(resolved_lang_mode=...), which uses "c++" instead of re-deriving C from header text (it never demotes, and an explicit lang still wins). A prefetched parse that ran -- or failed -- in C mode under a C++ decision is re-parsed once; HeaderGraphAst.resolved_force_cpp records the mode each parse ran or attempted, so a header set that fails as C++ too is not parsed twice. Because the decided mode, not the evidence behind it, is what the AST cache key hashes, the graph's parse reaches the primary pass's key: under --ast-frontend clang it reuses that pass's in-process AST instead of running clang again.

Found while building the fixtures for the header semantic-facts fix. The primary pass (castxml or clang) decides C or C++ from header text and the binary's mangled exports (extract/headers/toolchain._resolve_force_cpp, correlated by ast_config._exported_symbols_indicate_cpp). The header graph's parse, often prefetched before that decision exists, re-decided from header text alone (ast_config._detect_cpp_headers); neither a non-const reference (void fire(Slot& s);), a std:: name, an overload nor a default argument matches _CPP_ONLY_PATTERNS, so such a .h header was parsed as -x c -std=gnu11 and failed.

hygiene (was) — The failure was honest (the graph fell back to declaration-only and header_call_graph was stamped degraded), but the graph lost every type, call and semantic edge for a header the primary pass parsed as C++.

Reproduction (now a regression test)

tests/test_header_graph_language_mode.py dumps four such C++ headers (non-const reference, std::function, overload, default argument) under the castxml, clang and hybrid frontends, plus two C controls that must stay C. Before the fix, abicheck dump libapi.so -H api.h logs header graph: clang AST parse failed and the stored graph has degraded_passes: {"header_call_graph": true}.

Direction taken

Agree with the primary pass (the first direction below was recorded as an option); the text heuristic was left as it is, since the export evidence already decides these headers.

Directions recorded

• Agree with the primary pass: if it parsed the headers as C++ (castxml always does), or the binary's exports are Itanium-mangled, parse the graph's headers as C++ too.
• Or widen the text heuristic (& in a declarator, std::), with a C-mode retry kept for genuine C headers.