Records that contain a union keep Fil-C's union handling when they come from a precompiled header. Clang did not serialize RecordDecl's HasUnion bit, so std::string temporaries lost their zhas_union markers under a PCH; SROA then split a literal copy into a misaligned pointer load that trapped (Nix's unit tests, which build with a PCH). The C case writes a plain C record with a union through the PCH record abbreviation.