https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103502
Bug ID: 103502 Summary: -Wstrict-aliasing=3 doesn't warn on what is documented as UB Product: gcc Version: 11.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: stsp at users dot sourceforge.net Target Milestone: --- https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html --- Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.: int f() { double d = 3.0; return ((union a_union *) &d)->i; } --- Here is the test-case: --- union a_union { int i; double d; }; static int f() { double d = 3.0; return ((union a_union *) &d)->i; } int main() { return f(); } --- It only warns at -Wstrict-aliasing=2 or 1, but doesn't on 3. 3 is documented as the most precise option. So obviously it should warn on what the official gcc manual declares as an UB. Note: I very much wish such construct to not be UB. Because of the lack of a warning on -Wstrict-aliasing=3 I was freely using such construct for type-punning for years... until now I've found it invalid in gcc manual. If it is actually valid, please fix the docs, not gcc! :)