https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121603
--- Comment #1 from Pascal Cuoq <pascal_cuoq at hotmail dot com> --- On further investigation, if we change _BitInt(64) to long long, then GCC continues to produce different results and this time Clang agrees: https://gcc.godbolt.org/z/KacYdcces At first sight it looks like _Alignof(lvalue) is evaluating to the same value as __alignof(t) where t is the type of lvalue. Is this intended? It is pretty confusing (even if Clang implemented the same behavior for long long).