https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104944
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:d0b938a7612fb7acf1f181da9577235c83ede59e commit r12-7815-gd0b938a7612fb7acf1f181da9577235c83ede59e Author: Marek Polacek <pola...@redhat.com> Date: Wed Mar 23 17:12:29 2022 -0400 c++: alignas and alignof void [PR104944] I started looking into this PR because in GCC 4.9 we were able to detect the invalid struct alignas(void) S{}; but I broke it in r210262. It's ill-formed code in C++: [dcl.align]/3: "An alignment-specifier of the form alignas(type-id) has the same effect as alignas(alignof(type-id))", and [expr.align]/1: "The operand shall be a type-id representing a complete object type, or an array thereof, or a reference to one of those types." and void is not a complete type. It's also invalid in C: 6.7.5: _Alignas(type-name) is equivalent to _Alignas(_Alignof(type-name)) 6.5.3.4: "The _Alignof operator shall not be applied to a function type or an incomplete type." We have a GNU extension whereby we treat sizeof(void) as 1, but I assume it doesn't apply to alignof, at least in C++. However, __alignof__(void) is still accepted with a -Wpedantic warning. We still say "invalid application of 'alignof'" rather than 'alignas' in the void diagnostic, but I felt that fixing that may not be suitable as part of this patch. The "incomplete type" diagnostic still always prints '__alignof__'. PR c++/104944 gcc/cp/ChangeLog: * typeck.cc (cxx_sizeof_or_alignof_type): Diagnose alignof(void). (cxx_alignas_expr): Call cxx_sizeof_or_alignof_type with complain == true. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/alignas20.C: New test.
