c++: alignas and alignof void [PR104944]

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.c (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.

(cherry picked from commit d0b938a7612fb7acf1f181da9577235c83ede59e)