tree: Don't reuse types if TYPE_USER_ALIGN differ [PR94775]

tree: Don't reuse types if TYPE_USER_ALIGN differ [PR94775]

A year ago I submitted this patch:

~~
Here we trip on the TYPE_USER_ALIGN (t) assert in strip_typedefs: it
gets "const d[0]" with TYPE_USER_ALIGN=0 but the result built by
build_cplus_array_type is "const char[0]" with TYPE_USER_ALIGN=1.

When we strip_typedefs the element of the array "const d", we see it's
a typedef_variant_p, so we look at its DECL_ORIGINAL_TYPE, which is
char, but we need to add the const qualifier, so we call
cp_build_qualified_type -> build_qualified_type
where get_qualified_type checks to see if we already have such a type
by walking the variants list, which in this case is:

  char -> c -> const char -> const char -> d -> const d

Because check_base_type only checks TYPE_ALIGN and not TYPE_USER_ALIGN,
we choose the first const char, which has TYPE_USER_ALIGN set.  If the
element type of an array has TYPE_USER_ALIGN, the array type gets it too.

So we can make check_base_type stricter.  I was afraid that it might make
us reuse types less often, but measuring showed that we build the same
amount of types with and without the patch, while bootstrapping.
~~

However, the patch broke a few tests on STRICT_ALIGNMENT platforms and
had to be reverted.  This is another try.  The original patch is kept
unchanged, but I added the finalize_type_size hunk that ought to fix the
STRICT_ALIGNMENT issues.

The problem is that finalize_type_size can clear TYPE_USER_ALIGN on the
main variant of a type, but doesn't clear it on any of the variants.
Then we end up with types which share the same TYPE_MAIN_VARIANT, but
their TYPE_CANONICAL differs and then the usual "canonical types differ
for identical types" follows.

I've created alignas19.C to exercise this scenario.  What happens is:
- when parsing the class S we create a type S in xref_tag,
- we see alignas(8) so common_handle_aligned_attribute sets T_U_A in S,
- we parse the member function fn and build_memfn_type creates a copy
  of S to add const; this variant has T_U_A set,
- we finish_struct S which calls layout_class_type -> finish_record_type
  -> finalize_size_type where we reset T_U_A in S (but const S keeps it),
- finish_non_static_data_member for arr calls maybe_dummy_object with
  type = S,
- maybe_dummy_object calls same_type_ignoring_top_level_qualifiers_p
  to check if S and TREE_TYPE (current_class_ref), which is const S,
  are the same,
- same_type_ignoring_top_level_qualifiers_p creates cv-unqualified
  versions of the passed types.  Previously we'd use our main variant
  S when stripping "const S" of const, but since the T_U_A flags don't
  match (check_base_type), we create a new variant S'.  Then we crash in
  comptypes because S and S' have the same TYPE_MAIN_VARIANT but
  different TYPE_CANONICALs.

With my patch we'll clear T_U_A for S's variants too, and then instead
of S' we'll just use S.

gcc/ChangeLog:

	PR c++/94775
	* stor-layout.c (finalize_type_size): If we reset TYPE_USER_ALIGN in
	the main variant, maybe reset it in its variants too.
	* tree.c (check_base_type): Return true only if TYPE_USER_ALIGN match.
	(check_aligned_type): Check if TYPE_USER_ALIGN match.

gcc/testsuite/ChangeLog:

	PR c++/94775
	* g++.dg/cpp0x/alignas19.C: New test.
	* g++.dg/warn/Warray-bounds15.C: New test.