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.

1 files changed, 13 insertions, 3 deletions

diff --git a/gcc/stor-layout.c b/gcc/stor-layout.c
index 7d6b1b5..784f131 100644
--- a/gcc/stor-layout.c
+++ b/gcc/stor-layout.c
@@ -1926,6 +1926,7 @@ finalize_type_size (tree type)
      However, where strict alignment is not required, avoid
      over-aligning structures, since most compilers do not do this
      alignment.  */
+  bool tua_cleared_p = false;
   if (TYPE_MODE (type) != BLKmode
       && TYPE_MODE (type) != VOIDmode
       && (STRICT_ALIGNMENT || !AGGREGATE_TYPE_P (type)))
@@ -1937,7 +1938,9 @@ finalize_type_size (tree type)
       if (mode_align >= TYPE_ALIGN (type))
 	{
 	  SET_TYPE_ALIGN (type, mode_align);
-	  TYPE_USER_ALIGN (type) = 0;
+	  /* Remember that we're about to reset this flag.  */
+	  tua_cleared_p = TYPE_USER_ALIGN (type);
+	  TYPE_USER_ALIGN (type) = false;
 	}
     }
 
@@ -1991,14 +1994,21 @@ finalize_type_size (tree type)
 
       /* Copy it into all variants.  */
       for (variant = TYPE_MAIN_VARIANT (type);
-	   variant != 0;
+	   variant != NULL_TREE;
 	   variant = TYPE_NEXT_VARIANT (variant))
 	{
 	  TYPE_SIZE (variant) = size;
 	  TYPE_SIZE_UNIT (variant) = size_unit;
 	  unsigned valign = align;
 	  if (TYPE_USER_ALIGN (variant))
-	    valign = MAX (valign, TYPE_ALIGN (variant));
+	    {
+	      valign = MAX (valign, TYPE_ALIGN (variant));
+	      /* If we reset TYPE_USER_ALIGN on the main variant, we might
+		 need to reset it on the variants too.  TYPE_MODE will be set
+		 to MODE in this variant, so we can use that.  */
+	      if (tua_cleared_p && GET_MODE_ALIGNMENT (mode) >= valign)
+		TYPE_USER_ALIGN (variant) = false;
+	    }
 	  else
 	    TYPE_USER_ALIGN (variant) = user_align;
 	  SET_TYPE_ALIGN (variant, valign);