c++: Fix up build_cplus_array_type [PR121524]

The following testcase is miscompiled since my r15-3046 change
to properly apply std attributes after closing ] for arrays to the
array type.
Array type is not a class type, so when cplus_decl_attribute is
called on the ARRAY_TYPE, it doesn't do ATTR_FLAG_TYPE_IN_PLACE.
Though, for alignas/gnu::aligned/deprecated/gnu::unavailable/gnu::unused
attributes the handlers of those attributes for non-ATTR_FLAG_TYPE_IN_PLACE
on types call build_variant_type_copy and modify some flags on the new
variant type.  They also usually don't clear *no_add_attrs, so the caller
then checks if the attributes are present on the new type and if not, calls
build_type_attribute_variant.
On the following testcase, it results in the B::foo type to be properly
32 byte aligned.
The problem happens later when we build_cplus_array_type for C::a.
elt_type is T (typedef, or using works likewise), we get as m
main variant type with unsigned int element type but because elt_type
is different, build_cplus_array_type searches the TYPE_NEXT_VARIANT chain
to find if there isn't already a useful ARRAY_TYPE to reuse.
It checks for NULL TYPE_NAME, NULL TYPE_ATTRIBUTES and the right TREE_TYPE.
Unfortunately this is not good enough, build_variant_type_copy above created
a variant type on which it modified TYPE_USER_ALIGN and TYPE_ALIGN, but
TYPE_ATTRIBUTES is still NULL, only the build_type_attribute_variant call
later adds attributes.
The problem is that the intermediate type is found in the TYPE_NEXT_VARIANT
chain and reused.

The following patch adds conditions to prevent problems with the affected
attributes (except gnu::unused, I think whether TREE_USED is set or not
shouldn't prevent sharing).  In particular, if TYPE_USER_ALIGN is not
set on the variant, it wasn't user realigned, if it is set, it verifies
it has it set because the elt_type has been user aligned and TYPE_ALIGN
is the expected one.  For deprecated it punts on the flag being set and
for gnu::unavailable as well.

2025-08-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/121524
	* tree.cc (build_cplus_array_type): Don't reuse variant type
	if it has TREE_DEPRECATED or TREE_UNAVAILABLE flags set or,
	unless elt_type has TYPE_USER_ALIGN set and TYPE_ALIGN is
	TYPE_ALIGN of elt_type, TYPE_USER_ALIGN is not set.

	* g++.dg/cpp0x/gen-attrs-89.C: New test.

2 files changed, 14 insertions, 1 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index d56d73f..c66d8bf 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -1186,7 +1186,12 @@ build_cplus_array_type (tree elt_type, tree index_type, int dependent)
       for (t = m; t; t = TYPE_NEXT_VARIANT (t))
 	if (TREE_TYPE (t) == elt_type
 	    && TYPE_NAME (t) == NULL_TREE
-	    && TYPE_ATTRIBUTES (t) == NULL_TREE)
+	    && TYPE_ATTRIBUTES (t) == NULL_TREE
+	    && (!TYPE_USER_ALIGN (t)
+		|| (TYPE_USER_ALIGN (elt_type)
+		    && TYPE_ALIGN (t) == TYPE_ALIGN (elt_type)))
+	    && !TREE_DEPRECATED (t)
+	    && !TREE_UNAVAILABLE (t))
 	  break;
       if (!t)
 	{
diff --git a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-89.C b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-89.C
new file mode 100644
index 0000000..6ec29e1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-89.C
@@ -0,0 +1,8 @@
+// PR c++/121524
+// { dg-do compile { target c++11 } }
+
+typedef unsigned int T;
+struct A { unsigned a[8]; unsigned b; };
+struct B { T foo[8] [[gnu::aligned (32)]]; };
+struct C { T a[8]; T b; };
+static_assert (sizeof (C) == sizeof (A), "");