https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121524
--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:b125eee2d8526538476834773fb8216035d08598 commit r16-3207-gb125eee2d8526538476834773fb8216035d08598 Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Aug 14 22:30:45 2025 +0200 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 <ja...@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.
