g++-12.0.0-alpha20210801 snapshot (g:6f0c43e97825ee54e3779afbedcd0def12443001), g++ 11.1, g++ 10.3 all ICE when compiling the following testcase, reduced from test/SemaCXX/sizeless-1.cpp from the clang 12.0.0 test suite:

void cxx_only() {
  __SVInt8_t bad_brace_init_int8_5{{__SVInt8_t()}};
}

% aarch64-linux-gnu-g++-12.0.0 -c s4yu6ufy.cpp
s4yu6ufy.cpp: In function 'void cxx_only()':
s4yu6ufy.cpp:2:50: internal compiler error: tree check: expected record_type or union_type or qual_union_type, have vector_type in digest_init_r, at cp/typeck2.c:1206
    2 |   __SVInt8_t bad_brace_init_int8_5{{__SVInt8_t()}};
      |                                                  ^
0x8e7638 tree_check_failed(tree_node const*, char const*, int, char const*, ...)
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/tree.c:8686
0x794630 tree_check3(tree_node*, char const*, int, char const*, tree_code, tree_code, tree_code)
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/tree.h:3413
0x794630 digest_init_r
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/typeck2.c:1206
0xc508fd store_init_value(tree_node*, tree_node*, vec<tree_node*, va_gc, vl_embed>**, int)
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/typeck2.c:742
0xa55f07 check_initializer
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/decl.c:7185
0xa7d9aa cp_finish_decl(tree_node*, tree_node*, bool, tree_node*, int)
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/decl.c:8119
0xb70b67 cp_parser_init_declarator
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:22344
0xb4aeed cp_parser_simple_declaration
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:14887
0xb4d09d cp_parser_declaration_statement
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:13987
0xb4d87a cp_parser_statement
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:12117
0xb4e82e cp_parser_statement_seq_opt
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:12484
0xb4e920 cp_parser_compound_statement
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:12433
0xb6de8f cp_parser_function_body
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:24534
0xb6de8f cp_parser_ctor_initializer_opt_and_function_body
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:24585
0xb6fbfa cp_parser_function_definition_after_declarator
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:30658
0xb710cd cp_parser_function_definition_from_specifiers_and_declarator
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:30574
0xb710cd cp_parser_init_declarator
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:22106
0xb4aeed cp_parser_simple_declaration
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:14887
0xb7bfe0 cp_parser_declaration
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:14592
0xb7caa3 cp_parser_toplevel_declaration
	/var/tmp/portage/cross-aarch64-linux-gnu/gcc-12.0.0_alpha20210801/work/gcc-12-20210801/gcc/cp/parser.c:14613

Removing the extraneous braces around __SVInt8_t initializer yields PR99867, though.