[Bug c++/126811] [14/15/16/17 Regression] c++: ICE in tsubst when a constexpr init contains an NSDMI

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126811

--- Comment #1 from Iain Sandoe <iains at gcc dot gnu.org> ---
better backtrace:
 internal compiler error: in tsubst_expr, at cp/pt.cc:23634
    2 | template<class T> void g() { constexpr S s = {}; (void) s; }
      |                                               ^
0x2ddc95f internal_error(char const*, ...)
        ../../src/gcc/diagnostic-global-context.cc:787
0xb7a51d fancy_abort(char const*, int, char const*)
        ../../src/gcc/diagnostics/context.cc:1813
0x8926ae tsubst_expr(tree_node*, tree_node*, int, tree_node*)
        ../../src/gcc/cp/pt.cc:23634
0xe1e422 tsubst_expr(tree_node*, tree_node*, int, tree_node*)
        ../../src/gcc/cp/pt.cc:23033
0xc066e1 fold_non_dependent_expr_template
        ../../src/gcc/cp/constexpr.cc:11562
0xf07a89 store_init_value(tree_node*, tree_node*, vec<tree_node*, va_gc,
vl_embed>**, int)
        ../../src/gcc/cp/typeck2.cc:1079
0xc90763 check_initializer
        ../../src/gcc/cp/decl.cc:8759
0xc973ca cp_finish_decl(tree_node*, tree_node*, bool, tree_node*, int,
cp_decomp*)
        ../../src/gcc/cp/decl.cc:9879
0xdd896a cp_parser_init_declarator
        ../../src/gcc/cp/parser.cc:26515
0xddae3a cp_parser_simple_declaration
        ../../src/gcc/cp/parser.cc:18196
0xddedde cp_parser_declaration_statement
        ../../src/gcc/cp/parser.cc:17052
0xd9f0b6 cp_parser_statement
        ../../src/gcc/cp/parser.cc:14559
0xda2b60 cp_parser_statement_seq_opt
        ../../src/gcc/cp/parser.cc:15129
0xda2e6f cp_parser_compound_statement
        ../../src/gcc/cp/parser.cc:14976
0xddef65 cp_parser_function_body
        ../../src/gcc/cp/parser.cc:28999
0xddef65 cp_parser_ctor_initializer_opt_and_function_body
        ../../src/gcc/cp/parser.cc:29050
0xda31c7 cp_parser_function_definition_after_declarator
        ../../src/gcc/cp/parser.cc:36351
0xdd96fc cp_parser_function_definition_from_specifiers_and_declarator
        ../../src/gcc/cp/parser.cc:36266
0xdd96fc cp_parser_init_declarator
        ../../src/gcc/cp/parser.cc:26179
0xdd9c75 cp_parser_single_declaration
        ../../src/gcc/cp/parser.cc:36770