[Bug c++/125457] New: ICE when using modules with templates

[d7d1cd at mail dot ru via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125457

            Bug ID: 125457
           Summary: ICE when using modules with templates
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d7d1cd at mail dot ru
  Target Milestone: ---

Create two modules and a file with the main function:
    $ cat foo.ixx
    export module foo;

    template <typename T>
    struct Foo {};

    export using FooInst = Foo<int>;


    $ cat bar.ixx
    export module bar;

    import foo;

    export struct Bar { FooInst f; };


    $ cat main.cc
    import bar;

    int main() { Bar b; }


Try compiling this into a program and you'll get ICE:
    $ g++ -std=c++23 -fmodules foo.ixx bar.ixx main.cc
    main.cc: In function ‘int main()’:
    main.cc:3:18: internal compiler error: Segmentation fault
    3 | int main() { Bar b; }
    |                  ^
    0x2388cff internal_error(char const*, ...)
            ../../gcc/diagnostic-global-context.cc:787
    0x10a1cbf crash_signal
            ../../gcc/toplev.cc:325
    0x7db83f vec<tree_node*, va_gc, vl_embed>::iterate(unsigned int,
tree_node**) const
            ../../gcc/vec.h:960
    0x7db83f is_really_empty_class(tree_node*, bool)
            ../../gcc/cp/class.cc:9511
    0x7db8e0 is_really_empty_class(tree_node*, bool)
            ../../gcc/cp/class.cc:9519
    0x86a992 cp_finish_decl(tree_node*, tree_node*, bool, tree_node*, int,
cp_decomp*)
            ../../gcc/cp/decl.cc:9713
    0x974017 cp_parser_init_declarator
            ../../gcc/cp/parser.cc:26415
    0x9761ba cp_parser_simple_declaration
            ../../gcc/cp/parser.cc:18099
    0x9796de cp_parser_declaration_statement
            ../../gcc/cp/parser.cc:16955
    0x93ce46 cp_parser_statement
            ../../gcc/cp/parser.cc:14462
    0x9411e0 cp_parser_statement_seq_opt
            ../../gcc/cp/parser.cc:15032
    0x94147f cp_parser_compound_statement
            ../../gcc/cp/parser.cc:14879
    0x9797e4 cp_parser_function_body
            ../../gcc/cp/parser.cc:28880
    0x9797e4 cp_parser_ctor_initializer_opt_and_function_body
            ../../gcc/cp/parser.cc:28931
    0x9419a5 cp_parser_function_definition_after_declarator
            ../../gcc/cp/parser.cc:36154
    0x974dfa cp_parser_function_definition_from_specifiers_and_declarator
            ../../gcc/cp/parser.cc:36069
    0x974dfa cp_parser_init_declarator
            ../../gcc/cp/parser.cc:26079
    0x9761ba cp_parser_simple_declaration
            ../../gcc/cp/parser.cc:18099
    0x981eeb cp_parser_declaration
            ../../gcc/cp/parser.cc:17636
    0x984b1f cp_parser_toplevel_declaration
            ../../gcc/cp/parser.cc:17740


Here's an example on godbolt: https://godbolt.org/z/E5dbo4a9r