Comment 1 for bug 1915640

seen with trunk 20210207, with an installed compiler, empty directory:

$ g++-11 -std=c++20 -fmodules-ts -x c++-system-header iostream
$ g++-11 -std=c++20 -fmodules-ts -x c++-system-header utility
$ g++-11 -std=c++20 -fmodules-ts -x c++-system-header algorithm
In file included from /usr/include/c++/11/bits/stl_iterator_base_types.h:71,
                 from /usr/include/c++/11/bits/stl_algobase.h:65,
                 from /usr/include/c++/11/algorithm:61:
/usr/include/c++/11/bits/iterator_concepts.h:919:54: internal compiler error: tree check: expected binding_vector, have overload in maybe_record_mergeable_decl, at cp/name-lookup.c:3562
  919 | operator==(unreachable_sentinel_t, const _It&) noexcept
      | ^~~~~~~~
0xe2c617 tree_check_failed(tree_node const*, char const*, int, char const*, ...)
        ../../src/gcc/tree.c:9814
0xbfc27c tree_check(tree_node*, char const*, int, char const*, tree_code)
        ../../src/gcc/tree.h:3349
0xbfc27c maybe_record_mergeable_decl
        ../../src/gcc/cp/name-lookup.c:3562
0x5ea96a do_pushdecl
        ../../src/gcc/cp/name-lookup.c:3864
0x12cad8d do_pushdecl_with_scope
        ../../src/gcc/cp/name-lookup.c:4886
0x12cd607 pushdecl_namespace_level(tree_node*, bool)
        ../../src/gcc/cp/name-lookup.c:6251
0x142aeac push_template_decl(tree_node*, bool)
        ../../src/gcc/cp/pt.c:6054
0x192f6fe do_friend(tree_node*, tree_node*, tree_node*, tree_node*, overload_flags, bool)
        ../../src/gcc/cp/friend.c:608
0x13418db grokdeclarator(cp_declarator const*, cp_decl_specifier_seq*, decl_context, int, tree_node**)
        ../../src/gcc/cp/decl.c:13613
0x139e169 grokmethod(cp_decl_specifier_seq*, cp_declarator const*, tree_node*)
        ../../src/gcc/cp/decl.c:17489
0x139d880 cp_parser_save_member_function_body
        ../../src/gcc/cp/parser.c:30587
0x1331fcc cp_parser_init_declarator
        ../../src/gcc/cp/parser.c:21557
0x1406b5f cp_parser_single_declaration
        ../../src/gcc/cp/parser.c:30441
0x761a2f cp_parser_template_declaration_after_parameters
        ../../src/gcc/cp/parser.c:30013
0x7544b0 cp_parser_explicit_template_declaration
        ../../src/gcc/cp/parser.c:30279
0x133bef4 cp_parser_member_specification_opt
        ../../src/gcc/cp/parser.c:25740
0x133bef4 cp_parser_class_specifier_1
        ../../src/gcc/cp/parser.c:24818
0x132c600 cp_parser_class_specifier
        ../../src/gcc/cp/parser.c:25134
0x132c600 cp_parser_type_specifier
        ../../src/gcc/cp/parser.c:18381
0x132b40e cp_parser_decl_specifier_seq
        ../../src/gcc/cp/parser.c:15003
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