https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99284
--- Comment #1 from Alexander Lelyakin <alexander.lelyakin at googlemail dot com> --- There is a shorter sequence: g++ -std=c++20 -fmodules-ts -x c++-system-header unordered_map g++ -std=c++20 -fmodules-ts -x c++-system-header regex ---- /usr/local/include/c++/11.0.1/regex: internal compiler error: in key_mergeable, at cp/module.cc:10456 0x6e1dd9 trees_out::key_mergeable(int, merge_kind, tree_node*, tree_node*, tree_node*, depset*) ../../gcc/gcc/cp/module.cc:10456 0xa5a8f1 trees_out::decl_value(tree_node*, depset*) ../../gcc/gcc/cp/module.cc:7769 0xa625c3 depset::hash::find_dependencies(module_state*) ../../gcc/gcc/cp/module.cc:13237 0xa62c28 module_state::write(elf_out*, cpp_reader*) ../../gcc/gcc/cp/module.cc:17652 0xa640cf finish_module_processing(cpp_reader*) ../../gcc/gcc/cp/module.cc:19922 0x9f776b c_parse_final_cleanups() ../../gcc/gcc/cp/decl2.c:5175 Please submit a full bug report, with preprocessed source if appropriate. Please include the complete backtrace with any bug report. See <https://gcc.gnu.org/bugs/> for instructions. ---- g++ (GCC) 11.0.1 20210228 (experimental)