The following code causes an ICE with GCC 12.2.0 until trunk, on -std=c++{20,23}: template<class T> using lambda = decltype([]() {}); template<class R, class F = lambda<R>> void test() {} int main() { test<int>(); } godbolt: https://godbolt.org/z/Mx1z77hTn Error message: PS> g++ -std=c++20 "main_ice.cpp" -o"bin\main_ice.exe" main_ice.cpp: In substitution of 'template<class T> using lambda = decltype (<lambda>) [with T = R]': main_ice.cpp:4:36: required from here main_ice.cpp:2:25: internal compiler error: Segmentation fault 2 | using lambda = decltype([]() {}); | ^~~~~~~ libbacktrace could not find executable to open Please submit a full bug report, with preprocessed source (by using -freport-bug). See <https://github.com/msys2/MINGW-packages/issues> for instructions.
Confirmed. GCC has trouble with lambdas inside decltype especially with alias's and that is definitely what is being used here.
Another test for the same problem: ``` template <typename Type> using LambdaType = decltype([] {}); template <typename Type, typename = LambdaType<Type>> class S; ``` 107457.C:3:25: internal compiler error: Segmentation fault 3 | using lambda = decltype([]() {}); | ^~~~~~~ 0x1ac234a crash_signal /home/mpolacek/src/gcc/gcc/toplev.cc:319 0x7fd3f489099f ??? /usr/src/debug/glibc-2.38-16.fc39.x86_64/signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0 0xeda764 begin_function_body() /home/mpolacek/src/gcc/gcc/cp/decl.cc:18398 0xf4eb05 start_lambda_function(tree_node*, tree_node*) /home/mpolacek/src/gcc/gcc/cp/lambda.cc:1700 0x1106d94 tsubst_lambda_expr(tree_node*, tree_node*, int, tree_node*) /home/mpolacek/src/gcc/gcc/cp/pt.cc:19748 0x11119fc tsubst_expr(tree_node*, tree_node*, int, tree_node*) /home/mpolacek/src/gcc/gcc/cp/pt.cc:21684 0x10f5523 tsubst(tree_node*, tree_node*, int, tree_node*) /home/mpolacek/src/gcc/gcc/cp/pt.cc:16895 0x10ef6e0 tsubst_decl /home/mpolacek/src/gcc/gcc/cp/pt.cc:15531 0x1113f4e instantiate_template(tree_node*, tree_node*, int) /home/mpolacek/src/gcc/gcc/cp/pt.cc:22100 0x1114a20 instantiate_alias_template /home/mpolacek/src/gcc/gcc/cp/pt.cc:22198 0x10f21c7 tsubst(tree_node*, tree_node*, int, tree_node*) /home/mpolacek/src/gcc/gcc/cp/pt.cc:16193 0x10cea1f lookup_template_class(tree_node*, tree_node*, tree_node*, tree_node*, int, int) /home/mpolacek/src/gcc/gcc/cp/pt.cc:10204 0x116c376 finish_template_type(tree_node*, tree_node*, int) /home/mpolacek/src/gcc/gcc/cp/semantics.cc:3948 0x103d7eb cp_parser_template_id /home/mpolacek/src/gcc/gcc/cp/parser.cc:19048 0x104d762 cp_parser_class_name /home/mpolacek/src/gcc/gcc/cp/parser.cc:26655 0x1023508 cp_parser_qualifying_entity /home/mpolacek/src/gcc/gcc/cp/parser.cc:7400 0x10229d6 cp_parser_nested_name_specifier_opt /home/mpolacek/src/gcc/gcc/cp/parser.cc:7082 0x1040456 cp_parser_simple_type_specifier /home/mpolacek/src/gcc/gcc/cp/parser.cc:20491 0x103f70d cp_parser_type_specifier /home/mpolacek/src/gcc/gcc/cp/parser.cc:20142 0x104a146 cp_parser_type_specifier_seq /home/mpolacek/src/gcc/gcc/cp/parser.cc:25175
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:081c1e93d56d35c7314ed68e6d87628b430de917 commit r14-9938-081c1e93d56d35c7314ed68e6d87628b430de917 Author: Patrick Palka <ppalka@redhat.com> c++: templated substitution into lambda-expr [PR114393] The below testcases use a lambda-expr as a template argument and they all trip over the below added tsubst_lambda_expr sanity check ultimately because current_template_parms is empty which causes push_template_decl to return error_mark_node from the call to begin_lambda_type. Were it not for the sanity check this silent error_mark_node result leads to nonsensical errors down the line, or silent breakage. In the first testcase, we hit this assert during instantiation of the dependent alias template-id c1_t<_Data> from instantiate_template, which clears current_template_parms via push_to_top_level. Similar story for the second testcase. For the third testcase we hit the assert during partial instantiation of the member template from instantiate_class_template which similarly calls push_to_top_level. These testcases illustrate that templated substitution into a lambda-expr is not always possible, in particular when we lost the relevant template context. I experimented with recovering the template context by making tsubst_lambda_expr fall back to using scope_chain->prev->template_parms if current_template_parms is empty which worked but seemed like a hack. I also experimented with preserving the template context by keeping current_template_parms set during instantiate_template for a dependent specialization which also worked but it's at odds with the fact that we cache dependent specializations (and so they should be independent of the template context). So instead of trying to make such substitution work, this patch uses the extra-args mechanism to defer templated substitution into a lambda-expr when we lost the relevant template context. PR c++/114393 PR c++/107457 PR c++/93595 gcc/cp/ChangeLog: * cp-tree.h (LAMBDA_EXPR_EXTRA_ARGS): Define. (tree_lambda_expr::extra_args): New field. * module.cc (trees_out::core_vals) <case LAMBDA_EXPR>: Stream LAMBDA_EXPR_EXTRA_ARGS. (trees_in::core_vals) <case LAMBDA_EXPR>: Likewise. * pt.cc (has_extra_args_mechanism_p): Return true for LAMBDA_EXPR. (tree_extra_args): Handle LAMBDA_EXPR. (tsubst_lambda_expr): Use LAMBDA_EXPR_EXTRA_ARGS to defer templated substitution into a lambda-expr if we lost the template context. Add sanity check for error_mark_node result from begin_lambda_type. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/lambda-targ2.C: New test. * g++.dg/cpp2a/lambda-targ3.C: New test. * g++.dg/cpp2a/lambda-targ4.C: New test. Reviewed-by: Jason Merrill <jason@redhat.com>
Fixed for GCC 14.