https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101315
Bug ID: 101315 Summary: C++20 lambdas in unevaluated context: erroneously fails with "incomplete type" Product: gcc Version: 11.1.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: janpmoeller at gmx dot de Target Milestone: --- gcc 11.1 and gcc (trunk) fail to compile the following code with --std=c++20: (1) //////////////////////////////////////////////////////////////////////////// #include <concepts> #include <vector> template <typename Ptr, std::regular_invocable<int> Fn> struct foo_t { foo_t(Ptr ptr) {} }; template <typename T> using alias = foo_t<T, decltype([](int) { return 0; })>; template <typename T> auto fun(T const& t) { return alias<T>{t}; } int main() { std::vector<int> v; auto const error = fun(v); } //////////////////////////////////////////////////////////////////////////////// Both versions of gcc state: <source>: In function 'int main()': <source>:19:14: error: 'const void error' has incomplete type 19 | auto const error = fun(v); | ^~~~~ Compiler returned: 1 The issue seems to somehow be related to the alias. Replacing the definition of fun() with this code results in successful compilation: (2) //////////////////////////////////////////////////////////////////////////// template <typename T> auto fun(T const& t) { return foo_t<T, decltype([](int) { return 0; })>{t}; } //////////////////////////////////////////////////////////////////////////////// Specifying the return type of fun() explicitly results in a different error: (3) //////////////////////////////////////////////////////////////////////////// template <typename T> auto fun(T const& t) -> alias<T> { return alias<T>{t}; } //////////////////////////////////////////////////////////////////////////////// <source>: In function 'int main()': <source>:19:22: error: 'fun' was not declared in this scope 19 | auto const error = fun(v); | ^~~ Compiler returned: 1 I believe all three variants to be valid c++20. clang (trunk) compiles versions (1) and (2) of the code, (3) results in a frontend crash (I'm going to file a bug report against clang for that as well. I'll comment it here for reference after I'm done). Also see this link to godbolt: https://godbolt.org/z/r9GMfxzEo