https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111419
Bug ID: 111419 Summary: Eager instantiation of function return type in concept causes compilation error Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: ldionne.2 at gmail dot com Target Milestone: --- The following code fails to compile on GCC: ------------------------------------------------------------ template <class F> auto invoke(F f) -> decltype(f()); template <class F> concept invocable = requires(F&& f) { ::invoke(f); }; struct Incomplete; template<class T> struct Holder { T t; }; static_assert(invocable<Holder<Incomplete>& ()>); ------------------------------------------------------------ It produces the following error on GCC: <source>: In instantiation of 'struct Holder<Incomplete>': <source>:6:11: required from here <source>:10:37: error: 'Holder<T>::t' has incomplete type 10 | template<class T> struct Holder { T t; }; | ^ <source>:9:8: note: forward declaration of 'struct Incomplete' 9 | struct Incomplete; | ^~~~~~~~~~ Compiler returned: 1 My understanding is that this should be valid, because nothing actually requires instantiating the return type of `f()` here, but I'm not sufficiently well-versed in the details of concepts to know for sure. Clang and MSVC accept the code. Godbolt: https://godbolt.org/z/9G5zj47an