https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101006
Bug ID: 101006 Summary: Request diagnostic for likely concept syntax errors Product: gcc Version: 10.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: barry.revzin at gmail dot com Target Milestone: --- Consider the following: template <typename T> concept Thing = true; template <typename T> concept MemberThing = requires (T t) { t.member() -> Thing; // #1 !requires { t.member(); }; // #2 }; These are likely intended to be (obviously not at the same time, this is just an example): template <typename T> concept MemberThing = requires (T t) { { t.member() } -> Thing; requires !requires { t.member(); }; }; But #1 is very likely to be a bug, and #2 is completely pointless as a requirement since it's tautologically true. It would be nice if gcc could produce warnings in such cases. #2 is a similar case to the P2092 fixup of: template <typename T> concept MemberThing = requires (T t) { requires { t.member(); }; }; which is now ill-formed. However, gcc's diagnostic here could be more helpful too (perhaps including a fixup for an extra requires?) <source>:6:14: error: expected primary-expression before '{' token 6 | requires { t.member(); }; | ^ #1 might be harder to warn about since that could *theoretically* be intended, but if unqualified lookup for Thing actually finds a concept, seems like a good bet for a diagnostic maybe?