[Bug c++/101006] New: Request diagnostic for likely concept syntax errors

[barry.revzin at gmail dot com]

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?