[Bug c++/98768] New: Improve diagnostics for incorrect result type checking "-> Type" in concepts

[Bug c++/98768] New: Improve diagnostics for incorrect result type checking "-> Type" in concepts

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98768

            Bug ID: 98768
           Summary: Improve diagnostics for incorrect result type checking
                    "-> Type" in concepts
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the example:


template <typename Function, typename T>
concept Callable0Arg = requires(Function func) {
    func() -> T;
};


The expression "-> T" is valid only if the "func()" returns pointer to a type
that has member "T". At the same time there is an unused "typename T" in the
concept definition.

For such cases a warning like "Unused `T` in concept definition. Did you mean
`-> std::same_as<T>`"  would be really helpful.

[Bug c++/98768] Improve diagnostics for incorrect result type checking "-> Type" in concepts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98768

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Antony Polukhin from comment #0)
> For such cases a warning like "Unused `T` in concept definition. Did you
> mean `-> std::same_as<T>`"  would be really helpful.

That wouldn't fix it though, the correct form would be:

  { func() } -> T;

[Bug c++/98768] Improve diagnostics for incorrect result type checking "-> Type" in concepts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98768

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oops! I meant:

  { func() } -> std::same_as<T>;