[Bug c++/112267] New: "inline function constexpr used but never defined" warning in SFINAE context

[Bug c++/112267] New: "inline function constexpr used but never defined" warning in SFINAE context

[falemagn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112267

            Bug ID: 112267
           Summary: "inline function constexpr used but never defined"
                    warning in SFINAE context
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: falemagn at gmail dot com
  Target Milestone: ---

I am expecting no warnings from the following code snippets, especially because
SFINAE is being correctly triggered, and indeed clang doesn't provide one for
the specific case at hand, alas g++ does.

I am left wondering whether this is a bug or a "overreaction". The warning
itself cannot be turned off with a pragma.

    #if defined(__clang__)
        _Pragma("GCC diagnostic ignored \"-Wundefined-inline\"");       
    #endif

    #define DEFINE_FUNC 0

    constexpr bool func()
    #if DEFINE_FUNC
    {
        return true;
    }
    #else
    ;
    #endif

    template <int Dummy = 0, bool = func()>
    constexpr bool is_defined(int) {
        return true;
    }


    template <int Dummy = 0>
    constexpr bool is_defined(double) {
        return false;
    }

    int a = is_defined(0);

See it working on Compiler Explorer: https://godbolt.org/z/dvTKdGfbv

[Bug c++/112267] "inline function constexpr used but never defined" warning in SFINAE context

[falemagn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112267

--- Comment #1 from Fabio Alemagna <falemagn at gmail dot com> ---
Changing the return type of the function func() from int to auto makes the
warning disappear.

Incidentally it causes an error on clang, though, which prompts the question:
is this a clang bug, for erroring out, or a gcc bug, for not erroring out?

See it on Compiler Explorer: https://godbolt.org/z/4ETs4ca3d

[Bug c++/112267] "inline function constexpr used but never defined" warning in SFINAE context

[falemagn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112267

--- Comment #2 from Fabio Alemagna <falemagn at gmail dot com> ---
Sorry, "from int to auto" should have been "from bool to auto". You get the
point.

[Bug c++/112267] "inline function constexpr used but never defined" warning in SFINAE context

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112267

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Fabio Alemagna from comment #1)
> Changing the return type of the function func() from int to auto makes the
> warning disappear.
> 
> Incidentally it causes an error on clang, though, which prompts the
> question: is this a clang bug, for erroring out, or a gcc bug, for not
> erroring out?

I tested MSVC also. It errors out with /std:c++20 but not with /std:c++17 so
maybe there was a change for C++20 that clang decided it applies for previous
C++ too (and GCC did not implement that rule yet).

[Bug c++/112267] "inline function constexpr used but never defined" warning in SFINAE context

[falemagn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112267

--- Comment #4 from Fabio Alemagna <falemagn at gmail dot com> ---
With a tweak(In reply to Andrew Pinski from comment #3)
> (In reply to Fabio Alemagna from comment #1)
> > Changing the return type of the function func() from int to auto makes the
> > warning disappear.
> > 
> > Incidentally it causes an error on clang, though, which prompts the
> > question: is this a clang bug, for erroring out, or a gcc bug, for not
> > erroring out?
> 
> I tested MSVC also. It errors out with /std:c++20 but not with /std:c++17 so
> maybe there was a change for C++20 that clang decided it applies for
> previous C++ too (and GCC did not implement that rule yet).


Making the function a template, while keeping the return type as 'auto', and
making its template parameter depend on the template parameter of the caller,
solves the error and silences the warning on gcc.


Making the return value 'bool', even if the function is a template, still makes
the warning appear.


On Compiler Explorer: https://godbolt.org/z/E5qMT55MW