[Bug c++/110160] New: g++ rejects concept as cyclical with non-matching function signature

[Bug c++/110160] New: g++ rejects concept as cyclical with non-matching function signature

[danakj at orodu dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110160

            Bug ID: 110160
           Summary: g++ rejects concept as cyclical with non-matching
                    function signature
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: danakj at orodu dot net
  Target Milestone: ---

Godbolt: https://godbolt.org/z/d4PhfMqvq

Code:
```
#include <sstream>
#include <string>

template <class T>
concept StreamCanReceiveString = requires(T& t, std::string s) {
    { t << s };
};

struct NotAStream {};
struct UnrelatedType {};

template <StreamCanReceiveString S>
S& operator<<(S& s, UnrelatedType) {
    return s;
}

static_assert(!StreamCanReceiveString<NotAStream>);

static_assert(StreamCanReceiveString<std::stringstream>);
```

What happens here is GCC fails to be able to resolve the expression
`StreamCanReceiveString<NotAStream>`.

1. StreamCanReceiveString<NotAStream> tries to do NotAStream << std::string.
2. There is a templated operator<< that takes `StreamCanReceiveString` and
`UnrelatedType`
3. Since `UnrelatedType` is not std::string, this is not an overload candidate.
4. Clang and MSVC therefore do not try to recursively solve
`StreamCanReceiveString<NotAStream>` and reject the code. But GCC tries to
solve the concept and then fails due to recursion.

[Bug c++/110160] g++ rejects concept as cyclical with non-matching function signature

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110160

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=99599

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think this is a dup of bug 99599, specifically bug 99599 comment #2 .

[Bug c++/110160] g++ rejects concept as cyclical with non-matching function signature

[danakj at orodu dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110160

--- Comment #2 from danakj at orodu dot net ---
Ugh, yeah, I guess it is. It means you can't redirect through a template
function that uses concepts with G++.

[Bug c++/110160] g++ rejects concept as cyclical with non-matching function signature

[danakj at orodu dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110160

danakj at orodu dot net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #3 from danakj at orodu dot net ---
Okay I've got a workaround based on
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99599#c6. It's probably worse for
compile times, but it is what it is.

Thanks for the link.

*** This bug has been marked as a duplicate of bug 99599 ***