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From: "novulae at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112749] New: GCC accepts invalid code in concepts (requires
 clause incorrectly satisfied)
Date: Tue, 28 Nov 2023 18:22:08 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.0
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X-Bugzilla-Who: novulae at hotmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112749

            Bug ID: 112749
           Summary: GCC accepts invalid code in concepts (requires clause
                    incorrectly satisfied)
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: novulae at hotmail dot com
  Target Milestone: ---

The below example is ill-formed - the requires clause should not be satisfi=
ed
due to the failure to instantiate W1<int>, but GCC accepts the code
nonetheless.

Clang rejects the code.

$ cat gcc-bug.cpp
template<class> concept C1 =3D true;

template<class T> using W1 =3D decltype(nonexistent<T>(0));

template<class T>
requires C1<W1<T>>
void func(T);

void test() {
  func(0);
}

$ g++ -c -std=3Dc++20 gcc-bug.cpp

$ clang++ -c -std=3Dc++20 gcc-bug.cpp
gcc-bug.cpp:10:3: error: no matching function for call to 'func'
   10 |   func(0);
      |   ^~~~
gcc-bug.cpp:7:6: note: candidate template ignored: constraints not satisfied
[with T =3D int]
    7 | void func(T);
      |      ^
gcc-bug.cpp:3:39: note: because substituted constraint expression is
ill-formed: use of undeclared identifier 'nonexistent'
    3 | template<class T> using W1 =3D decltype(nonexistent<T>(0));
      |                                       ^
1 error generated.=