[Bug c++/112749] New: GCC accepts invalid code in concepts (requires clause incorrectly satisfied)

[Bug c++/112749] New: GCC accepts invalid code in concepts (requires clause incorrectly satisfied)

[novulae at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112749

            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 satisfied
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 = true;

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

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

void test() {
  func(0);
}

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

$ clang++ -c -std=c++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 = 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 = decltype(nonexistent<T>(0));
      |                                       ^
1 error generated.

[Bug c++/112749] GCC accepts invalid code in concepts (requires clause incorrectly satisfied)

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112749

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the bug report, I think this is pretty much a dup of PR102419.

GCC is behaving correctly here: the normal form of the constraint C1<W1<T>> is
just 'true (with empty parameter mapping)' which is independent of the template
parameter, so the constraint is trivially satisfied for any choice of T.  To
get the behavior you desire, define C1 in a way that depends on its template
parameter e.g.

template<class T> concept C1 = requires { typename T; };

*** This bug has been marked as a duplicate of bug 102419 ***