[clang] Incorrect "error: copy constructor must pass its first argument by reference" with unsatisfied constraint

[avikivity]

#include <concepts>

template <typename T>
struct a {
    T v;
    a(a<int> x) requires (!std::same_as<T, int>) : v(x.v) {} 
};

yields (clang 21.1.0)

<source>:7:14: error: copy constructor must pass its first argument by reference

However, the constructor should be ignored since the constraint is not satisfied. It is meant as a converting constructor when T != int, so requiring it to pass the first argument by reference is not sensible.

clang 20.1.8 works.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Avi Kivity (avikivity)

```c++ #include <concepts>

template <typename T>
struct a {
T v;
a(a<int> x) requires (!std::same_as<T, int>) : v(x.v) {}
};

yields (clang 21.1.0)

<source>:7:14: error: copy constructor must pass its first argument by reference

However, the constructor should be ignored since the constraint is not satisfied. It is meant as a converting constructor when T != int, so requiring it to pass the first argument by reference is not sensible.

clang 20.1.8 works.
</details>

[keinflue]

Probably introduced by #130866. As mentioned there at the end of the discussion, this new behavior has already been noticed. (I do not have access to the mentioned core reflector discussion.)

However, I think this is behavior in line with the current language rules. The relevant rule applies to the declaration directly and does not qualify that it only applies to eligible constructors. As mentioned in the link above, this may however be something that should be fixed in the standard.

Both GCC and MSVC agree, only EDG disagrees: https://godbolt.org/z/KcPTjcfYf

[avikivity]

Yes, #130866 (comment) says that they mailed the core reflector.

Relevant wording:

A declaration of a constructor for a class X is ill-formed if its first parameter is of type cv X and either there are no other parameters or else all other parameters have default arguments. A member function template is never instantiated to produce such a constructor signature.
[Example 5: 

struct S {
  template<typename T> S(T);
  S();
};

S g;

void h() {
  S a(g);           // does not instantiate the member template to produce S​::​S<S>(S);
                    // uses the implicitly declared copy constructor
}

— end example]

https://eel.is/c++draft/class.copy.ctor#5

[cor3ntin]

Relevant core issue https://cplusplus.github.io/CWG/issues/2837.html
I don't think WG21 came to a conclusion yet @hubert-reinterpretcast @zygoloid @t3nsor

[t3nsor]

thanks for the bump, I need to work on that issue