Full name of submitter: Brian Bi

Reference (section label): [class.copy.ctor], [namespace.udecl]

Issue description: [class.copy.ctor]/5 states:

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.

This wording does not explain what happens if such a constructor signature is inherited from a base class; MSVC currently treats the using-declaration as ill-formed (https://godbolt.org/z/6x47rf4vT). We should say that such a constructor is excluded from the inherited set.

Also, the wording "never instantiated" doesn't explain at what step of the process instantiation stops. I doubt there would be any implementation divergence, but even still, we should be a bit more explicit and say the result is a substitution failure.

Suggested resolution:

Add a paragraph before [namespace.udecl]/13:

The set of declarations named by a using-declarator that inhabits a class C does not include any constructor whose first parameter has type cv C and all of whose remaining parameters (possibly none) have default arguments.

Edit [class.copy.ctor]/5:

A declaration of a constructor for a class X is ill-formed if its first parameter is of type 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. During type deduction for a constructor template of X ([temp.deduct.general]), if substitution produces such a constructor signature, type deduction fails.

In Example 5 to [class.copy.ctor]/5, edit the comment:

// does not instantiate the member template to produce S​::​S<S>(S); no S::S<S>(S) candidate generated from the member template;
// uses the implicitly declared copy constructor is used