Clang accepts invalid parenthesized constructor declarator

[zygoloid]

Per [class.ctor.general]/1, a constructor declarator cannot be parenthesized:

struct A {
  // OK, constructor
  (A)(int);

  // Error, not a constructor due to enclosing parens.
  (A(float));
};
// OK, constructor
(A::A(int)) {}
// Error, not a constructor due to enclosing parens.
(A::A(float)) {}

But Clang accepts anyway, as do all other major compilers. We should reject under -pedantic-errors, but given the widespread support it probably makes sense to continue to accept this by default.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Richard Smith (zygoloid)

Per [[class.ctor.general]/1](https://eel.is/c++draft/class.ctor.general#1), a constructor declarator cannot be parenthesized:

struct A {
  // OK, constructor
  (A)(int);

  // Error, not a constructor due to enclosing parens.
  (A(float));
};
// OK, constructor
(A::A(int)) {}
// Error, not a constructor due to enclosing parens.
(A::A(float)) {}

But Clang accepts anyway, as do all other major compilers. We should reject under -pedantic-errors, but given the widespread support it probably makes sense to continue to accept this by default.