[PATCH] D119544: Deferred Concept Instantiation Implementation

[Erich Keane via Phabricator via cfe-commits]

erichkeane added a comment.
Herald added a reviewer: dang.

@rsmith pointed out https://eel.is/c++draft/temp#friend-9 which I think is supposed to fix the friend function issues:

> A non-template friend declaration with a requires-clause shall be a definition.
> A friend function template with a constraint that depends on a template parameter from an enclosing template shall be a definition.
> Such a constrained friend function or function template declaration does not declare the same function or function template as a declaration in any other scope.

To me, this says that

  template<typename T>
  struct S {
      friend void foo() requires true {}
  };
  void bar() {
  S<int> s;
  S<float> s2;
  }

Should be perfectly fine, and NOT a redefinition?  At least this: https://godbolt.org/z/PaK1Yhn1E seems to show that all 3 compilers get this wrong currently?  Or I'm misreading this.

It ALSO means that:

  template<typename T>
  struct S {
  template<typename U>
  friend void foo() requires constriant<T> {}
  };
  void bar() {
  S<int> s;
  S<float> s2;
  }

is perfectly fine, and not a redefinition. (CURRENT Clang and GCC get this right, but MSVC seems to get it wrong: https://godbolt.org/z/a7dYezoPW)

HOWEVER, By reading the rule, a slightly DIFFERENT version of that

  template<typename T>
  struct S {
  template<typename U>
  friend void foo() requires constriant<U> {} // No longer relies on the enclosing template
  };
  void bar() {
  S<int> s;
  S<float> s2;
  }

Is a redefinition.  All 3 compilers diagnose this. https://godbolt.org/z/7qbYsb635

Do I have this right?  I'm having trouble figuring out the implementation strategy here.  It seems my SemaOverload.cpp changes would need to do special work to get example #1 to work, and I got close on #2 (with the exception of some assert in libc++).  And #3  I probably had right?

Can someone else read the standard bit there and make sure I got a good spread of the issues and interpreted the prose correctly?

Thanks!
-Erich


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