clang-16 regression in friend functions with concept-constrained template arguments

[DeadlyRedCube]

Godbolt link: https://godbolt.org/z/j86T8dzMY

In Clang 16 (tested in both 16.0.0 via Godbolt and 16.0.6 via the standard LLVM Ubuntu distro), if you friend a function with a template argument that is concept-constrained:

template <typename T> concept some_concept = true;

template <some_concept T>
class Foo
{ 
private:
  Foo();
  template <some_concept U> friend Foo<U> Friended(U a);
};

template <some_concept T>
Foo<T> Friended(T a)
  { return {}; }

void Bar()
  { Friended(1); }

it fails to compile with a spurious error:

<source>:13:12: error: calling a private constructor of class 'Foo<int>'
  { return {}; }
           ^
<source>:16:5: note: in instantiation of function template specialization 'Friended<int>' requested here
  { Friended(1); }
    ^
<source>:7:3: note: declared private here
  Foo();
  ^

If the function uses typename instead of some_concept it compiles fine (the supplied Godbolt link has both versions) This compiles fine in Clang 14 and 15 (as well as GCC and MSVC).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

CC @erichkeane

[zygoloid]

This also compiles fine in Clang trunk, probably fixed by 6db007a0654ed7a6ed5c3aa3b61a937c19a6bc6b.

[brunodf-snps]

We encounter this issue but are still confined to clang-16 for the moment.

Are there any hopes to fix this regression on release/16.x ? @alexander-shaposhnikov is 6db007a0654ed7a6ed5c3aa3b61a937c19a6bc6b something suitable to cherry pick there?

The regression seems introduced during LLVM 16 development in commit babdef27c503c0bbbcc017e9f88affddda90ea4e (by @erichkeane).