[Clang][concepts][regression] Construction of a class X nested in class template is ill-formed when X has class template friend with specific constraints

[JMazurkiewicz]

As in title. Repro:

template <class> struct First {
  struct Fail {
    template <bool C>
      requires C
    friend class Second;
  };
};

template <bool C>
  requires C
struct Second {};

auto x = First<void>::Fail{};

This code was well-formed until Clang-16. Build:

> E:\llvm-project\build-llvm\bin\clang.exe --version
clang version 17.0.0 (https://github.com/JMazurkiewicz/llvm-project.git 3b7d29832296b2f48869c9dbee7a5be060fef0c5)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: E:\llvm-project\build-llvm\bin
> E:\llvm-project\build-llvm\bin\clang.exe .\clang-reduced.cpp -std=c++20
.\clang-reduced.cpp:4:16: error: requires clause differs in template redeclaration
      requires C
               ^
.\clang-reduced.cpp:13:10: note: in instantiation of member class 'First<void>::Fail' requested here
auto x = First<void>::Fail{};
         ^
.\clang-reduced.cpp:10:12: note: previous template declaration is here
  requires C
           ^
1 error generated.

Last commit: 3b7d298
Compiler explorer: https://godbolt.org/z/d6zn9sh67
Discovered while testing ranges::adjacent_view from MSVC-STL.

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

@erichkeane looks like this behavior changed between clang-15 and clang-16, it looks valid to me, wdyt?

[alexander-shaposhnikov]

fwiw - this one compiles with https://reviews.llvm.org/D146178

[JMazurkiewicz]

Fixed in 6db007a
Compiler explorer: https://godbolt.org/z/d9nErKsYM