[clang-18 regression] Compile error in out-of-line definition of constrained class template nested in a class template

[ozaktash]

The following code doesn't compile on Clang 18 and further:

template<typename>
concept Constrained = true;

template <typename T>
class C
{
    template<Constrained>
    class D;
};

template <typename T>
template <Constrained>
class C<T>::D
{
};

With the following message:

<source>:12:11: error: type constraint differs in template redeclaration
   12 | template <Constrained>
      |           ^
<source>:7:14: note: previous template declaration is here
    7 |     template<Constrained>
      |              ^
1 error generated.

It compiles with Clang 17, GCC and MSVC (https://godbolt.org/z/r65o8sPjs).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (ozaktash)

The following code doesn't compile on Clang 18 and further: ```cpp template<typename> concept Constrained = true;

template <typename T>
class C
{
template<Constrained>
class D;
};

template <typename T>
template <Constrained>
class C<T>::D
{
};

With the following message:

<source>:12:11: error: type constraint differs in template redeclaration
12 | template <Constrained>
| ^
<source>:7:14: note: previous template declaration is here
7 | template<Constrained>
| ^
1 error generated.

It compiles with Clang 17, GCC and MSVC (https://godbolt.org/z/r65o8sPjs).

</details>

[AaronBallman]

Somewhat amusingly, Clang 18 fixed an issue present in Clang 17 regarding explicit specializations with similar code: https://godbolt.org/z/e7doYqe1d