Template matching incorrectly considers __attribute__((noreturn))?

```c++
#include

template
struct S;

template
struct S {};

template
struct S {};

static_assert(sizeof(S) != 0);
```

produces

```
:12:15: error: implicit instantiation of undefined template 'S'
12 | static_assert(sizeof(S) != 0);
| ^
:4:8: note: template is declared here
4 | struct S;
| ^
```

on Clang, but GCC only produces a warning:

```
:12:39: warning: ignoring attributes on template argument 'void() noexcept' [-Wignored-attributes]
12 | static_assert(sizeof(S) != 0);
| ^
```

The issue names no source files or tests; begin by compiling the provided C++ reproducer with Clang and GCC. Trace how Clang matches the two S specializations for decltype(abort), including noreturn and noexcept handling. Done requires an agreed expected result, corrected behavior, and regression coverage in the relevant compiler tests.

Written by the indexing model from the issue text.