[Bug c++/91592] New: `__is_assignable` fails for private assignment operators in certain contexts

[blelbach at cct dot lsu.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91592

            Bug ID: 91592
           Summary: `__is_assignable` fails for private assignment
                    operators in certain contexts
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blelbach at cct dot lsu.edu
  Target Milestone: ---

This code fails with GCC 8.2 and GCC trunk.

https://godbolt.org/z/IljfIw


```
class B { B& operator=(const B&); };

static_assert(!__is_assignable(B&, B const&), "");   // Ok.

void foo() {
  static_assert(!__is_assignable(B&, B const&), ""); // FAIL.
}
```

MSVC, Clang and EDG all accept this code.