[Bug c++/82299] New: -Wuseless-cast errors on typed enums used in member data initializers in c++1z

[matt at godbolt dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82299

            Bug ID: 82299
           Summary: -Wuseless-cast errors on typed enums used in member
                    data initializers in c++1z
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at godbolt dot org
  Target Milestone: ---

The follow code:

```
// compile with -Wuseless-cast -std=c++1z
enum Enum : char { A = 0, B = 1 };

struct S {
  Enum e { Enum::A };
};
```

Compiled with `-Wuseless-cast -std=c++1z` yields the following erroneous
warning:

```
5 : <source>:5:20: warning: useless cast to type 'enum class Enum'
[-Wuseless-cast]
   Enum e { Enum::A };
                    ^
```

This only happens if the `enum` is typed (e.g. the `char` above), and it's
compiled with std=c++1z.

Seems to have been introduced in GCC 7, still present in trunk.

(see https://godbolt.org/g/ehKEau )