[Bug c++/103084] New: Accepts invalid using enum declaration with an invalid elaborated-type-specifier

[aaron at aaronballman dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103084

            Bug ID: 103084
           Summary: Accepts invalid using enum declaration with an invalid
                    elaborated-type-specifier
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aaron at aaronballman dot com
  Target Milestone: ---

GCC (and MSVC) currently accepts the following snippet while Clang and ICC
reject:
```
enum class Pig { Oink };
using Hog = Pig;
using enum Hog; // Clang & ICC rejects, GCC & MSVC accepts
```
I think Clang and ICC are correct to reject this code.
http://eel.is/c++draft/enum.udecl says that the production needs to parse an
elaborated-enum-specifier. One of the requirements for a valid
elaborated-enum-specifier is that the elaboration has to agree in kind with the
given declaration (https://eel.is/c++draft/dcl.type.elab#6). In this case, the
given declaration is not an enum, it's a type alias.

Note that all compilers reject the following slightly different example due to
the elaborated type specifier being invalid:
```
enum class Pig { Oink };
using Hog = Pig;
enum Hog H; // Everyone rejects
```