https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107164
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang says it is a GCC extension:
<source>:2:6: warning: redeclaration of already-defined enum 'E' is a GNU
extension [-Wgnu-redeclared-enum]
enum E;
^
<source>:1:6: note: previous definition is here
enum E {a,b,c};
^
GCC does have a forward declaration extension for enum types so it makes sense
to allow this as an extension too.
That is:
```
enum E;
enum E {a,b,c};
```
(which GCC does have a pedantic diagnostic about).
