[Bug c/63543] incomplete type error should suppress duplicates

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63543

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |manu at gcc dot gnu.org

--- Comment #2 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
It would also be nice to mention the type.

test.c:6:12: error: incomplete definition of type 'struct undefined'
  int x = f->a;
          ~^
test.c:2:8: note: forward declaration of 'struct undefined'
struct undefined;
       ^