an enum must be compatible with one or more integer type,
so adjust the test accordingly. That means the following
redeclarations should work:
enum e6 { E1 = -1, E0 };
void f3(enum e6);
void f3(int); // should work as int and e6 are compatible
while the following should not:
void f4(enum e6 e);
void f4(unsigned e); // should error as unsigned and e6 are incompatible
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