a) Formally, the code is correct. As p->what can never be < 0 according to its type.
I am assuming this is C code. C and C++ have different rules for enums.
This isn't strictly true. The C standard does not say that enums are unsigned. It only says that they are equivalent to an integral type with a few restrictions, and an implementation can choose to make it signed or unsigned when all enum values are non-negative. So depending on the compiler, it could be possible for p->what to be negative.
b) GCC _silently_ allows the {-1} initialization for that type, even
with -Wall.
This is because there is no such thing as an enum type in C, enums are just an alias for an integral type, and it is never wrong to store -1 in an integral type, even if it is unsigned. (C++ on the other hand will complain here because C++ has actual enum types distinct from integral types.)
Uncommenting the c= -1 member of enum, or explicit casting p->what to int solves the problem, of course. But maybe some warning would be appropriate in such a situation?
You will get a warning if you compile with -W. tmp.c:22: warning: comparison of unsigned expression >= 0 is always true -- Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
