Or can it be a separate unsigned integer type?
I have different specializations of a template function for different (unsigned) integer types. Do I need to provide a separate specialization for size_t?
The C++ Standard says:
18.2/2 The contents are the same as the Standard C library header , with the following changes:
18.2/6 The type size_t is an implementation-defined unsigned integer type that is large enough to contain the size in bytes of any object.
18.2/7 [ Note: It is recommended that implementations choose types for ptrdiff_t and size_t whose integer conversion ranks (4.13) are no greater than that of signed long int unless a larger size is necessary to contain all the possible values. —end note ]
So, it doesn't say explicitly whether the implementation-defined unsigned integer type will be one of unsigned short, int, long, long long. The fact that 18.2/6 exists and specifies an "implementation-defined unsigned integer type" may be seen to override 18.2/2's default of following C, so any answer for C can't be trusted for C++.
The recommendation re conversion ranks implies the size_t will be expected to be one of the types mentioned in 4.13, where size_t isn't explicitly mentioned but the obvious candidates are, but that's no guarantee.
Do I need to provide a separate specialization for size_t?
You could use std::is_same and std::enable_if to do so when size_t is a distinct type....