[Bug libstdc++/125439] std::in_range doesn't accept const-qualified result type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125439

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard defines 'signed integer types' and 'unsigned integers types', then
defined 'integer type' as a set containing all the character types, bool, all
the signed and unsigned integer types, **and cv-qualified versions** of those. 

So bool and const bool and const int and const char are all integer types, but
they are not in the set of signed integer types or unsigned integer types.