[Bug c++/80544] New: result of const_cast<T* cv> should by cv-unqualified

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80544

            Bug ID: 80544
           Summary: result of const_cast<T* cv> should by cv-unqualified
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

This is valid:

template<typename T> struct check { };
template<typename T> struct check<const T>; // incomplete

template<typename T> check<T> f(T&&) { return {}; }

int main() {
  int i;
  f(const_cast<int* const>(&i));
}

Clang accepts it, EDG accepts with a helpful warning:

"v.cc", line 8: warning: type qualifier is meaningless on cast type
    f(const_cast<int* const>(&i));
                 ^

G++ gives a bogus error:

v.cc: In function ‘int main()’:
v.cc:8:31: error: invalid use of incomplete type ‘struct check<int* const>’
   f(const_cast<int* const>(&i));
                               ^
v.cc:1:29: note: declaration of ‘struct check<int* const>’
 template<typename T> struct check { };
                             ^~~~~
v.cc: In instantiation of ‘check<T> f(T&&) [with T = int* const]’:
v.cc:8:31:   required from here
v.cc:4:31: error: return type ‘struct check<int* const>’ is incomplete
 template<typename T> check<T> f(T&&) { return {}; }
                               ^

The result of a const_cast to a pointer type is a prvalue, and since a pointer
is not a class or array type the type of the prvalue should be cv-unqualified.