[Bug c++/50732] New: [type_traits] is_base_of unnecessarily instantiates Base (which shouldn't be instantiated)

[mimomorin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50732

             Bug #: 50732
           Summary: [type_traits] is_base_of<Base, Derived> unnecessarily
                    instantiates Base (which shouldn't be instantiated)
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: mimomo...@gmail.com


Created attachment 25504
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25504
Test case (which fails to be compiled)

Let `Base` be an instance of a class template. By FDIS 20.9.4.3 p3, 
`Base` can be incomplete type and its instantiation should be avoided 
in `is_base_of<Base, Derived>`. But, as can be seen below, g++ erroneously 
instantiates `Base` when its definition is visible. 

The following code compiles fine:

    #include <iostream>
    #include <type_traits>

    template <typename T> struct non_instantiatable;

    int main (int argc, char* argv[])
    {
        std::cout << std::is_base_of<non_instantiatable<int>, void>::value <<
std::endl;
        return 0;
    }

But if we add the definition, the code fails to compile, 
since g++ tries to instantiate `non_instantiatable<int>`: 

    #include <iostream>
    #include <type_traits>

    template <typename T>
    struct non_instantiatable
    {
        typedef typename T::THIS_TYPE_CANNOT_BE_INSTANTIATED type;
    };

    int main (int argc, char* argv[])
    {
        std::cout << std::is_base_of<non_instantiatable<int>, void>::value <<
std::endl;

        return 0;
    }

Tested on g++ 4.7.0 20111008 (experimental).