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: [email protected]
ReportedBy: [email protected]
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).
