gcc/testsuite/g++.dg/cpp0x/pr81246.C

// { dg-do compile { target c++11 } }

namespace N
{ 
  template < typename T > class A
  { 
    template < T > friend class B;  // { dg-error "not a valid type" "" { target c++17_down } }
  };

  A < float > a;
}