Compiler version and platform:
Intel(R) C Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 17.0.0.098 Build 20160721
$ g++-6.2 small.cpp
$ clang++-3.8 small.cpp
$
$ icc small.cpp
small.cpp(9): error: declaration is incompatible with function template "void S<T>::f(U) [with T=int]" (declared at line 4)
template < typename U > friend void S < T >::f (U);
^
detected during:
instantiation of class "C<T> [with T=int]" at line 4
instantiation of "void S<T>::f(U) [with T=int, U=double]" at line 16
small.cpp(4): error #308: member "C<T>::i [with T=int]" (declared at line 10) is inaccessible
template < typename U > void f (U u) { C < T > ct; ct.i = 0; }
^
detected during instantiation of "void S<T>::f(U) [with T=int, U=double]" at line 16
compilation aborted for small.cpp (code 2)
$
$ cat small.cpp
template < typename T > class C;
template < typename T > struct S
{
template < typename U > void f (U u) { C < T > ct; ct.i = 0; }
};
template < typename T > class C
{
template < typename U > friend void S < T >::f (U);
int i;
};
int main ()
{
S < int > si;
si.f (0.0);
return 0;
}
$
