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Compiler Warning (level 3) C4686

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'user-defined type' : possible change in behavior, change in UDT return calling convention

A class template specialization was not is defined before it was used in a return type. Anything that instantiates the class will resolve C4686; declaring an instance or accessing a member (C<int>::anything) are also options.

This warning is the result of work to make the Visual C++ .NET 2003 compiler conform to the ISO C++ standard.

This warning is off by default. See Compiler Warnings That Are Off by Default for more information.

Try the following instead,

// C4686.cpp
// compile with: /W3
#pragma warning (default : 4686)
template <class T>
class C;

template <class T>
C<T> f(T);

template <class T>
class C {};

int main() {
   f(1);   // C4686
}

template <class T>
C<T> f(T) {
   return C<int>();
}