You may receive a "C3767" error message when you access a friend function with an argument of type nested class in Visual C++ .NET 2003 or in Visual C++ 2005 (814793)

	friend bool operator==(B, B){return false;}

struct A { 

	struct B { 
		friend bool operator==(B, B){return false;} 
	}; 

}; 

bool operator!=(A::B b0, A::B b1) 
{ 
	return !(b0==b1); 
}

MORE INFORMATION

With Argument-dependent (Koenig) lookup, when a function is called, the namespaces where the function parameters reside are taken into account to determine if that function is visible in the current scope.

Argument-dependent lookup examines types of all arguments in a function, and the associated namespaces and classes or structs are collected. The name of the function is then looked up in all the namespaces and classes or structs.

The following code example compiles in Microsoft Visual C++ .NET 2002 because the friend function declaration is exported to the enclosing nonclass scope. In this case, the enclosing nonclass scope is the global scope:

#include <iostream>

struct A
{
				struct B
			{
		
			};
	friend bool operator==(B, B){return false;}
};

	bool operator!=(A::B b0, A::B b1)
	{ 
		return !(b0==b1);
 //error C3767: '==' matching function is not accessible could be the friend function [may be found via argument-dependent lookup]
	}

	
	int main() {
		
		return 0;
	}

Note You must add the common language runtime support compiler option (/clr:oldSyntax) in Visual C++ 2005 to successfully compile the previous code sample. To add the common language runtime support compiler option in Visual C++ 2005, follow these steps:

1. Click Project, and then click <ProjectName> Properties.
   
   Note <ProjectName> is a placeholder for the name of the project.
2. Expand Configuration Properties, and then click General.
3. Click to select Common Language Runtime Support, Old Syntax (/clr:oldSyntax) in the Common Language Runtime support project setting in the right pane, click Apply, and then click OK.

For more information about the common language runtime support compiler option, visit the following Microsoft Web site:To reproduce this behavior, compile the code sample in the "More Information" section of this article by using Visual C++ .NET 2003 or Visual C++ 2005. You receive the error that is described in the "Symptoms" section of this article. This error occurs for the following reasons:

• Visual C++ .NET 2003 complies with 98 percent of the ISO C++ standards. That is a higher level than by any previous version of Visual C++. Visual C++ .NET 2003 also contains new language support for features such as Koenig lookup.
• In Visual C++ .NET 2003, no more friend declarations are exported to the nonclass scope. Instead, overloaded friend operators are resolved with the help of Argument-dependent lookup.

Therefore, in Visual C++ .NET 2003, the compiler strongly adheres to the rule that is specified in the "Resolution" section of this article.