What does it mean when the compiler says that it can’t convert something to itself?

A customer encountered a very strange error message from the Visual C++ compiler:

oops.cpp(7): Error C2664: 'void something(blah)' cannot convert argument 1 from 'blah' to 'blah'

Why is the compiler complaining that it cannot convert a blah to a blah? How is it not possible to convert something to itself?

The answer is given in the next line of the error message:

oops.cpp(7): note: use of undefined type 'blah'

Here’s a sample program that demonstrates the problem.

struct blah;

void something(blah);

void test(blah& b)
{
    something(b); // error here
}

The problem is that the test function is passing a blah object by value to the something function. This requires the compiler to convert the thing you passed (a blah object) to the thing the function accepts (a blah object), which would normally be accomplished by using the copy constructor. But the compiler can’t find a copy constructor for blah, so it complains.

Now, the reason it can’t find a copy constructor is that the type blah has never been defined. All that exists is a forward reference. That’s what the second error message is trying to tell you: “You said blah, but I don’t know anything about blah.”

Bonus chatter: Other compilers produce slightly less confusing error messages by complaining about the incompleteness first.

// gcc
oops.cpp:7:15: error: invalid use of incomplete type 'struct blah'
oops.cpp:1:8: note: forward declaration of 'struct blah'

// clang
oops.cpp:11:15: error: argument type 'blah' is incomplete
oops.cpp:1:8: note: forward declaration of 'blah'

// icc
oops.cpp(11): error: cannot convert to incomplete class "blah"