Defining a single opCast disables explicit cast to base interfaces

[Ali Çehreli via Digitalmars-d]

The following program compiles fine:

interface I
{}
class B : I
{}

class C : B
{
    int i;
}

void main()
{
    auto c = new C;

    auto i = cast(I)c;    // compiles
    auto b = cast(B)c;    // compiles
}

Let's add an unrelated opCast to C:

class C : B
{
    int i;

    int opCast(T : int)()
    {
        return i;
    }
}

Now the last two lines of main fail to compile:

Error: template instance opCast!(I) does not match template declaration 
opCast(T : int)()
Error: template instance opCast!(B) does not match template declaration 
opCast(T : int)()

Is this per spec? (Actually, where is the spec? (Trick question. ;) )

There is a workaround: Add a catch-all opCast that forwards to the 
all-powerful std.conv.to:

    T opCast(T)()
    {
        import std.conv;
        return this.to!T;
    }

Now it compiles and works as expected.

However, the question remains...

Thank you,
Ali