out parameter with default value

[AsmMan via Digitalmars-d-learn]

Why it does works:

void f(out int c)
{
    if(some_cond)
	c = 10;
}

but it doesn't?

void f(out int c = 1)
{
    if(some_cond)
	c = 10;
}

it give compiler error:

Error: constant 1 is not an lvalue