At runtime, a failing downcast evaluates to null.
At compile time, it gives an error:
"Error: cannot reinterpret class from C to test.B at compile time"
It should just evaluate to null, too.

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class A {}
class B : A {}
class C : A {}

bool test(T)() {
	A a = new T;
	return (cast(B) a) !is null;
}

void main() {
	assert(test!B()); // ok
	assert(!test!C()); // ok
	static assert(test!B()); // ok
	static assert(!test!C()); // error
}
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