TemplateParameterList in is expression

[simendsjo]

I'm having some problems understanding the (complex) is expression.

 // Example from the documentation
 static if (is(int[10] X : X[Y], int Y : 5))
 {
        assert(0);      // should not match, Y should be 10
 }

 // but this doesn't match either.. Shouldn't it?
 static if (is(int[10] X : X[Y], int Y : 10))
 {
        assert(0);
 }