[cs] invalid cast methods generated

[nadako]

reduced from our test suite, which compiles only because we have a name clash between a type parameter name and some class name

class Base<T> {}
class Child1<A, B> extends Base<B> {}
class Child2<T> extends Child1<T, T> {}

error CS0246: The type or namespace name 'A' could not be found (are you missing a using directive or an assembly reference?)

problematic generated code:

public class Child2<T> : global::Child1<T, T>, global::Child2 {

	// ...snip...
		
	public override object Base_cast<T_c>() {
		return this.Child2_cast<A>();
	}

	// ...snip...
	
}

[nadako]

I spent several hours trying to understand what's going on there in RealTypeParamsModf.create_stub_casts but type parameter handling is just beyond my comprehension.
It seems that when it's processing Child1<A,B> while generating cast for the Child2<T> there's only Child1.A in reverse_params, but when it tries to process further and gets to Base<T>, it tries to find a mapping from Child1.B to Base.T and fails because reverse_params only has Child1.A... So to say, I don't even understand what did I just write :-P

@waneck pls halp

[nadako]

Note that 94809e3 surfaced this issue in our unit tests (because some types were eliminated by DCE that accidentally had the same name as type params), so I added #if !cs at TestOverloads.hx with a TODO note for now. Please, don't forget to remove that after fixing this issue :)

[waneck]

Yeah that type parameter handling gave me a headache :P The issue was that reverse_params should store the type parameters that the current cast function should be called, when in reference to the current parent class