[cpp] Support type parameters on extern classes

[Aidan63]

Hello,
I've added support for type parameters / generic extern classes for cpp. Main change is that TCppInst now carries a list of tcpp type parameters around with it which are used if the class is extern. This allows you to write externs like in the following example, where previously you'd need to write externs for each type of the native templated type (or do stuff like have a generic extern but then extern it with another extern specifying the type).

function main() {
	final ivec = new StdVector();
	ivec.push_back(1);

	final fvec = new StdVector();
	fvec.push_back(1.4685);
}

@:keep
@:unreflective
@:structAccess
@:include('vector')
@:native('std::vector')
extern class StdVector<T> {
	function new();

	function size() : Int;

	function push_back(_v : T) : Void;

	function data() : cpp.RawPointer<T>;

	function at(_idx : Int) : T;
}

which generates cpp code looking like this.

void Main_Fields__obj::main(){
            	HX_STACKFRAME(&_hx_pos_195033e4c87195a0_1_main)
HXLINE(   2)		std::vector<int> ivec = std::vector<int>();
HXLINE(   3)		ivec.push_back(1);
HXLINE(   5)		std::vector<Float> fvec = std::vector<Float>();
HXLINE(   6)		fvec.push_back(((Float)1.4685));
            	}

One thing you can't do with generic externs is something like @:native('::cpp::Struct<std::vector>') for templated classes like you can with non templated classes as the parameters are simply appened to the end (maybe we need a new meta like @:structWrap to auto wrap the type in a cpp.Struct?).

[Aidan63]

Hmm, was hoping the failing cppia test was something being flakey, but seems I have broken something (and the mac ci seems stuck running as well)

[Aidan63]

There we are, all green now. That's what I get for changing code I thought I understood.

[Aidan63]

Had an issue where if you tried to use another extern class for the T parameter, passing an object of that type into an extern function would cause it be casted to dynamic and refuse to compile. The issue seemed to be when a function call is retyped it uses the types of the TFun which in the case of generics will be something like StdVector.T if you want to use T as a function argument and cpp_type_of resorts to dynamic for KTypeParameter instances.

I worked around this by using the types of the arguments being passed in instead of the argument types of the function definition when retyping the function for InstStruct functions. Not sure if this is the correct solution, but it works.

Also, these two new CI failures seem unrelated.