cpp.Function fields don't compile with HXCPP_GC_GENERATIONAL #167

With HXCPP_GC_GENERATIONAL, Haxe treats a cpp.Function field as an object and writes a barrier on _hx_v.mPtr in its setter:

inline ::cpp::Function< int (int) > _hx_set_callback(::hx::StackContext *_hx_ctx,::cpp::Function< int (int) > _hx_v) { HX_OBJ_WB(this,_hx_v.mPtr) return callback=_hx_v; }

cpp::Function (include/cpp/Pointer.h) has no mPtr, so any class with a cpp.Function field fails to compile in a generational build:

error: 'class cpp::Function<int(int)>' has no member named 'mPtr'

The generator is at fault (HaxeFoundation/haxe#13001), and HaxeFoundation/haxe#13002 fixes it, but that fix is still open and only helps future compilers. HaxeFoundation/hxcpp#1425 works around it in the runtime: under HXCPP_GC_GENERATIONAL, cpp::Function gets a static null mPtr, so the barrier current compilers emit compiles and does nothing. That's correct, since a function pointer isn't a GC object, and HX_OBJ_WB already skips a null value.

libhxcpp has the same cpp::Function. The plan is to take HaxeFoundation/hxcpp#1425 with its test: a cpp.Function field in test/haxe/TestTypes.hx, which already builds with HXCPP_GC_GENERATIONAL.