Here is the code:

package;

class Main {
	public static function main() {

		System.init({}, function(){}); //c++ does not without this line in clean new build...

		var s = 'test';
		trace(s.substr(0, 1) + s.substr(1) + '_');
	}
}

It works on most platforms and configurations but crashes in windows when using Visual C++ 2015 with /O2 (default for Kha project) or /Ox optimizations.
It crashes somewhere inside memcpy, which is called from String + operator. Not sure which of the two because adding logging between them fixes the problem

This is generated c++

HXLINE(   6)		 ::Dynamic _hx_tmp = ::haxe::Log_obj::trace;
HXDLIN(   6)		::String _hx_tmp1 = HX_("test",52,c8,f9,4c).substr((int)0,(int)1);
HXDLIN(   6)		::String _hx_tmp2 = ((_hx_tmp1 + HX_("test",52,c8,f9,4c).substr((int)1,null())) + HX_("_",5f,00,00,00)); // crashes here
HXDLIN(   6)		_hx_tmp(_hx_tmp2,hx::SourceInfo(HX_("Main.hx",05,5c,7e,08),6,HX_("Main",59,64,2f,33),HX_("main",39,38,56,48)));

I've tried to run similar code with plain hxcpp without Kha and it worked as expected, but I'm not aware of how to change c++ optimization options with hxcpp. I'm not even sure if I built debug or release...

PS Added some logging to String class:

//added this to String::operator+
Kore::log(Kore::LogLevel::Info, "%i %i %i", inRHS.length, inRHS.__s, *((int*)(&inRHS.__s)+1));

//added this to String::substr() (result is temp variable that stores the result of substr before returning it)
Kore::log(Kore::LogLevel::Info, "%i %i", result.length, result.__s );

and the output was:

 3 50990268 - result of substr
 20315128 3 50990268 - received by operator+ (pointer moved 4 bytes backwards)

This looks like Visual C++ compiler bug.

One more: this code crashes in release build with /O2 optimization

static public var instance:ClassDescription = Type.createInstance(Type.resolveClass('typedEval.ClassDescriptionImpl'), []);

but this works fine

static public var instance:ClassDescription = {
	var cls = Type.resolveClass('typedEval.ClassDescriptionImpl');
	trace(cls)
	Type.createInstance(cls, []);
}

In C++ code that crashes looks like this

HXLINE(  19)		instance = ( ( ::typedEval::ClassDescription)	(::Type_obj::createInstance(::Type_obj::resolveClass(HX_("typedEval.ClassDescriptionImpl",0c,ad,5a,64)),::cpp::VirtualArray_obj::__new(0))) );

But if I modify it to

				auto temp = ::Type_obj::resolveClass(HX_("typedEval.ClassDescriptionImpl", 0c, ad, 5a, 64));
HXLINE(  19)		instance = ( ( ::typedEval::ClassDescription)(::Type_obj::createInstance(temp,::cpp::VirtualArray_obj::__new(0))) );

It begins to work.

I wonder why there were no such problems before. May be haxe 3.3 was generating different c++ code or may be there were different compilation options in kha? And what to do with this?