import std.stdio;

struct S(alias Fun){
// public:	// uncomment to break compilation
	int x;
	void f() {
		writefln("%s, %d", Fun()? "yes":"no", x);
	}
}

void f1(int a, int b){
	auto s = S!({ return a < b; })(3);
	s.f();
}

void main(){
	f1(10, 20);
}
This compiles and works as expected.
But as soon as I added any access qualifier (even one redundant 'public:') to structure S, compilation breaks with message:

bug001.d(9): Error: function bug01.f1.S!(delegate nothrow bool()

{

return a < b;

}

).S.f cannot get frame pointer to __dgliteral1