stonemaster/phobos — comment on commit 9920a832272188cd24b948fc4dd7fce77866dd39
File: std/variant.d line 1534
By blackwhale:

For 2.
the best thing I know is __traits(compiles, (){
T dummy=void;
visitor(dummy);
});

But anyway you are right, I was looking at the diff only and was just wondering what a template without constraints is doing here.