Usually it goes like this:
`foreach(T v; lwr..upr)` -> `for(T key = low, limit = upr; key < limit; ++key) { T = key; ... }`

What this change does, is if `upr` doesn't implicitly convert to `T`, it tries `upr-1` (which will work thanks to VRP) and if it's all ok, it makes:
`foreach(T v; lwr..upr)` -> `for(typeof(upr) key = low, limit = upr; key < limit; ++key) { T = cast(T)key; ... }`

I'll put this in a comment.