std.conv.roundTo does not accept const/immutable/shared for its Source.
These attributes won't pass static asserts in std.conv.roundTo.
e.g.:

immutable real a = -2.3;
int b = roundTo!(int)(a);  // failure
int c = cast(int)std.math.round(a);  // success

It seems that static asserts in std.conv.roundTo should be replaced by std.traits.isIntegral and std.traits.isFloatingPoint.

p.s.
I suppose roundTo is able to be pure function.