https://issues.dlang.org/show_bug.cgi?id=13683
--- Comment #5 from Nick Treleaven <[email protected]> --- static assert is an improvement over the constraint, but it still hides the actual error message which should be: Error: use `!is` instead of `!=` when comparing with `null` Removing the constraint allows the correct error to be seen. But then the constraint may be needed to disambiguate overloads. Without a compiler solution, we could have a new trait - __traits(canPass, pred, range.front) which ignores the body of pred: https://forum.dlang.org/post/[email protected] In the absence of that, the best I've come up with is: if (__traits(isTemplate, pred) || is(typeof(pred(range.front)))) Which doesn't check that pred can take that argument when pred is a lambda, but it doesn't hide the actual error unlike the constraint status quo or the static assert way. A compiler solution might not work in the presence of overloads, because currently failing constraints are only shown when there are no overloads. --
