Bug 13571 – Overload of std.range.tee which accepts a functions does not accept structs or classes with opCall

The following code does not work currently with tee: struct Callable { void opCall(int n) { writeln(n); } } void main() { auto ints = [1, 2, 3]; //Error auto res = ints.tee!Callable; } Note that the following code *does* work: struct Callable { void opCall(int n) { writeln(n); } } void main() { Callable callable; auto ints = [1, 2, 3]; auto r = ints.tee(callable); foreach (_; r) {} //Prints 1 2 3 } So I don't know how useful adding support for doing `tee!Callable` would be.

I tried adding support for opCall to tee, and I ran into some problems. As you can just pass the struct/object with opCall to the overload of tee that takes an output range as a function argument, the extra complexity that's necessary to support taking structs/objects with opCall as a template argument is not worth it IMO. Therefore, I'm going to close this.

Probably the simplest way to implement this would be to make `tee` use `unaryFun`, and, if necessary, add `opCall` support to `unaryFun` instead.