ElementType doesn't work with a class/struct that defines an opApply, this prints 'void' (dmd 2.043):


import std.stdio: writeln;
import std.range: ElementType;

struct Foo {
    char stop;
    int opApply(int delegate(ref long) dg) {
        int result;
        for (long i = 0; i < stop; i++) {
            result = dg(i);
            if (result)
                break;
        }
        return result;
    }
}

void main() {
    writeln(typeid(ElementType!Foo)); // Output: void
}



A naive way to find the type given by the opApply:

template IterType(alias iterable) {
    alias ReturnType!({ foreach(x; iterable)
                            return x;
                        assert(0);
                      }) IterType;
}


A more refined way (this template is named BaseType1, because it goes down just one level):

...
static if ( is(typeof(T.opApply)) )
        alias OpApplyType!(T) BaseType1;
...


Where:

template OpApplyType(T) {
    static if (ParameterTypeTuple!(ParameterTypeTuple!(T.opApply)[0]).length == 1)
        alias ParameterTypeTuple!(ParameterTypeTuple!(T.opApply)[0])[0] OpApplyType;
    else
        alias ParameterTypeTuple!(ParameterTypeTuple!(T.opApply)[0]) OpApplyType;
}