import std.algorithm: reduce;
void main() {
    const data = [2, 3, 4];
    int r1 = reduce!q{a * b}(0, data); // OK
    int r2 = reduce!q{a * b}(data); // error
}


DMD 2.059head gives:

...\dmd2\src\phobos\std\algorithm.d(731): Error: can only initialize const member _field_field_0 inside constructor
...\dmd2\src\phobos\std\conv.d(3334): Error: *chunk is not mutable
...\dmd2\src\phobos\std\algorithm.d(743): Error: template instance std.conv.emplace!(const(int),const(int)) error instantiating
test.d(5):        instantiated from here: reduce!(const(int[]))
test.d(5): Error: template instance std.algorithm.reduce!("a * b").reduce!(const(int[])) error instantiating


I think  reduce!q{a*b}(data)  too should compile and work.

--------------

Extra note: in std.algorithm.reduce there is also this (now bug 2443 is fixed) at about line 723:


            // For now, just iterate using ref to avoid unnecessary copying.
            // When Bug 2443 is fixed, this may need to change.
            foreach(ref elem; r)
            {
                if(initialized)