import std.algorithm, std.typecons;
int add1(int x, int y) {
    return x + y;
}
alias Tuple!int T;
T add2(T x, T y) {
    return T(x[0] + y[0]);
}
void main() {
    T[] items1 = [T(0), T(1)];
    const t0 = T(0);
    immutable r1 = reduce!add2(t0, items1); // OK
    const(int[]) items2 = [0, 1];
    const int zero = 0;
    immutable r2 = reduce!add1(zero, items2); // OK
    const(T[]) items3 = items1.idup;
    immutable r3 = reduce!add2(t0, items3); // Error
}



DMD 2.059head gives:

...\dmd2\src\phobos\std\typecons.d(399): Error: template std.typecons.Tuple!(int).Tuple.opAssign(R) if (isTuple!(R) && allSatisfy!(isIdentityAssignable,Types)) does not match any function template declaration
...\dmd2\src\phobos\std\typecons.d(399): Error: template std.typecons.Tuple!(int).Tuple.opAssign(R) if (isTuple!(R) && allSatisfy!(isIdentityAssignable,Types)) cannot deduce template function from argument types !()(const(int))
...\dmd2\src\phobos\std\algorithm.d(702): Error: template instance std.typecons.Tuple!(Tuple!(int)).Tuple.__ctor!(const(Tuple!(int))) error instantiating
test.d(17):        instantiated from here: reduce!(const(Tuple!(int)),const(Tuple!(int)[]))
test.d(17): Error: template instance std.algorithm.reduce!(add2).reduce!(const(Tuple!(int)),const(Tuple!(int)[])) error instantiating


I think just as reduce works on an const(int[]) it should also work on an const(Tuple!(int)[]).