I am not sure what's the cause of this problem.


import std.algorithm, std.typecons;
alias Tuple!int T;
void main() {
    const T del = T(1);
    T[] items1 = [T(0), T(1), T(2)];
    auto r1 = filter!(x => x != del)(items1); // OK
    auto r2 = filter!(x => del != x)(items1); // error
    const(T[]) items2 = [T(0), T(1), T(2)];
    auto r3 = filter!(x => x != del)(items2); // error
}



test.d(7): Error: function std.typecons.Tuple!(int).Tuple.opEquals!(Tuple!(int)).opEquals (Tuple!(int) rhs) is not callable using argument types (Tuple!(int)) const
...\dmd2\src\phobos\std\algorithm.d(1089): Error: template instance test.main.__lambda4!(Tuple!(int)) error instantiating
test.d(7):        instantiated from here: filter!(Tuple!(int)[])
test.d(7): Error: template instance test.main.filter!(__lambda4).filter!(Tuple!(int)[]) error instantiating
test.d(9): Error: function std.typecons.Tuple!(int).Tuple.opEquals!(const(Tuple!(int))).opEquals (const(Tuple!(int)) rhs) is not callable using argument types (const(Tuple!(int))) const
...\dmd2\src\phobos\std\algorithm.d(1089): Error: template instance test.main.__lambda6!(const(Tuple!(int))) error instantiating
test.d(9):        instantiated from here: filter!(const(Tuple!(int))[])
test.d(9): Error: template instance test.main.filter!(__lambda6).filter!(const(Tuple!(int))[]) error instantiating


If  x => x != del  works then I think  x => del != x  too should work.
The third case is a filter done on a const(Tuple!(int)[]), I think it's legit code.