assumeSorted can't access private function when compiling with -debug

[FreeSlave via Digitalmars-d-learn]

Here's code:

private {
    import std.algorithm;
    import std.range;
    import std.typecons;
    alias Tuple!(int, string) Data;
}

private bool myCmp(Data a, Data b) {
    return a[0] < b[0];
}

auto bar() {
    return [Data(1, "one"), Data(2, "two")].assumeSorted!myCmp;
}

void main()
{
    bar();
}

Build it with -debug and without. It gives me error when building 
with -debug.

/usr/include/dmd/phobos/std/algorithm/sorting.d(154): Error: 
function sorttest.myCmp is not accessible from module sorting

Is it bug?