Why is sort allocating in this case?

[Jack Stouffer via Digitalmars-d-learn]

The docs explicitly say that SwapStrategy.unstable is 
non-allocating, but this code (which is for finding the 
statistical mode of a range) will fail to compile.

auto mode(alias pred = "a == b", R)(R r) @nogc
     if (is(ElementType!R : real) &&
         isInputRange!R &&
         !isInfinite!R)
{
     import core.stdc.stdlib : malloc;

     import std.algorithm.iteration : group;
     import std.algorithm.sorting : sort, SwapStrategy;
     import std.algorithm.mutation : copy;
     import std.typecons : Tuple;

     alias LT = Tuple!(Unqual!(ElementType!R), size_t);

     if (r.empty)
     {
         return real.nan;
     }

     auto grouping = r.group!pred;

     // Because the struct Group does not have swappable elements, 
it cannot be
     // sorted, so copy it to another array
     auto buffer = (cast(LT*) malloc(r.length * LT.sizeof))[0 .. 
r.length];
     copy(grouping, buffer);

     sort!("a[1] > b[1]", SwapStrategy.unstable)(buffer);

     return buffer[0][0];
}


$ dmd/src/dmd -unittest test.d
test.d(439): Error: @nogc function 'test.mode!("a == b", 
int[]).mode' cannot call non- at nogc function 
'std.algorithm.sorting.sort!("a[1] > b[1]", cast(SwapStrategy)0, 
Tuple!(int, ulong)[]).sort'