Following code does not compile, 

import std.algorithm;
void main()
{
    auto r1 = sort!("a<b", SwapStrategy.unstable)([1,3,2,4]);
    assert(equal(r1, [1,2,3,4]));

    auto r2 = sort!("a<b", SwapStrategy.semistable)([1,3,2,4]);  // Line 7
    assert(equal(r2, [1,2,3,4]));

    auto r3 = sort!("a<b", SwapStrategy.stable)([1,3,2,4]);
    assert(equal(r3, [1,2,3,4]));
}

Output:
C:\dmd2\src\phobos\std\algorithm.d(7001): Error: static assert  (cast(SwapStrategy)1 != cast(SwapStrategy)1) is false
C:\dmd2\src\phobos\std\algorithm.d(6710):        instantiated from here: sortImpl!(result,ss,int[])
test.d(7):        instantiated from here: sort!("a<b",cast(SwapStrategy)1,int[])

Semi-stable sort is stable sort.
So if stable sort is supported, we can implement it with stable sort, at least.