Test case:

struct A1
{
    int x;
    ref int opIndex() { return x; }
    ref int opSlice() { assert(0); }
}
void main()
{
    A1 a = A1(1);

    auto x = a[];       // a.opIndex(), OK
    assert(x == a.x);

    // When a.opIndexUnary!"-" is not found,
    // it should be rewritten to: -a.opIndex() rather than -a.opSlice()
    auto y = -a[];      // asserts in opSlice(), NG
    assert(y == -a.x);

    // When a.opIndexAssign(int) is not found,
    // it should be rewritten to: a.opIndex() = 1; rather than a.opSlice() = 1;
    a[] = 1;            // asserts in opSlice(), NG
    assert(a.x == 1);

    // When a.opIndexOpAssign!"+"(int) is not found,
    // it should be rewritten to: a.opIndex() += 1; rather than a.opSlice() += 1;
    a[] += 1;           // asserts in opSlice(), NG
    assert(a.x == 2);
}

I caught the issue in the d.learn forum thread that was posted a half year ago:
http://forum.dlang.org/post/luadir$t0g$1@digitalmars.com