struct S

{

    int value;

​

    alias opApply = opApplyImpl!(int delegate(ref int) @system);

    int opApplyImpl(DG : int delegate(ref int))(scope DG callback)

        // int opApply(int delegate(ref int) callback)

    {

        if (auto result = callback(value)) return result;

        return 0;

    }

}

​

@safe pure nothrow @nogc unittest

{

    S xs = S(3);

    int sum = 0;

    foreach (x; xs) sum += x;

    assert(sum == 3);

}

​

version(failure1)

@safe pure nothrow @nogc unittest

{

    S xs = S(3);

    int sum = 0;

    xs.opApply((ref int x) { sum += x; return 0; });

    assert(sum == 3);

}

​

static bool b = false;  // some global state to break pure

static foreach (i; 0 .. 4)

{

    version(failure2)

    @safe pure nothrow @nogc unittest

    {

        static immutable exc = new immutable Exception("");

        S xs = S(3);

        int* p = &xs.value; // some pointer to break @safe-ty

        foreach (x; xs)

        {

            static if (i == 0) { p = new int(x); } // allocates

            static if (i == 1) { ++p; } // un-@safe

            static if (i == 2) { b = true; } // impure

            static if (i == 3) { throw exc; } // throws Exceptions

        }

    }

}

​