I have the rather strange question: Why does this work?

    struct S
    {
        int value0;

        alias opApply = opApplyImpl!(int delegate(ref int)); //1
        int opApplyImpl(DG : int delegate(ref int))(scope DG callback) //2
        {
            if (auto result = callback(value0)) return result;
            return 0;
        }
    }

    @safe pure nothrow @nogc unittest
    {
        S xs = S(3);
        int sum = 0;
        foreach (x; xs) sum += x;
        assert(sum == 3);
    }

But replacing Lines //1 and //2 with

    int opApply(int delegate(ref int) callback)

(replacing the alias to a template instance by equivalent(?) non-template code) does not. As far as I know, template instantiation should make them exactly the same thing.

The spec does not say anything to this.

Even after replacing //1 by
    alias opApply = opApplyImpl!(int delegate(ref int) @system); //1
i.e. marking the callback @system explicitly,
the @safe unittest will still compile!

Using
    xs.opApply((ref int x) { sum += x; return 0; });
in the unittest directly makes it fail. The compiler claims as expected that the template instance is not satisfy the conditions for either attribute on the unittest.

But somehow, it does respect attributes. An un-@safe unittest like this

    static bool b = false;  // some global state to break pure
    static foreach (i; 0 .. 4)
    {
        version(failure)
        @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
            }
        }
    }

actually shows that any single attribute can be failed, suggesting it is the same for all.

Is there some attribute magic in the compiler's opApply rewriting? It should be mentioned.

I've filed a bug report [1] some time ago. I'm posting here, because I want to understand what's going on here.

A runnable example is here: https://run.dlang.io/is/KQ0tmL

[1] https://issues.dlang.org/show_bug.cgi?id=19706

On Tuesday, 14 May 2019 at 14:36:21 UTC, Q. Schroll wrote:
> [snip]
> I've filed a bug report [1] some time ago. I'm posting here, because I want to understand what's going on here.

I read through this post and while I don't know precisely what the compiler is thinking, it seems for me that your reasoning that this is a bug is correct.

I quess that either the compiler instantiates opApplyImpl with something else than alias opApply specifies (it shouldn't) or (more likely) template attribute inference malfunctions.