[Issue 17953] inout-like mechanism to infer function attributes from callback attributes

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17953

Bolpat <qs.il.paperinik at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |qs.il.paperinik at gmail.com

--- Comment #3 from Bolpat <qs.il.paperinik at gmail.com> ---
This can be done using templates:
```d
int f(DG)(scope DG callback)
{
    return callback("Hello, World");
}

void main() @safe
{
    import std.stdio;
    f((string str) { writeln(str); return 0; });
}
```

For opApply, for some reason, this works:
```d
struct S
{
    private int opApplyImpl(DG)(scope DG dg)
    {
        static if (is(DG : int delegate(string)))
                return dg("Hello, World!");
                else
            return dg(0, "Hello, World!");
    }
    alias opApply = opApplyImpl!(int delegate(string));
    alias opApply = opApplyImpl!(int delegate(size_t, string));
}

void main() @safe
{
    import std.stdio;
    foreach (s; S())
    {
        static assert(is(typeof(s) == string));
        writeln(s);
    }
    foreach (i, s; S())
    {
        static assert(is(typeof(i) == size_t));
        static assert(is(typeof(s) == string));
        writeln(i, " ", s);
    }
}
```

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