inout auto ref escaping a reference to parameter, only errors with vibe Json type, or if inout is there.

[aliak via Digitalmars-d-learn]

Hi,

I have an error that says "Error: returning `match1(opt)` escapes 
a reference to parameter `opt`, perhaps annotate with `return`".

The code is here: https://run.dlang.io/is/ESZDW4 (It's copied at 
the end of this post as well)

1) If you remove the inout from line 11. It works.
2) If you *do not* call match2 and call match1 instead, it also 
works.
3) If you return something other than vibe's Json type, it works 
(afaict)

Am I using inout wrong? Why is it only happen with Json type so 
far. If I return any other random struct it works fine. And why 
does it work if I don't go through the "match2" template? Any 
help would be much appreciated.

---

template match1(handlers...) {
    auto ref match1(T)(inout auto ref Optional!T opt) { // remove 
inout, it works
        if (opt.empty) {
            return  handlers[1]();
        } else {
            return  handlers[0](opt.front);
        }
    }
}

template match2(handlers...) {
    auto match2(T)(auto ref Optional!T opt) {
        return match1!handlers(opt);
    }
}

void main() {
    some(1)
        .match2!( // use match1, it works
            (int i) {return Json(1);}, // return anything else, 
it works
            () {return Json(1);}
        )
        .writeln;
}

Cheers,
- Ali