Calling static array based vararg function with a value causes extra destructor call

[s-ludwig]

The following simplified automatic reference counting implementation erroneously counts below zero. Passing a value of type S[1] to bar instead works correctly.

int counter = 0;

struct S {
    bool constructed;
    this(bool) { constructed = true; counter++; }
    this(this) { if (constructed) counter++; }
    ~this() {
        if (constructed) {
            assert(counter > 0);
            counter--;
        }
    }
}

void bar(S[1] s...) { S s2 = s[0]; }
void foo(S s) { bar(s); }
void main() { S s = S(true); foo(s); }

Output:

core.exception.AssertError@onlineapp.d(10): Assertion failure

[oCHIKIo]

I ve been testing something like this for a week now. I think the solution could be that the compiler is incorrectly adding an extra destructor hook on the caller side when packing the varargs. since static arrays are passed by value, the callee already handles the cleanup. we probably just need to restrict that hook to dynamic arrays only and make sure the items are properly copied.

[ibuclaw]

There's already an open issue to remove callee destruction from codegen.

#20570