[Issue 19904] move semantics fail through the `emplace` pipeline

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19904

--- Comment #2 from kinke at gmx.net ---
This is what I mean:

import core.stdc.stdio;

struct S
{
    this(this) { printf("postblit\n"); }
    ~this() { printf("dtor\n"); }
}

void foo(S s) { printf("rvalue\n"); }
void foo(ref S s) { printf("lvalue\n"); }

void forward(Args...)(auto ref Args args) { foo(args); }

void main()
{
    printf("directly:\n");
    foo(S());
    printf("forward:\n");
    forward(S());
}

=>

directly:
rvalue
dtor
forward:
lvalue
dtor

The rvalue-ness is lost when forwarding `auto ref` params this way (but still
detectible via isRef trait).

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