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

[d-bugmail at puremagic.com]

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

--- Comment #3 from kinke at gmx.net ---
Ah, due to this rval->lval conversion when calling an `auto ref` function, the
ctor is called with an lvalue arg, even if the emplace-arg was originally an
rvalue, and that then leads to a superfluous copy (and later destruction)
instead of a move at that end of the chain.

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