Binding rvalues to ref parameters redux

[Andrei Alexandrescu]

On 3/27/19 12:33 AM, Nicholas Wilson wrote:
> It would be far better to describe this through lambda lowering.

One important realization working on this was that insertion of 
constructors and destructors cannot be realized via lambda lowering 
because lifetimes don't follow some natural scoping. For example, the 
temporaries created on the left-hand side of && survive through the end 
of the full expression, whereas those created on the right-hand side do 
not. The "?:" operator is a hot mess including additional hidden state 
variables, conditional destruction thus breaking the language rules etc. 
I'll make sure the document mentions why lowering is not the appropriate 
mechanism to describe insertion of constructors and destructors.

One other thing is that these temporary construction and destruction 
rules preexisted. In fact probably the nicest thing about the DIP is 
that no new rules are introduced - temporaries are created and destroyed 
exactly as if there was no "ref" for the parameter. That way we could 
migrate some of the DIP into the spec, thus simplifying the DIP. I've 
already done that with the definitions of "full expression", "lvalue", 
and "rvalue".