CWG3049 Implicitly deleted move operation should not disable trivial relocation

[jensmaurer]

Some trivially copyable types turn out not to be trivially relocatable, per the current Working Draft.

EWG to confirm design intent here.

See CWG3049 for details.

[phalpern]

Some background:

The revision history listing R7 changes states:

• Simplified presentation and discussion of trivial relocatability (for detailed history, consult [P2786R6])

(Aside: Never again will I allow a paper I work on to strip exposition from later versions of a paper)

The second paragraph of the Introduction of P2786R6 says, clearly, "Any trivially copyable type is trivially relocatable by default." This principle was taken for granted from the very early days of discussion --- so much so that very little text needed to be written about it. It is something that P2786 and P1144 had in common, and, unfortunately, the bulk of EWG discussion revolved around the differences between P2786 and P1144. Although trivially copyable types are not always trivially relocatable, they should be by default, unless an explicit user-provided operation forbids it.

Moreover, the first paragraph of section 5.3 of the approved paper says:

A class will be implicitly trivially relocatable if all its bases and members are trivially relocatable and none of its eligible special member functions are user provided; a contextual keyword will signify that a class might still be trivially relocatable even if it has user-provided special member functions.

I think the intention is pretty clear that a class should implicitly be trivially relocatable unless it contains a subpart that is not trivially relocatable or contains a user-declared special member that would disable it.

The undesirable behavior in the status quo was inadvertently included in P2786R7, via cutting
and pasting, when replaceable was added. The error was compounded during
wording review when the cut-and-pasted text was unified into the default
movable term in P2786R13. Note that the suggested resolution to CWG3049 does not change the meaning of replaceable.