RU007 [basic.life].8.3 Relax pointer value / aliasing rules

[wg21bot]

In many cases it is impossible to use a pointer returned from placement new or std::launder (for example in std::vector, std::variant, std::optional, std::uninitialized_+std::destroy).
Because of that issue all the standard libraries have undefined behaviors in widely used types. The only way to fix that issue is to adjust the lifetime rules to auto-launder the placement new.
Dropping the “const“ and “reference” requirement from paragraph 8 [basic.life] removes UB from std::vector and std::optional. Additional removing of the “potentially-overlapping” requirement removes UB from std::variant.

Proposed change:
Apply the following changes to the [basic.life] paragraph 8 bullet 3:
– the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and

Optionally remove the to the bullet 4 from [basic.life] paragraph 8:
– neither the original object nor the new object is a potentially-overlapping subobject ([intro.object]).

[jfbastien]

From #41 (closed as duplicate):

The restriction on automatically referring to the new object when a class type contains a non-static data member of const-qualified or reference type renders use of the pointer returned by data() of a std::vector of such class types unsafe without applying std::launder. In particular, if a pointer returned by data() on a one-element vector of a class type that is subject to the restriction is stored and then followed by a pop_back() and a call to push_back, then the stored pointer would not be usable to access the new element without applying launder. The need to apply launder is not currently indicated by the library wording and is presumably unwanted.

Proposed change:
In the bullet, strike:
• the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and
Remove, from subclause 6.6.2 [intro.object] paragraph 2, the note beginning with “[i]f the subobject contains a reference member or [ … ]” and the associated example.

Please send to EWG with SG12 guidance, if appropriate.

[jfbastien]

CWG to recommend a fix, send it back to SG12, trickle back to EWG, then back to CWG.

[jensmaurer]

CWG in Belfast: See wiki notes for RU007. Summary: Leave p8.4 alone; change p8.3 to read "the original object is neither a complete object that is const-qualified nor a subobject of such an object".

[jfbastien]

EWG on Thursday: http://wiki.edg.com/bin/view/Wg21belfast/RU007

Accept RU007 with modification. Update basic.life p8.3 to read “the original object is neither a complete object that is const-qualified nor a subobject of such an object.

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