[PATCH] D124571: Avoid strict aliasing violation on type punning inside llvm::PointerIntPair

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

> No, wait... We can have a PunnedPointer<PointerIntPair<int*>>, and the constructor of PointerIntPair is not trivial, because it holds a PunnedPointer which does not have a trivial constructor. Back to the drawing board.

If you want to support calling getPointerAddress() in that case, then yes, I think you need to use placement new or a union.  Anything that involves "implicitly creating objects" will only work with implicit-lifetime types.

In D124571#3495535 <https://reviews.llvm.org/D124571#3495535>, @brenoguim wrote:

>> [cstring.syn] says memcpy implicitly creates objects, so I think it's effectively the same result.
>
> @efriedma Do you have  reference to that?

[cstring.syn] is a section in the standard?  (You can search for the string "cstring.dyn" in the standard/a standard draft.  Or online version: https://timsong-cpp.github.io/cppwp/cstring.syn .)


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  https://reviews.llvm.org/D124571/new/

https://reviews.llvm.org/D124571