https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101631
Bug ID: 101631
Summary: gcc permits object reference to object outside of its
lifetime during constant evaluation
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: fsb4000 at yandex dot ru
Target Milestone: ---
Hi.
The bug has come up during the implementation of SSO for constexpr std::string
which can be found here:
https://github.com/microsoft/STL/pull/1735#discussion_r674285711
The issue stems from an obscure
rule(https://eel.is/c++draft/class.union.general#6) that allows switching the
active member of a union by assigning to an element of the array member.
(Obligatory godbolt: https://godbolt.org/z/6qG7v9eYx)
While the usage on line 12 is indeed correct, for std::string we need to go
through char_traits which is imitated by perform_assignment. However, the
lifetime of the subobject buf[5] only starts right before the assignment inside
of perform_assignment, so on line 25 we form an object reference to an object
outside of its lifetime which is ill formed.
gcc should emit an appropriate diagnostic.
Found by: Michael Schellenberger Costa
I hope it helps.
