[Bug c++/110619] Dangling pointer returned from constexpr function converts in nullptr

[peter at cordes dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110619

Peter Cordes <peter at cordes dot ca> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |peter at cordes dot ca

--- Comment #7 from Peter Cordes <peter at cordes dot ca> ---
(In reply to Andrew Pinski from comment #2)
> >but it is not nullptr.
> 
> Or is it just undefined so it could be considered a nullptr ...


Implementation-defined behaviour, according to answers on
https://stackoverflow.com/questions/76843246/why-does-the-address-of-an-out-of-scope-variable-equal-zero-with-constexpr


https://eel.is/c++draft/basic.compound#def:value,invalid_pointer

https://eel.is/c++draft/basic.stc.general#4

>  Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior.
> **Any other use of an invalid pointer value has implementation-defined behavior.**

So this wasn't a bug, but the new behaviour is also allowed.

This commit could be reverted or kept, depending on maintainability and/or
quality-of-life for users of GCC.  Having it pick the other
implementation-defined behaviour from clang (GCC's previous behaviour) is maybe
a *good* thing, to help programmers catch dependence on an invalid pointer
being either null or non-null if they try their code with both compilers.