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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/96765] Base class constructor cast to derived should cause
 a warning
Date: Wed, 04 May 2022 16:11:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96765

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
Hmm, is the cast undefined, actually?  I thought you were right, but now ca=
n't
find any wording to that effect.  I see

[expr.static.cast] If the prvalue of type =E2=80=9Cpointer to cv1 B=E2=80=
=9D points to a B that
is actually a base class subobject of an object of type D, the resulting
pointer points to the enclosing object of type D. Otherwise, the behavior is
undefined.

But this B is a base of a D.  Then

[class.cdtor] For an object with a non-trivial constructor, referring to any
non-static member or base class of the object before the constructor begins
execution results in undefined behavior.

but D() has begun execution.  Then

[class.cdtor] During the construction of an object, if the value of the obj=
ect
or any of its subobjects is accessed through a glvalue that is not obtained,
directly or indirectly, from the constructor=E2=80=99s this pointer, the va=
lue of the
object or subobject thus obtained is unspecified.

but 'this' in the Base ctor seems to count as obtained indirectly from 'thi=
s'
in the Derived ctor.

Then [class.cdtor] paragraph 4 talks about virtual functions, which this is
not.

What rule makes this undefined?=