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From: "iamsupermouse at mail dot ru" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111771] Incorrect "is used uninitialized" warning, as if
 zero-initialization didn't propagate through user-provided default
 constructors
Date: Thu, 12 Oct 2023 05:17:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111771

--- Comment #2 from Egor <iamsupermouse at mail dot ru> ---
Before calling A's constructor, it will zero `x` anyway.

I was also surprised when I learned this yesterday, but it's what the stand=
ard
says.

1. `()` performs value-initialization on B:
http://eel.is/c++draft/dcl.dcl#dcl.init.general-16.4

2. Since B's ctor is not user-provided, that resolves to zero-initialization
followed by default-initialization:
http://eel.is/c++draft/dcl.dcl#dcl.init.general-9.1.2

3. Zero-initialization of B propagates to A, then propagates to `x` and zer=
oes
it, regardless of A having a user-provided constructor or not:
http://eel.is/c++draft/dcl.dcl#dcl.init.general-6.2

4. Lastly default-initialization of B calls B's constructor and in turn cal=
ls
A's constructor.=