[Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[iamsupermouse at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

            Bug ID: 111771
           Summary: Incorrect "is used uninitialized" warning, as if
                    zero-initialization didn't propagate through
                    user-provided default constructors
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iamsupermouse at mail dot ru
  Target Milestone: ---

Here's the code. GCC with `-Wall -O3` warns that `x` is used uninitialized,
even though it's zeroed.

    struct A
    {
        A() {}
        int x;
    };

    struct B : A {};

    int main()
    {
        B b = B();
        return b.x;
    }

Since `B` doesn't have a user-provided default constructor, value-initializing
it like this performs zero-initialization, which propagates recursively
(http://eel.is/c++draft/dcl.dcl#dcl.init.general-6.2) over all members, zeroing
everything. Yet GCC incorrectly warns about `x` being uninitialized.

[Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Did you miss that the implicit B constructor will just call A's constructor ?

[Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[iamsupermouse at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

--- 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 standard
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 zeroes
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 calls
A's constructor.

[Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-10-12
           Keywords|                            |diagnostic, wrong-code
             Status|UNCONFIRMED                 |NEW
                 CC|                            |jason at gcc dot gnu.org
      Known to fail|                            |13.2.1, 7.5.0

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
We have

int main ()
{
  int D.2848;

  {
    struct B b;

    try
      {
        b.D.2778.x = 0;
        B::B (&b);
        D.2848 = b.D.2778.x;
        return D.2848;

but:

void B::B (struct B * const this)
{
  _1 = &this->D.2778;
  A::A (_1);
}


void A::A (struct A * const this)
{
  *this = {CLOBBER};

so A::A invoked by B::B invalidates the initialized storage.  Maybe a
"different" B::B should have been called (one not invoking A::A?).

[Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
-fno-lifetime-dse fixes the issue (and the diagnostic)

[Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 108993.

*** This bug has been marked as a duplicate of bug 108993 ***