[Bug c++/97128] New: Uninitialized members of base class wrongly allowed in constexpr constructor

[Bug c++/97128] New: Uninitialized members of base class wrongly allowed in constexpr constructor

[feodor.alexeev+gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97128

            Bug ID: 97128
           Summary: Uninitialized members of base class wrongly allowed in
                    constexpr constructor
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: feodor.alexeev+gcc at gmail dot com
  Target Milestone: ---

Given this code: (https://godbolt.org/z/qsaeM7)

struct Base {
    int x;
};

struct Derived : Base {
    constexpr Derived() { }
};

int main () {
    constexpr Derived kDerived;
    return kDerived.x;
}

g++ -std=c++17 -pedantic compiles the program that exits with code 0.
clang rejects it as I believe in c++17 for a constructor to be constexpr all
members must be initialized before execution enters the body of the
constructor.

I also believe that the program is invalid as of c++20 as all must be
initialized by the end of the constexpr constructor. Still g++ -std=c++2a
compiles it.

Note that if there was an uninitialized member of Derived itself, g++ would
generate an error as expected.

There is a similar bug filed where g++ wrongly allows uninitialized member of
anonymous struct inside a union member:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86581 . Not sure if this counts as
a duplicate.

[Bug c++/97128] Uninitialized members of base class wrongly allowed in constexpr constructor

[feodor.alexeev+gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97128

--- Comment #1 from Fedor Alekseev <feodor.alexeev+gcc at gmail dot com> ---
Sorry, the link in the initial comment led to a slightly less minimal example.
Here's a link to the code from the initial comment in CE:
https://godbolt.org/z/dWP1sh

[Bug c++/97128] Uninitialized members of base class wrongly allowed in constexpr constructor in c++17 mode

[feodor.alexeev+gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97128

--- Comment #2 from Fedor Alekseev <feodor.alexeev+gcc at gmail dot com> ---
Also my initial claim about c++20 was wrong, gcc 10+ generates the error when
provided with -std=c++2a switch.
Still there is a problem in c++17 mode.

[Bug c++/97128] Uninitialized members of base class wrongly allowed in constexpr constructor in c++17 mode

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97128

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-04-15
     Ever confirmed|0                           |1
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=99700

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  If it's a regression I can't tell when it happened.  See also
pr99700 for a similar test case in C++ 20 mode.