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From: "fchelnokov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/104512] New: [c++20] consteval constructor does not need to
 initialize all data members
Date: Sat, 12 Feb 2022 09:52:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104512

            Bug ID: 104512
           Summary: [c++20] consteval constructor does not need to
                    initialize all data members
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

In the next program struct B has immediate consteval default constructor, w=
hich
does not initialize i field. Then this constructor is used to make a tempor=
ary
and its i field remains untouched:
```
struct B {
    bool b =3D true;
    int i;
    consteval B() {}
};
static_assert( B{}.b );
```
Clang and MSVC are fine with it. But GCC complains:
 error: 'B{true}' is not a constant expression
    7 | static_assert( B{}.b );
      |                  ^
error: 'B()' is not a constant expression because it refers to an incomplet=
ely
initialized variable
Demo: https://gcc.godbolt.org/z/x4n6ezrhT

The requirement that "every non-variant non-static data member must be
initialized" in constexpr/consteval constructor is removed since c++20. Rel=
ated
discussion: https://stackoverflow.com/a/71085832/7325599=