[Bug c++/104512] New: [c++20] consteval constructor does not need to initialize all data members

[Bug c++/104512] New: [c++20] consteval constructor does not need to initialize all data members

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104512

            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, which
does not initialize i field. Then this constructor is used to make a temporary
and its i field remains untouched:
```
struct B {
    bool b = 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 incompletely
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. Related
discussion: https://stackoverflow.com/a/71085832/7325599

[Bug c++/104512] [c++20] consteval constructor does not need to initialize all data members

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104512

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
constexpr works but consteval does not.

Also I notice ICC rejects the consteval version but accepts the constexpr one.

[Bug c++/104512] [c++20] consteval constructor does not need to initialize all data members

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104512

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://github.com/llvm/llv
                   |                            |m-project/issues/57627,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=106485

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
```
struct B {
    bool b = true;
    int i;
    consteval B() {}
};
static_assert( []() consteval{ return B{}.b; }() );
```
Works for GCC. I think this is a bug in clang ...
static_assert is an immediate function context so B{} needs to be a evaluated
and such.
See https://github.com/llvm/llvm-project/issues/57627 for the clang bug.

See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106485#c2 .

[Bug c++/104512] [c++20] consteval constructor does not need to initialize all data members

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104512

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> static_assert is an immediate function context
static_assert is NOT an immediate function context sorry for the typo.

[Bug c++/104512] [c++20] consteval constructor does not need to initialize all data members

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104512

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> constexpr works but consteval does not.
> 
> Also I notice ICC rejects the consteval version but accepts the constexpr
> one.

This gives the hint that both MSVC and clang are incorrect really. EDG also
implements correctly static_assert not being an immediate function context.

So closing as invalid.