[Bug c++/106613] New: GCC rejects valid program involving std::invariant saying incomplete type

[Bug c++/106613] New: GCC rejects valid program involving std::invariant saying incomplete type

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106613

            Bug ID: 106613
           Summary: GCC rejects valid program involving std::invariant
                    saying incomplete type
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following valid(afaik) program is rejected by gcc.
https://godbolt.org/z/e73Yf4s47

```
#include <variant>

struct LiteralExpr
{
    int value;
};

template <typename T>
struct BaseSpace
{
    struct AddExpr;
    struct AddExprBox
    {
        const AddExpr &_impl;

    public:
        AddExprBox(const AddExpr &obj) {}
    };
    using Expr = std::variant<AddExprBox, LiteralExpr>;
    struct AddExpr
    {
        std::variant<AddExprBox, LiteralExpr> lhs;  // error:
'BaseSpace<T>::AddExpr::lhs' has incomplete type
    };
};

auto main() -> int
{
    auto expr = BaseSpace<int>::Expr(LiteralExpr{2});
}
```

The error says:

```
error: 'BaseSpace<T>::AddExpr::lhs' has incomplete type
   22 |         std::variant<AddExprBox, LiteralExpr> lhs;
      |                                               ^~~
In file included from <source>:1:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/variant:1336:11: note:
declaration of 'class std::variant<BaseSpace<int>::AddExprBox, LiteralExpr>'
 1336 |     class variant
      |           ^~~~~~~
```

[Bug c++/106613] GCC rejects valid program involving std::invariant saying incomplete type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106613

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang rejects it with the same error message.

Both GCC and clang are correct because the inner types are not complete until
after the outer type is complete.

[Bug c++/106613] GCC rejects valid program involving std::invariant saying incomplete type

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106613

--- Comment #2 from Jason Liam <jlame646 at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> clang rejects it with the same error message.
> 
> Both GCC and clang are correct because the inner types are not complete
> until after the outer type is complete.

I don't think what you said about " inner types are not complete until after
the outer type is complete" is true.

In particular, the following program is well-formed. Note in the below program
the nested type named `A` is complete when its `};` is encountered and not when
the containing class `C`'s `};` is encountered as opposed to your claim.

```
struct C 
{
    struct A
    {

    };
    static constexpr A a{}; //this is fine because A is complete here as
opposed to Andrew's claim

};
```

[Bug c++/106613] GCC rejects valid program involving std::invariant saying incomplete type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106613

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |DUPLICATE

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 96645.

*** This bug has been marked as a duplicate of bug 96645 ***

[Bug c++/106613] GCC rejects valid program involving std::invariant saying incomplete type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106613

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=102199

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also see PR 102199.

Also see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88165#c10 (which
references when the inner class type is complete).

This is one of the dark corners of C++ which have issues with many things.