[Bug c++/97569] New: Declaring a struct in a field declaration of another struct.

[Bug c++/97569] New: Declaring a struct in a field declaration of another struct.

[anders.granlund.0 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97569

            Bug ID: 97569
           Summary: Declaring a struct in a field declaration of another
                    struct.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: anders.granlund.0 at gmail dot com
  Target Milestone: ---

Consider the following c++ program:

  int main()
  {
      struct A
      {
        struct B *b;
      };

      using U = B;   
  }

Compile it with "-std=c++20 -pedantic-errors".

The gcc compiler accepts it.

Clang however rejects it with the following error 
message: "unknown type name 'B': using U = B;".

The interesting thing is that if we replace  struct S  with  struct S {}  both
compilers agree on rejecting the program.

I'm not sure if this is a bug in gcc or clang, but I think this is a gcc bug.

[Bug c++/97569] Declaring a struct in a field declaration of another struct. gcc and clang difference.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97569

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Anders Granlund from comment #0)
> The interesting thing is that if we replace  struct S  with  struct S {} 
> both compilers agree on rejecting the program.

I don't see any struct S in the example program.

[Bug c++/97569] Declaring a struct in a field declaration of another struct. gcc and clang difference.

[anders.granlund.0 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97569

--- Comment #2 from Anders Granlund <anders.granlund.0 at gmail dot com> ---
(In reply to Jonathan Wakely from comment #1)
> (In reply to Anders Granlund from comment #0)
> > The interesting thing is that if we replace  struct S  with  struct S {} 
> > both compilers agree on rejecting the program.
> 
> I don't see any struct S in the example program.

struct B  it should be.

[Bug c++/97569] Declaring a struct in a field declaration of another struct. gcc and clang difference.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97569

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Ah right, so

  int main()
  {
      struct A
      {
        struct B *b;
        struct C {} *c;
      };

      using U = B;
      using V = C;   
  }

For the `struct C {}` case that explicitly defines (and declares) a new struct
in the scope of A, i.e. A::C, and so it's definitely correct that it isn't
declared in the enclosing block scope. That case is substantially different,
and all compilers agree on that.

For `struct B *b` the standard says that implicitly declares a new type, but
not as a member of A. [basic.scope.pdecl] p7 (7.2) says that the the identifier
B "is declared in the smallest namespace or block scope that contains the
declaration." And that's the block scope of main. So I think GCC is correct,
and Clang is wrong. EDG agrees with GCC FWIW.

[Bug c++/97569] Declaring a struct in a field declaration of another struct. gcc and clang difference.

[anders.granlund.0 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97569

Anders Granlund <anders.granlund.0 at gmail dot com> changed:

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

--- Comment #4 from Anders Granlund <anders.granlund.0 at gmail dot com> ---
Thanks for the explanation. I will close this bug report and report to bug to
clang instead.