[Bug c++/98885] New: [modules] forward declaration of classes prevent them from being exported at the point of actual declaration

[pilarlatiesa at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98885

            Bug ID: 98885
           Summary: [modules] forward declaration of classes prevent them
                    from being exported at the point of actual declaration
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pilarlatiesa at gmail dot com
  Target Milestone: ---

$ /home/pililatiesa/GCC-11/bin/g++ -v

Using built-in specs.
COLLECT_GCC=/home/pililatiesa/GCC-11/bin/g++
COLLECT_LTO_WRAPPER=/home/pililatiesa/GCC-11/bin/../libexec/gcc/x86_64-pc-linux-gnu/11.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-master/configure --prefix=/home/pililatiesa/GCC-11
--enable-languages=c++ --disable-bootstrap
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.0 20210129 (experimental) (GCC)


$ cat A.cpp

export module A;

class A; // forward declaration

export class A {}; // actual declaration


$ cat main.cpp

import A;

int main()
{
  A a;

  return 0;
}


$ /home/pililatiesa/GCC-11/bin/g++ -fmodules-ts A.cpp main.cpp -o foo
main.cpp: In function ‘int main()’:
main.cpp:6:3: error: ‘A’ was not declared in this scope
    6 |   A a;
      |   ^


It can be worked around by typing "export" in the forward declaration as well.

Not sure this is a bug, but it that's the specified behaviour, I’d say it’s
very counter-intuitive. For example, this won’t work:

$ cat A.cpp

export module A;

class B;

export
class A
{
  void
  f(B const &) const;
};

$ cat B.cpp

export module B;

class A;

export
class B
{
  void
  f(A const &) const;
};

$ cat A-impl.cpp

module A;

import B;

void
A::f(B const &) const {}

$ cat B-impl.cpp

module B;

import A;

void
B::f(A const &) const {}