[Bug c++/102028] New: std::ostream& used as std::stringstream&

[Bug c++/102028] New: std::ostream& used as std::stringstream&

[lars.maier at tefax dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102028

            Bug ID: 102028
           Summary: std::ostream& used as std::stringstream&
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lars.maier at tefax dot net
  Target Milestone: ---

The following code compiles, although it shouldn't:
```
#include <sstream>
#include <iostream>

void foo(std::stringstream const&);

struct A {
    friend auto operator<<(std::ostream& os, A const&) -> std::ostream&;
};


int main() {
    foo(std::stringstream{} << A{});
    return 0;
}
```
Reason: the result of operator<< with std::stringstream and A const& is just an
std::ostream& and you should not be able to call foo on such an reference.

[Bug c++/102028] std::ostream& used as std::stringstream&

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102028

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is the template which gets matched:
  /**
   *  @brief  Generic inserter for rvalue stream
   *  @param  __os  An input stream.
   *  @param  __x  A reference to the object being inserted.
   *  @return  __os
   *
   *  This is just a forwarding function to allow insertion to
   *  rvalue streams since they won't bind to the inserter functions
   *  that take an lvalue reference.
  */
  template<typename _Ostream, typename _Tp>
    inline __rvalue_stream_insertion_t<_Ostream, _Tp>
    operator<<(_Ostream&& __os, const _Tp& __x)
    {
      __os << __x;
      return std::move(__os);
    }

That is the code is same as:
    std::stringstream &&a =  std::stringstream{};
    std::ostream &b = a;
    b<<A{};
    foo(std::move(a));

The original code is able to compile on GCC, clang (with libc++) and MSVC.

[Bug c++/102028] std::ostream& used as std::stringstream&

[lars.maier at tefax dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102028

--- Comment #2 from Lars Maier <lars.maier at tefax dot net> ---
You are right. Thank you.

[Bug c++/102028] std::ostream& used as std::stringstream&

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102028

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

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Not a bug.

[Bug c++/102028] std::ostream& used as std::stringstream&

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102028

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Lars Maier from comment #0)
> Reason: the result of operator<< with std::stringstream and A const& is just
> an std::ostream&

No it isn't, https://cplusplus.github.io/LWG/issue1203 changed that.