[Bug libstdc++/80676] New: basic_stringbuf does not use initial capacity of SSO string

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80676

            Bug ID: 80676
           Summary: basic_stringbuf does not use initial capacity of SSO
                    string
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

The following prints "overflow":

#include <sstream>
#include <iostream>

struct SB : std::stringbuf
{
  int_type overflow(int_type c) override
  {
    std::cout << "overflow\n";
    return std::stringbuf::overflow(c);
  }
};

int main()
{
  SB sb;
  std::ostringstream s;
  s.std::ios::rdbuf(&sb);
  s.put('a');
}

The call to the virtual function should not be necessary when using the new
ABI, because the SSO string has an initial non-zero capacity. The stringbuf
could use it.