Proposed resolution:

This wording is relative to N4810.

1. Modify [ostream.sentry], class basic_ostream::sentry synopsis, as indicated:

   namespace std {
     template<class charT, class traits = char_traits<charT>>
     class basic_ostream<charT, traits>::sentry {
       bool ok_; // exposition only
       int uncaught_ = uncaught_exceptions(); // exposition only
     public:
       explicit sentry(basic_ostream<charT, traits>& os);
       ~sentry();
       explicit operator bool() const { return ok_; }
       sentry(const sentry&) = delete;
       sentry& operator=(const sentry&) = delete;
     };
   }
   

   […]

   ~sentry();
   

   -4- If (os.flags() & ios_base::unitbuf) && !uncaught_exceptions() <= uncaught_ && os.good() is true, calls os.rdbuf()->pubsync(). If that function returns -1, sets badbit in os.rdstate() without propagating an exception.

[ 2019-03-21; Daniel comments and provides wording ]

The wording below implements Roger's suggestion.

[ 2015-06, Telecon ]

JW: I already added an 's' here to make it use the new function, but that doesn't resolve Roger's suggestion to capture the value on entry and check it.

In the current [ostream.sentry], p4 refers to the now deprecated std::uncaught_exception(): D.9 [depr.uncaught].

If ((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good()) is true, calls os.rdbuf()->pubsync().

This needs to be changed, for example to use std::uncaught_exceptions() and to capture the value on entry and compare with the saved value on exit.