LWG-1203 More useful rvalue stream insertion #1225
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Fixes #538 by completing LWG-1203 "More useful rvalue stream insertion".
We had already implemented most of this LWG issue resolution, so only the constraints needed to be adjusted. Replacing
is_base_of<ios_base, Stream>withis_convertible<Stream*, ios_base*>implements the "publicly and unambiguously derived" wording in WG21-N4861 [istream.rvalue]/1 and [ostream.rvalue]/1 thanks to [conv.ptr]/3. As a bonus, it also allows us to remove the "avoid infinite recursion" constraintnegation<is_lvalue_reference<Stream>>, because attempting to form a pointer to a reference will SFINAE due to [dcl.ptr]/4 and [dcl.ref]/5.Finally, I crafted a test that fails without this change and passes with it.