Re: [committed] libstdc++: Implement LWG 1203 for rvalue iostreams

[Stephan Bergmann <sbergman@redhat.com>]

On 30/04/2021 15:48, Jonathan Wakely via Libstdc++ wrote:
> This implements the resolution of LWG 1203 so that the constraints for
> rvalue stream insertion/extraction are simpler, and the return type is
> the original rvalue stream type not its base class.
> 
> Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/istream (operator>>(Istream&&, x&)): Simplify, as
> 	per LWG 1203.
> 	* include/std/ostream (operator<<(Ostream&&, const x&)):
> 	Likewise.
> 	* testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc:
> 	Adjust dg-error pattern.
> 	* testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc:
> 	Likewise.
> 	* testsuite/27_io/basic_istream/extractors_other/char/4.cc: Define
> 	is_extractable trait to replace std::__is_extractable. Make it
> 	work with rvalue streams as well as lvalues, to replace f() and
> 	g() helper functions.
> 	* testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc:
> 	Likewise.
> 	* testsuite/27_io/basic_ostream/inserters_other/char/6.cc:
> 	Define is_insertable trait to replace std::__is_insertable. Make
> 	it work with rvalue streams as well as lvalues, to replace f()
> 	and g() helper functions.
> 	* testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc:
> 	Likewise.
> 	* testsuite/27_io/filesystem/path/io/dr2989.cc: Prune additional
> 	errors from new constraints.
> 	* testsuite/27_io/rvalue_streams-2.cc: Remove PR 80675 checks,
> 	which are no longer expected to compile.
> 	* testsuite/27_io/rvalue_streams.cc: Adjust existing test.
> 	Verify LWG 1203 changes.
> 
> Tested powerpc64le-linux. Committed to trunk.

FWIW, it looks like this is causing issues for Clang (at least Clang 11 
and recent Clang 13 trunk):

> $ cat test.cc
> #include <ostream>
> int i = 1 << std::ios::erase_event;

(i.e., using and enum in namespace std),

> $ clang++ --gcc-toolchain=~/gcc/trunk/inst -fsyntax-only test.cc
> In file included from test.cc:1:
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/ostream:727:33: error: cannot initialize a parameter of type 'std::ios_base *' with an rvalue of type 'int *'
>     __rval_streamable(ios_base* = (_Tp*)nullptr);
>                                 ^ ~~~~~~~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/ostream:733:25: note: in instantiation of default function argument expression for '__rval_streamable<int, void>' required here
>            typename = decltype(std::__rval_streamable<_Os>()
>                                ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/ostream:748:12: note: in instantiation of default argument for '__rvalue_stream_insertion_t<int, std::ios_base::event>' required here
>     inline __rvalue_stream_insertion_t<_Ostream, _Tp>
>            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> test.cc:2:11: note: while substituting deduced template arguments into function template 'operator<<' [with _Ostream = int, _Tp = std::ios_base::event]
> int i = 1 << std::ios::erase_event;
>           ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/ostream:727:33: note: passing argument to parameter here
>     __rval_streamable(ios_base* = (_Tp*)nullptr);
>                                 ^
> 1 error generated.