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

[Jonathan Wakely <jwakely@redhat.com>]

On 06/05/21 17:55 +0200, Stephan Bergmann wrote:
>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.

It looks like the failed conversion with the default argument is not
in the immediate context, so is an error not a substitution failure.
Clang is probably right, so I'll change it.

The reason I did it that way was to save instantiating
std::is_convertible but also because it seemed like an easy way to
avoid confusing diagnostics that say:

error: forming pointer to reference type 'std::basic_ostream<char>&'

for overload resolution failures for operator<< (because those
diagnostics are already hundreds of lines long and confusing enough
already).

This seems to work:

--- a/libstdc++-v3/include/std/ostream
+++ b/libstdc++-v3/include/std/ostream
@@ -722,9 +722,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      __rval_streamable();
  #else
    template<typename _Tp,
-          typename = _Require<__not_<__is_one_of<_Tp, _Tp&, ios_base>>>>
+          typename = _Require<__not_<is_same<_Tp, ios_base>>,
+                              is_convertible<_Tp*, ios_base*>>>
      _Tp&
-    __rval_streamable(ios_base* = (_Tp*)nullptr);
+    __rval_streamable();
  #endif

    // SFINAE helper to check constraints for operator<<(Ostream&&, const T&).


I'll finish testing that.