Re: [committed] libstdc++: Fix value categories used by ranges access CPOs [PR 100824]

[Jonathan Wakely <jwakely@redhat.com>]

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On 04/06/21 21:44 +0100, Jonathan Wakely wrote:
>On 04/06/21 18:03 +0100, Jonathan Wakely wrote:
>>The implementation of P2091R0 was incomplete, so that some range access
>>CPOs used perfect forwarding where they should not. This fixes it by
>>consistently operating on lvalues.
>>
>>Some additional changes that are not necessary to fix the bug:
>>
>>Modify the __as_const helper to simplify its usage. Instead of deducing
>>the value category from its argument, and requiring callers to forward
>>the argument as the correct category, add a non-deduced template
>>parameter which is used for the value category and accept the argument
>>as an lvalue. This means callers say __as_const<T>(t) instead of
>>__as_const(std::forward<T>(t)).
>>
>>Always use an lvalue reference type as the template argument for the
>>_S_noexcept helpers, so that we only instantiate one specialization for
>>lvalues and rvalues of the same type.
>>
>>Move some helper concepts and functions from namespace std::__detail
>>to ranges::__cust_access, to be consistent with the ranges::begin CPO.
>>This ensures that the __adl_begin concept and the _Begin::operator()
>>function are in the same namespace, so unqualified lookup is consistent
>>and the poison pills for begin are visible to both.
>>
>>Simplified static assertions for arrays, because the expression a+0 is
>>already ill-formed for an array of incomplete type.
>>
>>Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>>
>>libstdc++-v3/ChangeLog:
>>
>>	PR libstdc++/100824
>>	* include/bits/iterator_concepts.h (__detail::__decay_copy)
>>	(__detail::__member_begin, __detail::__adl_begin): Move to
>>	namespace ranges::__cust_access.
>>	(__detail::__ranges_begin): Likewise, and rename to __begin.
>>	Remove redundant static assertion.
>>	* include/bits/ranges_base.h (_Begin, _End, _RBegin, _REnd):
>>	Use lvalue in noexcept specifier.
>>	(__as_const): Add non-deduced parameter for value category.
>>	(_CBegin, _CEnd, _CRBegin, _CREnd, _CData): Adjust uses of
>>	__as_const.
>>	(__member_size, __adl_size, __member_empty, __size0_empty):
>>	(__eq_iter_empty, __adl_data): Use lvalue objects in
>>	requirements.
>>	(__sentinel_size): Likewise. Add check for conversion to
>>	unsigned-like.
>>	(__member_data): Allow non-lvalue types to satisfy the concept,
>>	but use lvalue object in requirements.
>>	(_Size, _SSize): Remove forwarding to always use an lvalue.
>>	(_Data): Likewise. Add static assertion for arrays.
>>	* testsuite/std/ranges/access/cdata.cc: Adjust expected
>>	behaviour for rvalues. Add negative tests for ill-formed
>>	expressions.
>>	* testsuite/std/ranges/access/data.cc: Likewise.
>>	* testsuite/std/ranges/access/empty.cc: Adjust expected
>>	behaviour for rvalues.
>>	* testsuite/std/ranges/access/size.cc: Likewise.
>
>An additional problem with ranges::data was pointed out in the PR,
>fixed with this patch.

And this implements the rest of LWG 3403. The change to the
ranges::ssize constraints was already done by the first patch in this
thread, this fixes the return type.

Tested powerpc64le-linux. Committed to trunk.

This should also be backported to gcc-11 and gcc-10.



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commit 621ea10ca060ba19ec693aa73b5e29d553cca849
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jun 4 20:28:04 2021

    libstdc++: Implement LWG 3403 for std::ranges::ssize
    
    I already changed the constraints for ranges::ssize to use ranges::size,
    this implements the rest of LWG 3403, so that the returned type is the
    signed type corresponding to the result of ranges::size.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ranges_base.h (_SSize): Return the result of
            ranges::size converted to the wider of make-signed-like-t<S> and
            ptrdiff_t, rather than the ranges different type.
            * testsuite/std/ranges/access/ssize.cc: Adjust expected result
            for an iota_view that uses an integer class type for its
            difference_type.

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 61d91eb8389..e3c3962bcd9 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -425,22 +425,32 @@ namespace ranges
 
     struct _SSize
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3403. Domain of ranges::ssize(E) doesn't match ranges::size(E)
       template<typename _Tp>
 	requires requires (_Tp& __t) { _Size{}(__t); }
 	constexpr auto
 	operator()(_Tp&& __t) const noexcept(noexcept(_Size{}(__t)))
 	{
-	  using __iter_type = decltype(_Begin{}(__t));
-	  using __diff_type = iter_difference_t<__iter_type>;
-	  using __gnu_cxx::__int_traits;
 	  auto __size = _Size{}(__t);
-	  if constexpr (integral<__diff_type>)
+	  using __size_type = decltype(__size);
+	  // Return the wider of ptrdiff_t and make-signed-like-t<__size_type>.
+	  if constexpr (integral<__size_type>)
 	    {
-	      if constexpr (__int_traits<__diff_type>::__digits
+	      using __gnu_cxx::__int_traits;
+	      if constexpr (__int_traits<__size_type>::__digits
 			    < __int_traits<ptrdiff_t>::__digits)
 		return static_cast<ptrdiff_t>(__size);
+	      else
+		return static_cast<make_signed_t<__size_type>>(__size);
 	    }
-	  return static_cast<__diff_type>(__size);
+#if defined __STRICT_ANSI__ && defined __SIZEOF_INT128__
+	  // For strict-ansi modes integral<__int128> is false
+	  else if constexpr (__detail::__is_int128<__size_type>)
+	    return static_cast<unsigned __int128>(__size);
+#endif
+	  else // Must be one of __max_diff_type or __max_size_type.
+	    return __detail::__max_diff_type(__size);
 	}
     };
 
diff --git a/libstdc++-v3/testsuite/std/ranges/access/ssize.cc b/libstdc++-v3/testsuite/std/ranges/access/ssize.cc
index b00ec5db1f8..f53f462580c 100644
--- a/libstdc++-v3/testsuite/std/ranges/access/ssize.cc
+++ b/libstdc++-v3/testsuite/std/ranges/access/ssize.cc
@@ -76,9 +76,12 @@ void
 test06()
 {
   auto i = std::views::iota(1ull, 5u);
-  auto s = std::ranges::ssize(i);
-  using R = std::ranges::range_difference_t<decltype(i)>;
-  static_assert( std::same_as<decltype(s), R> );
+  auto s = std::ranges::size(i);
+  auto ss = std::ranges::ssize(i);
+  // std::ranges::range_difference_t<decltype(i)> is larger than long long,
+  // but LWG 3403 says ranges::ssize(i) returns the signed version of the
+  // type that ranges::size(i) returns, not the range's difference_type.
+  static_assert( std::same_as<decltype(ss), std::make_signed_t<decltype(s)>> );
   VERIFY( s == 4 );
 }