[PATCH] Remove PR 92268 workaround and fix new test failures

[Jonathan Wakely <jwakely@redhat.com>]

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With the compiler bug fixed we can simplify the __sizable concept to use
a return-type-requirement again. I also realised it was redundantly
re-checking a subset of the sized_sentinel_for requirements.

The compiler fix also revealed bugs in two tests which started to fail
and are fixed by this patch.

	* include/bits/range_access.h (__sizable): Rename to __sentinel_size.
	Remove workaround for PR c++/92268 and remove redundant requirements
	that are already checked by sized_sentinel_for.
	* testsuite/std/ranges/access/cend.cc: Fix failures.
	* testsuite/std/ranges/access/end.cc: Likewise.

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commit 9faf3f15a81c9f4a575f2c6c72d88bcbdd476a6c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Oct 31 11:53:08 2019 +0000

    Remove PR 92268 workaround and fix new test failures
    
    With the compiler bug fixed we can simplify the __sizable concept to use
    a return-type-requirement again. I also realised it was redundantly
    re-checking a subset of the sized_sentinel_for requirements.
    
    The compiler fix also revealed bugs in two tests which started to fail
    and are fixed by this patch.
    
            * include/bits/range_access.h (__sizable): Rename to __sentinel_size.
            Remove workaround for PR c++/92268 and remove redundant requirements
            that are already checked by sized_sentinel_for.
            * testsuite/std/ranges/access/cend.cc: Fix failures.
            * testsuite/std/ranges/access/end.cc: Likewise.

diff --git a/libstdc++-v3/include/bits/range_access.h b/libstdc++-v3/include/bits/range_access.h
index f0a339277e9..de074460c16 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -689,19 +689,13 @@ namespace ranges
 	    -> __detail::__is_integer_like;
 	};
 
-    // FIXME: needed due to PR c++/92268
-    template<forward_iterator _It, sized_sentinel_for<_It> _End>
-      requires requires (_It __it, _End __end)
-      { { __end - __it } -> __detail::__is_integer_like; }
-      void
-      __subtractable_fwd_iter(_It, _End)
-      { }
-
     template<typename _Tp>
-      concept __sizable = requires(_Tp&& __t)
+      concept __sentinel_size = requires(_Tp&& __t)
 	{
-	  __subtractable_fwd_iter(_Begin{}(std::forward<_Tp>(__t)),
-				  _End{}(std::forward<_Tp>(__t)));
+	  { _Begin{}(std::forward<_Tp>(__t)) } -> forward_iterator;
+
+	  { _End{}(std::forward<_Tp>(__t)) }
+	    -> sized_sentinel_for<decltype(_Begin{}(std::forward<_Tp>(__t)))>;
 	};
 
     struct _Size
@@ -717,7 +711,7 @@ namespace ranges
 	    return noexcept(__decay_copy(std::declval<_Tp>().size()));
 	  else if constexpr (__adl_size<_Tp>)
 	    return noexcept(__decay_copy(size(std::declval<_Tp>())));
-	  else if constexpr (__sizable<_Tp>)
+	  else if constexpr (__sentinel_size<_Tp>)
 	    return noexcept(_End{}(std::declval<_Tp>())
 			    - _Begin{}(std::declval<_Tp>()));
 	}
@@ -725,7 +719,7 @@ namespace ranges
     public:
       template<typename _Tp>
 	requires is_array_v<remove_reference_t<_Tp>>
-	  || __member_size<_Tp> || __adl_size<_Tp> || __sizable<_Tp>
+	  || __member_size<_Tp> || __adl_size<_Tp> || __sentinel_size<_Tp>
 	constexpr auto
 	operator()(_Tp&& __e) const noexcept(_S_noexcept<_Tp>())
 	{
@@ -738,7 +732,7 @@ namespace ranges
 	    return std::forward<_Tp>(__e).size();
 	  else if constexpr (__adl_size<_Tp>)
 	    return size(std::forward<_Tp>(__e));
-	  else if constexpr (__sizable<_Tp>)
+	  else if constexpr (__sentinel_size<_Tp>)
 	    return __detail::__to_unsigned_like(
 		_End{}(std::forward<_Tp>(__e))
 		- _Begin{}(std::forward<_Tp>(__e)));
diff --git a/libstdc++-v3/testsuite/std/ranges/access/cend.cc b/libstdc++-v3/testsuite/std/ranges/access/cend.cc
index 789c6ca3837..94349c35d51 100644
--- a/libstdc++-v3/testsuite/std/ranges/access/cend.cc
+++ b/libstdc++-v3/testsuite/std/ranges/access/cend.cc
@@ -66,15 +66,18 @@ struct RR
   long l = 0;
   int a[4] = { 0, 1, 2, 3 };
 
-  const void* begin() const { return nullptr; }
-  friend const void* begin(const RR&&) noexcept { return nullptr; }
-
-  short* end() noexcept { return &s; }
-  const long* end() const { return &l; }
+  const void* begin() const; // return type not an iterator
 
   friend int* end(RR&) { throw 1; }
+  short* end() noexcept { return &s; }
+
+  friend const long* begin(const RR&) noexcept;
+  const long* end() const { return &l; }
+
+  friend int* begin(RR&&) noexcept;
   friend int* end(RR&& r) { return r.a + 1; }
-  friend const int* end(const RR&) { throw 1; }
+
+  friend const int* begin(const RR&&) noexcept;
   friend const int* end(const RR&& r) noexcept { return r.a + 3; }
 };
 
diff --git a/libstdc++-v3/testsuite/std/ranges/access/end.cc b/libstdc++-v3/testsuite/std/ranges/access/end.cc
index 7f09cc2a4ab..6638bb35721 100644
--- a/libstdc++-v3/testsuite/std/ranges/access/end.cc
+++ b/libstdc++-v3/testsuite/std/ranges/access/end.cc
@@ -61,11 +61,12 @@ struct R
 {
   int a[4] = { 0, 1, 2, 3 };
 
-  const int* begin() const { return nullptr; }
-  friend const int* begin(const R&& r) noexcept { return nullptr; }
+  const int* begin() const;
+  friend int* begin(R&&) noexcept;
+  friend const int* begin(const R&&) noexcept;
 
   // Should be ignored because it doesn't return a sentinel for int*
-  const long* end() const { return nullptr; }
+  const long* end() const;
 
   friend int* end(R& r) { return r.a + 0; }
   friend int* end(R&& r) { return r.a + 1; }
@@ -105,15 +106,20 @@ struct RR
   long l = 0;
   int a[4] = { 0, 1, 2, 3 };
 
-  const void* begin() const { return nullptr; }
-  friend const void* begin(const RR&&) noexcept { return nullptr; }
+  const void* begin() const; // return type not an iterator
 
+  friend const short* begin(RR&) noexcept;
   short* end() noexcept { return &s; }
+
+  friend const long* begin(const RR&) noexcept;
   const long* end() const { return &l; }
 
-  friend int* end(RR&) { throw 1; }
+  friend const int* begin(RR&&) noexcept;
+  friend int* end(RR&) { throw 1; } // not valid for rvalues
   friend int* end(RR&& r) { return r.a + 1; }
-  friend const int* end(const RR&) { throw 1; }
+
+  friend const int* begin(const RR&&) noexcept;
+  friend const int* end(const RR&) { throw 1; } // not valid for rvalues
   friend const int* end(const RR&& r) noexcept { return r.a + 3; }
 };