[PATCH v2 4/4] libstdc++: Make ref_view<R> statically sized if R has static size

[Tomasz Kamiński]

This patch introduces ranges::__static_size<_Range> helper functions,
that returns ranges::size(__rg) for __statically_sized_range.
This function is then used for ref_view<R>::size if R has static size,
avoiding derefence of pointer value that is not know at compile time.
Similary for ref_view<R>::empty() we compare the size with zero,
if it is know statically.

This implements revelant part of P3928R0: static_sized_range by Hewill Kang.
As standard does not specify when constexpr functions are usable at compile
time, such implementation are allowed (but not mandated) by current draft.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (ranges::__static_size): Define.
	* include/std/ranges (ref_view::size()): For ranges with static
	size return ranges::__static_size<_Range> == 0.
	(ref_view::empty): For ranges with static size, compare it
	against zero.
	* testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc:
	Expect errors from ref_view uses.
	* testsuite/23_containers/inplace_vector/cons/from_range_neg.cc:
	Expect errors from ref_view uses.
---
v2 also expands the empty() method, and adds __static_size helper funciton.
We could really use some consteval __builtin_unknown_ref(_Tp), that will
produce reference to unknown object of type _Tp. This will avoid nested
lambda and instantiating integer_constant.

Tested on x86_64-linux. OK for trunk?

 libstdc++-v3/include/bits/ranges_base.h        |  9 +++++++++
 libstdc++-v3/include/std/ranges                | 18 ++++++++++++++++--
 .../inplace_vector/cons/from_iota_neg.cc       |  6 ++----
 .../inplace_vector/cons/from_range_neg.cc      | 10 ++++------
 4 files changed, 31 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index fea4100eb09..e61ef75f857 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -562,6 +562,15 @@ namespace ranges
   template<typename _Tp>
     concept __static_sized_range = sized_range<_Tp> && requires (_Tp& __t)
       { static_cast<char(*)[size_t(ranges::size(__t) >= 0)]>(nullptr); };
+
+  template<__static_sized_range _Tp>
+    consteval range_size_t<_Tp>
+    __static_size()
+    {
+      auto __conjure = [](_Tp& __t)
+      { return integral_constant<range_size_t<_Tp>, ranges::size(__t)>{}; };
+      return decltype(__conjure(std::declval<_Tp&>()))::value;
+    }
 #endif // C++26
 
   template<typename _Derived>
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 4f2082487d1..c1e8ac3cb17 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1380,11 +1380,25 @@ namespace views::__adaptor
 
       constexpr bool
       empty() const requires requires { ranges::empty(*_M_r); }
-      { return ranges::empty(*_M_r); }
+      {
+#if __cplusplus > 202302L
+	if constexpr (__static_sized_range<_Range>)
+	  return ranges::__static_size<_Range>() == 0;
+        else
+#endif
+	  return ranges::empty(*_M_r);
+      }
 
       constexpr auto
       size() const requires sized_range<_Range>
-      { return ranges::size(*_M_r); }
+      {
+#if __cplusplus > 202302L
+	if constexpr (__static_sized_range<_Range>)
+	  return ranges::__static_size<_Range>();
+        else
+#endif
+	  return ranges::size(*_M_r);
+      }
 
       constexpr auto
       data() const requires contiguous_range<_Range>
diff --git a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
index ae0b96175b7..6b66b5d97ed 100644
--- a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
@@ -33,14 +33,12 @@ test_all()
   std::inplace_vector<int, 15> tr1(std::from_range, ref_view(m12));
 
   std::inplace_vector<int, 10> tm2(std::from_range, m12); // { dg-error "(from here|expansion of)" }
-  // ref_view is not statically sized due pointer dereference
-  std::inplace_vector<int, 10> tr2(std::from_range, ref_view(m12));
+  std::inplace_vector<int, 10> tr2(std::from_range, ref_view(m12)); // { dg-error "(from here|expansion of)" }
 
   StaticIota<__int128, 0> mm;
 
   std::inplace_vector<int, 10> tm3(std::from_range, mm); // { dg-error "(from here|expansion of)" }
-  // ref_view is not statically sized due pointer dereference
-  std::inplace_vector<int, 10> tr3(std::from_range, ref_view(mm));
+  std::inplace_vector<int, 10> tr3(std::from_range, ref_view(mm)); // { dg-error "(from here|expansion of)" }
 }
 
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
index 48d5b4c56f5..ea187761e5e 100644
--- a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
@@ -62,16 +62,14 @@ test_all()
 
   test_one(a1); // { dg-error "from here" }
   test_one(s1); // { dg-error "from here" }
-  // ref_view is not statically sized due pointer dereference
-  test_one(ref_view(a1));
-  test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
+  test_one(ref_view(a1)); // { dg-error "from here" }
+  test_one(a5 | std::views::adjacent<5> | std::views::elements<0>); // { dg-error "from here" }
   test_one(s5 | std::views::adjacent<5> | std::views::elements<0>); // { dg-error "from here" }
 
   test_five(a5); // { dg-error "from here" }
   test_five(s5); // { dg-error "from here" }
-  // ref_view is not statically sized due pointer dereference
-  test_five(ref_view(a5));
-  test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
+  test_five(ref_view(a5)); // { dg-error "from here" }
+  test_five(a7 | std::views::adjacent<3> | std::views::elements<0>); // { dg-error "from here" }
   test_five(s7 | std::views::adjacent<3> | std::views::elements<0>); // { dg-error "from here" }
 }
 
-- 
2.53.0