[PATCH 2/2] libstdc++: Make ref_view<R> statically sized if R is statically sized

[Tomasz Kamiński]

If the referenced range is statically sized, instead of referencing a
pointer whose value is not know at compile time, we invoke ranges::size
on static `__placeholder` variable  of _Range&. The result is then used
as initializer of constexpr variable, guaranteeing that evaluation happens
at compile time, and __placeholder var is never referenced from runtime.

libstdc++-v3/ChangeLog:

	* include/std/ranges (ref_view::_S_placeholder): Declare without
	definition.
	(ref_view::size()): Use ranges::size on _S_placeholder for
	statically sized ranges.
	* testsuite/23_containers/inplace_vector/cons/from_range_neg.cc:
	Expect errors from ref_view of array examples.
--
I wonder if that should not be something that is guaranteed in the
standsard. I again relly on complete lack of specification when constexpr
function are usable at compile time, and I think ref_view<S>::size() can
me made one in this case. We could apply the logic (using requires
clause) for all ref_view methods in future.

I think that this patch is GCC-17 material, as any issues with support 
of references to uknown, my affect existing code simply recompiled with
--std=c++26.

Testing on x86_64-linux. OK for GCC-17 when all test passes?

 libstdc++-v3/include/std/ranges                     | 13 ++++++++++++-
 .../inplace_vector/cons/from_range_neg.cc           |  6 ++----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 95e0109a10c..e3f28b1ae42 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1364,6 +1364,7 @@ namespace views::__adaptor
     private:
       _Range* _M_r;
 
+      static _Range& _S_placeholder;
       static void _S_fun(_Range&); // not defined
       static void _S_fun(_Range&&) = delete;
 
@@ -1395,7 +1396,17 @@ namespace views::__adaptor
 
       constexpr auto
       size() const requires sized_range<_Range>
-      { return ranges::size(*_M_r); }
+      {
+#if __cplusplus > 202302L
+	if constexpr (__static_sized_range<_Range>)
+	  {
+	    constexpr auto __size = ranges::size(_S_placeholder);
+	    return __size;
+	  }
+        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_range_neg.cc b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
index cf87129da51..15fd7b5c285 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
@@ -57,14 +57,12 @@ test_all()
   test_one(a1); // { dg-error "from here" }
   test_one(s1); // { dg-error "from here" }
   test_one(s5 | std::views::adjacent<5> | std::views::elements<0>); // { dg-error "from here" }
-  // ref_view is not statically sized due pointer dereference
-  test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
+  test_one(a5 | 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" }
   test_five(s7 | std::views::adjacent<3> | std::views::elements<0>); // { dg-error "from here" }
-  // ref_view is not statically sized due pointer dereference
-  test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
+  test_five(a7 | std::views::adjacent<3> | std::views::elements<0>); // { dg-error "from here" }
 }
 
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }
-- 
2.53.0