[gcc r12-5925] libstdc++: Make ranges::size and ranges::empty check for unbounded arrays

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:55823c5a0ba50022d7fcc95e74ec293143810ef6

commit r12-5925-g55823c5a0ba50022d7fcc95e74ec293143810ef6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sun Dec 12 21:16:25 2021 +0000

    libstdc++: Make ranges::size and ranges::empty check for unbounded arrays
    
    Passing IncompleteType(&)[] to ranges::begin produces an error outside
    the immediate context, which is fine for ranges::begin, but it means
    that we fail to enforce the SFINAE-able constraints for ranges::size and
    ranges::size. They should not be callable for any array of unknown
    bound, whether the type is complete or not. Because we don't enforce
    that in their constraints, we get a hard error when they try to use
    ranges::begin.
    
    This simply adds explicit checks for arrays of unknown bound to the
    constraints for ranges::size and ranges::empty. We only need to check it
    for the __sentinel_size and __eq_iter_empty concepts, because those are
    the ones that are relevant to arrays, and which try to use
    ranges::begin.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ranges_base.h (ranges::size, ranges::empty): Add
            explicit check for unbounded arrays before using ranges::begin.
            * testsuite/std/ranges/access/empty.cc: Check handling of unbounded
            arrays.
            * testsuite/std/ranges/access/size.cc: Likewise.

Diff:
---
 libstdc++-v3/include/bits/ranges_base.h           |  4 ++++
 libstdc++-v3/testsuite/std/ranges/access/empty.cc | 10 ++++++++++
 libstdc++-v3/testsuite/std/ranges/access/size.cc  | 10 ++++++++++
 3 files changed, 24 insertions(+)

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 43b0b9f7bf3..bf95823668b 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -383,6 +383,8 @@ namespace ranges
     template<typename _Tp>
       concept __sentinel_size = requires(_Tp& __t)
 	{
+	  requires (!is_unbounded_array_v<remove_reference_t<_Tp>>);
+
 	  { _Begin{}(__t) } -> forward_iterator;
 
 	  { _End{}(__t) } -> sized_sentinel_for<decltype(_Begin{}(__t))>;
@@ -466,6 +468,8 @@ namespace ranges
     template<typename _Tp>
       concept __eq_iter_empty = requires(_Tp& __t)
 	{
+	  requires (!is_unbounded_array_v<remove_reference_t<_Tp>>);
+
 	  { _Begin{}(__t) } -> forward_iterator;
 
 	  bool(_Begin{}(__t) == _End{}(__t));
diff --git a/libstdc++-v3/testsuite/std/ranges/access/empty.cc b/libstdc++-v3/testsuite/std/ranges/access/empty.cc
index 3cad4740124..83b40872bd4 100644
--- a/libstdc++-v3/testsuite/std/ranges/access/empty.cc
+++ b/libstdc++-v3/testsuite/std/ranges/access/empty.cc
@@ -121,6 +121,16 @@ test04()
   static_assert( ! noexcept(std::ranges::empty(E3{})) );
 }
 
+template<typename T>
+  concept has_empty = requires (T& t) { std::ranges::empty(t); };
+
+// If T is an array of unknown bound, ranges::empty(E) is ill-formed.
+static_assert( ! has_empty<int[]> );
+static_assert( ! has_empty<int(&)[]> );
+static_assert( ! has_empty<int[][2]> );
+struct Incomplete;
+static_assert( ! has_empty<Incomplete[]> );
+
 int
 main()
 {
diff --git a/libstdc++-v3/testsuite/std/ranges/access/size.cc b/libstdc++-v3/testsuite/std/ranges/access/size.cc
index c7e4f78f96a..24857975f7b 100644
--- a/libstdc++-v3/testsuite/std/ranges/access/size.cc
+++ b/libstdc++-v3/testsuite/std/ranges/access/size.cc
@@ -122,6 +122,16 @@ test06()
   static_assert( std::ranges::size(R{}) == 42 );
 }
 
+template<typename T>
+  concept has_size = requires (T& t) { std::ranges::size(t); };
+
+// If T is an array of unknown bound, ranges::size(E) is ill-formed.
+static_assert( ! has_size<int[]> );
+static_assert( ! has_size<int(&)[]> );
+static_assert( ! has_size<int[][2]> );
+struct Incomplete;
+static_assert( ! has_size<Incomplete[]> );
+
 int
 main()
 {