[libc++] Addresses LWG3103.

[Mordante]

Summary

LWG3103 Errors in taking subview of span should be ill-formed where possible

Note that the real work was already done before, including tests.

[Mordante]

CI fixes.

[Mordante]

Rebased to fix CI.

libcxx/docs/Status/Cxx20Issues.csv

 "`3241 <https://wg21.link/LWG3241>`__","``chrono-spec``\  grammar ambiguity in |sect|\ [time.format]","Belfast","|Complete|","16.0","|chrono| |format|"
 "`3257 <https://wg21.link/LWG3257>`__","Missing feature testing macro update from P0858","Belfast","",""
 "`3256 <https://wg21.link/LWG3256>`__","Feature testing macro for ``constexpr``\  algorithms","Belfast","|Complete|","13.0"
 "`3273 <https://wg21.link/LWG3273>`__","Specify ``weekday_indexed``\  to range of ``[0, 7]``\ ","Belfast","|Complete|","16.0","|chrono|"
 "`3070 <https://wg21.link/LWG3070>`__","``path::lexically_relative``\  causes surprising results if a filename can also be a  *root-name*","Belfast","",""
 "`3266 <https://wg21.link/LWG3266>`__","``to_chars(bool)``\  should be deleted","Belfast","|Complete|","14.0"
 "`3272 <https://wg21.link/LWG3272>`__","``%I%p``\  should parse/format ``duration``\  since midnight","Belfast","","","|chrono| |format|"
 "`3259 <https://wg21.link/LWG3259>`__","The definition of *constexpr iterators* should be adjusted","Belfast","|Nothing To Do|",""
-"`3103 <https://wg21.link/LWG3103>`__","Errors in taking subview of ``span``\  should be ill-formed where possible","Belfast","",""
+"`3103 <https://wg21.link/LWG3103>`__","Errors in taking subview of ``span``\  should be ill-formed where possible","Belfast","11.0","|Complete|"
 "`3274 <https://wg21.link/LWG3274>`__","Missing feature test macro for ``<span>``\ ","Belfast","|Complete|","11.0"
 "`3276 <https://wg21.link/LWG3276>`__","Class ``split_view::outer_iterator::value_type``\  should inherit from ``view_interface``\ ","Belfast","|Complete|","15.0","|ranges|"
 "`3277 <https://wg21.link/LWG3277>`__","Pre-increment on prvalues is not a requirement of ``weakly_incrementable``\ ","Belfast","|Nothing To Do|","","|ranges|"
 "`3149 <https://wg21.link/LWG3149>`__","``DefaultConstructible``\  should require default initialization","Belfast","|Complete|","13.0"
 "","","","","",""
 "`1203 <https://wg21.link/LWG1203>`__","More useful rvalue stream insertion","Prague","|Complete|","12.0"
 "`2859 <https://wg21.link/LWG2859>`__","Definition of *reachable* in [ptr.launder] misses pointer arithmetic from pointer-interconvertible object","Prague","",""
 "`3018 <https://wg21.link/LWG3018>`__","``shared_ptr``\  of function type","Prague","",""

libcxx/test/std/containers/views/views.span/span.sub/first.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
 
 // template<size_t Count>
 //  constexpr span<element_type, Count> first() const;
 //
 // constexpr span<element_type, dynamic_extent> first(size_type count) const;
 //
-//  Requires: Count <= size().
-
+// Mandates: Count <= Extent is true.
 
 #include <span>
 #include <cassert>
 #include <algorithm>
 #include <string>
 
 #include "test_macros.h"
 

libcxx/test/std/containers/views/views.span/span.sub/first.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
 
 // template<size_t Count>
 //  constexpr span<element_type, Count> first() const;
 //
-//  Requires: Count <= size().
+// Mandates: Count <= Extent is true.
 
 #include <span>
 #include <cstddef>
 
 void f() {
   int array[] = {1, 2, 3, 4};
   std::span<const int, 4> sp(array);
 
   //  Count too large
   [[maybe_unused]] auto s1 = sp.first<5>(); // expected-error@span:* {{span<T, N>::first<Count>(): Count out of range}}
 
   //  Count numeric_limits
   [[maybe_unused]] auto s2 = sp.first<std::size_t(-1)>(); // expected-error@span:* {{span<T, N>::first<Count>(): Count out of range}}
 }

libcxx/test/std/containers/views/views.span/span.sub/last.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
 
 // template<size_t Count>
 //  constexpr span<element_type, Count> last() const;
 //
 // constexpr span<element_type, dynamic_extent> last(size_type count) const;
 //
-//  Requires: Count <= size().
-
+// Mandates: Count <= Extent is true.
 
 #include <span>
 #include <cassert>
 #include <algorithm>
 #include <string>
 
 #include "test_macros.h"
 

libcxx/test/std/containers/views/views.span/span.sub/last.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
 
 // template<size_t Count>
 //  constexpr span<element_type, Count> last() const;
 //
-//  Requires: Count <= size().
+// Mandates: Count <= Extent is true.
 
 #include <span>
 #include <cstddef>
 
 void f() {
   int array[] = {1, 2, 3, 4};
   std::span<const int, 4> sp(array);
 
   //  Count too large
   [[maybe_unused]] auto s1 = sp.last<5>(); // expected-error@span:* {{span<T, N>::last<Count>(): Count out of range}}
 
   //  Count numeric_limits
   [[maybe_unused]] auto s2 = sp.last<std::size_t(-1)>(); // expected-error@span:* {{span<T, N>::last<Count>(): Count out of range}}
 }

libcxx/test/std/containers/views/views.span/span.sub/subspan.pass.cpp

 // <span>
 
 // template<size_t Offset, size_t Count = dynamic_extent>
 //   constexpr span<element_type, see below> subspan() const;
 //
 // constexpr span<element_type, dynamic_extent> subspan(
 //   size_type offset, size_type count = dynamic_extent) const;
 //
-//  Requires: (0 <= Offset && Offset <= size())
-//      && (Count == dynamic_extent || Count >= 0 && Offset + Count <= size())
+// Mandates: Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset) is true.
 
 #include <span>
 #include <cassert>
 #include <algorithm>
 #include <string>
 
 #include "test_macros.h"
 

libcxx/test/std/containers/views/views.span/span.sub/subspan.verify.cpp

 // with a very large extent (like size_t(-2)) -- silence those.
 // ADDITIONAL_COMPILE_FLAGS: -Xclang -verify-ignore-unexpected=warning
 
 // <span>
 
 // template<size_t Offset, size_t Count = dynamic_extent>
 //   constexpr span<element_type, see below> subspan() const;
 //
-//  Requires: offset <= size() &&
-//            (count == dynamic_extent || count <= size() - offset)
+// Mandates: Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset) is true.
 
 #include <span>
 #include <cstddef>
 
 void f() {
   int array[] = {1, 2, 3, 4};
   std::span<const int, 4> sp(array);