[libc++] Use static_asserts for span::front() and span::back() when possible

libcxx/include/span

@@ -369,12 +369,12 @@ public:
 #    endif
 
   _LIBCPP_HIDE_FROM_ABI constexpr reference front() const noexcept {
-    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "span<T, N>::front() on empty span");
+    static_assert(_Extent >= 1, "span<T, N>::front() on empty span");
     return __data_[0];
   }
 
   _LIBCPP_HIDE_FROM_ABI constexpr reference back() const noexcept {
-    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "span<T, N>::back() on empty span");
+    static_assert(_Extent >= 1, "span<T, N>::back() on empty span");
     return __data_[size() - 1];
   }
 

libcxx/test/libcxx/containers/views/views.span/span.elem/assert.back.pass.cpp

@@ -5,6 +5,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
@@ -23,17 +24,13 @@
 #include "check_assertion.h"
 
 int main(int, char**) {
-    {
-        std::array<int, 3> array{0, 1, 2};
-        std::span<int> const s(array.data(), 0);
-        TEST_LIBCPP_ASSERT_FAILURE(s.back(), "span<T>::back() on empty span");
-    }
-
-    {
-        std::array<int, 3> array{0, 1, 2};
-        std::span<int, 0> const s(array.data(), 0);
-        TEST_LIBCPP_ASSERT_FAILURE(s.back(), "span<T, N>::back() on empty span");
-    }
-
-    return 0;
+  {
+    std::array<int, 3> array{0, 1, 2};
+    std::span<int> const s(array.data(), 0);
+    TEST_LIBCPP_ASSERT_FAILURE(s.back(), "span<T>::back() on empty span");
+  }
+
+  // back() on a span with a static extent is caught statically and tested in front.verify.cpp
+
+  return 0;
 }

libcxx/test/libcxx/containers/views/views.span/span.elem/assert.front.pass.cpp

@@ -5,6 +5,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
@@ -23,17 +24,13 @@
 #include "check_assertion.h"
 
 int main(int, char**) {
-    {
-        std::array<int, 3> array{0, 1, 2};
-        std::span<int> const s(array.data(), 0);
-        TEST_LIBCPP_ASSERT_FAILURE(s.front(), "span<T>::front() on empty span");
-    }
-
-    {
-        std::array<int, 3> array{0, 1, 2};
-        std::span<int, 0> const s(array.data(), 0);
-        TEST_LIBCPP_ASSERT_FAILURE(s.front(), "span<T, N>::front() on empty span");
-    }
-
-    return 0;
+  {
+    std::array<int, 3> array{0, 1, 2};
+    std::span<int> const s(array.data(), 0);
+    TEST_LIBCPP_ASSERT_FAILURE(s.front(), "span<T>::front() on empty span");
+  }
+
+  // front() on a span with a static extent is caught statically and tested in front.verify.cpp
+
+  return 0;
 }

libcxx/test/libcxx/containers/views/views.span/span.elem/back.verify.cpp

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+//
+// constexpr reference back() const noexcept;
+
+// Make sure that accessing a statically-sized span out-of-bounds triggers a
+// compile-time error.
+
+#include <array>
+#include <span>
+
+int main(int, char**) {
+  std::array<int, 3> array{0, 1, 2};
+  {
+    std::span<int, 0> const s(array.data(), 0);
+    s.back(); // expected-error@span:* {{span<T, N>::back() on empty span}}
+  }
+  {
+    std::span<int, 3> const s(array.data(), 3);
+    s.back(); // nothing
+  }
+
+  return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.elem/front.verify.cpp

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+//
+// constexpr reference front() const noexcept;
+
+// Make sure that accessing a statically-sized span out-of-bounds triggers a
+// compile-time error.
+
+#include <array>
+#include <span>
+
+int main(int, char**) {
+  std::array<int, 3> array{0, 1, 2};
+  {
+    std::span<int, 0> const s(array.data(), 0);
+    s.front(); // expected-error@span:* {{span<T, N>::front() on empty span}}
+  }
+  {
+    std::span<int, 3> const s(array.data(), 3);
+    s.front(); // nothing
+  }
+
+  return 0;
+}