[libc++] Add missing assertion in std::span constructor (#118396)

The (iterator, size) constructor should ensure that it gets passed a
valid range when the size is not 0.

Fixes #107789

Author: ldionne

libcxx/include/span

@@ -267,6 +267,8 @@ public:
   _LIBCPP_HIDE_FROM_ABI constexpr explicit span(_It __first, size_type __count) : __data_{std::to_address(__first)} {
     (void)__count;
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(_Extent == __count, "size mismatch in span's constructor (iterator, len)");
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__count == 0 || std::to_address(__first) != nullptr,
+                                     "passed nullptr with non-zero length in span's constructor (iterator, len)");
   }
 
   template <__span_compatible_iterator<element_type> _It, __span_compatible_sentinel_for<_It> _End>
@@ -441,7 +443,10 @@ public:
 
   template <__span_compatible_iterator<element_type> _It>
   _LIBCPP_HIDE_FROM_ABI constexpr span(_It __first, size_type __count)
-      : __data_{std::to_address(__first)}, __size_{__count} {}
+      : __data_{std::to_address(__first)}, __size_{__count} {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__count == 0 || std::to_address(__first) != nullptr,
+                                     "passed nullptr with non-zero length in span's constructor (iterator, len)");
+  }
 
   template <__span_compatible_iterator<element_type> _It, __span_compatible_sentinel_for<_It> _End>
   _LIBCPP_HIDE_FROM_ABI constexpr span(_It __first, _End __last)

libcxx/test/libcxx/containers/views/views.span/span.cons/assert.iter_size.pass.cpp

@@ -25,13 +25,48 @@
 #include "check_assertion.h"
 
 int main(int, char**) {
-    std::array<int, 3> array{0, 1, 2};
+  std::array<int, 3> array{0, 1, 2};
 
-    auto too_large = [&] { std::span<int, 3> const s(array.data(), 4); (void)s; };
-    TEST_LIBCPP_ASSERT_FAILURE(too_large(), "size mismatch in span's constructor (iterator, len)");
+  // Input range too large (exceeds the span extent)
+  {
+    auto f = [&] {
+      std::span<int, 3> const s(array.data(), 4);
+      (void)s;
+    };
+    TEST_LIBCPP_ASSERT_FAILURE(f(), "size mismatch in span's constructor (iterator, len)");
+  }
 
-    auto too_small = [&] { std::span<int, 3> const s(array.data(), 2); (void)s; };
-    TEST_LIBCPP_ASSERT_FAILURE(too_small(), "size mismatch in span's constructor (iterator, len)");
+  // Input range too small (doesn't fill the span)
+  {
+    auto f = [&] {
+      std::span<int, 3> const s(array.data(), 2);
+      (void)s;
+    };
+    TEST_LIBCPP_ASSERT_FAILURE(f(), "size mismatch in span's constructor (iterator, len)");
+  }
 
-    return 0;
+  // Input range is non-empty but starts with a null pointer
+  {
+    // static extent
+    {
+      auto f = [&] {
+        int* p = nullptr;
+        std::span<int, 3> const s(p, 3);
+        (void)s;
+      };
+      TEST_LIBCPP_ASSERT_FAILURE(f(), "passed nullptr with non-zero length in span's constructor (iterator, len)");
+    }
+
+    // dynamic extent
+    {
+      auto f = [&] {
+        int* p = nullptr;
+        std::span<int, std::dynamic_extent> const s(p, 1);
+        (void)s;
+      };
+      TEST_LIBCPP_ASSERT_FAILURE(f(), "passed nullptr with non-zero length in span's constructor (iterator, len)");
+    }
+  }
+
+  return 0;
 }