[libc++] Add a static_assert for a Mandates in seed_seq (#86992)

Fixes #84843

Author: ldionne

libcxx/include/__random/seed_seq.h

@@ -14,6 +14,7 @@
 #include <__algorithm/max.h>
 #include <__config>
 #include <__iterator/iterator_traits.h>
+#include <__type_traits/is_unsigned.h>
 #include <cstdint>
 #include <initializer_list>
 #include <vector>
@@ -79,6 +80,11 @@ void seed_seq::__init(_InputIterator __first, _InputIterator __last) {
 
 template <class _RandomAccessIterator>
 void seed_seq::generate(_RandomAccessIterator __first, _RandomAccessIterator __last) {
+  using _ValueType = typename iterator_traits<_RandomAccessIterator>::value_type;
+  static_assert(is_unsigned<_ValueType>::value && sizeof(_ValueType) >= sizeof(uint32_t),
+                "[rand.util.seedseq]/7 requires the value_type of the iterator to be an unsigned "
+                "integer capable of accommodating 32-bit quantities.");
+
   if (__first != __last) {
     std::fill(__first, __last, 0x8b8b8b8b);
     const size_t __n = static_cast<size_t>(__last - __first);

libcxx/test/std/numerics/rand/rand.util/rand.util.seedseq/generate.mandates.verify.cpp

@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <random>
+
+// class seed_seq;
+
+// template<class RandomAccessIterator>
+//     void generate(RandomAccessIterator begin, RandomAccessIterator end);
+
+// Check the following requirement: https://eel.is/c++draft/rand.util.seedseq#7
+//
+//  Mandates: iterator_traits<RandomAccessIterator>::value_type is an unsigned integer
+//            type capable of accommodating 32-bit quantities.
+
+// UNSUPPORTED: c++03
+// REQUIRES: stdlib=libc++
+
+#include <random>
+#include <climits>
+
+#include "test_macros.h"
+
+void f() {
+  std::seed_seq seq;
+
+  // Not an integral type
+  {
+    double* p = nullptr;
+    seq.generate(p, p); // expected-error-re@*:* {{static assertion failed{{.+}}: [rand.util.seedseq]/7 requires{{.+}}}}
+    // expected-error@*:* 0+ {{invalid operands to}}
+  }
+
+  // Not an unsigned type
+  {
+    long long* p = nullptr;
+    seq.generate(p, p); // expected-error-re@*:* {{static assertion failed{{.+}}: [rand.util.seedseq]/7 requires{{.+}}}}
+  }
+
+  // Not a 32-bit type
+  {
+#if UCHAR_MAX < UINT32_MAX
+    unsigned char* p = nullptr;
+    seq.generate(p, p); // expected-error-re@*:* {{static assertion failed{{.+}}: [rand.util.seedseq]/7 requires{{.+}}}}
+#endif
+  }
+
+  // Everything satisfied
+  {
+    unsigned long* p = nullptr;
+    seq.generate(p, p); // no diagnostic
+  }
+}