[libcxx] implement LWG4148: unique_ptr::operator* should not allow dangling references

Author: elhewaty

libcxx/include/__memory/unique_ptr.h

@@ -261,7 +261,10 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
-      _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>())) {
+      _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>()))
+    // TODO: use reference_converts_from_temporary_v once implemented.
+    requires(!__reference_converts_from_temporary(__add_lvalue_reference_t<_Tp>, decltype(*declval<pointer>())))
+  {
     return *__ptr_;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer operator->() const _NOEXCEPT { return __ptr_; }

libcxx/test/libcxx/memory/unique_ptr.compile.fail.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+#include <iostream>
+#include <memory>
+
+using namespace std;
+
+struct deleter {
+  using pointer = long*;
+  void operator()(pointer) const {}
+};
+
+int main(int argc, char const *argv[]) {
+  long l = 0;
+  std::unique_ptr<const int, deleter> p(&l);
+  // Error: indirection requires pointer operand ('std::unique_ptr<const int, deleter>' invalid)
+  int i = *p;
+  cout << i << endl;
+  return 0;
+}