Index: libcxx/include/memory
===================================================================
--- libcxx/include/memory
+++ libcxx/include/memory
@@ -443,6 +443,11 @@
     template<class U> bool owner_before(weak_ptr<U> const& b) const noexcept;
 };
 
+template<class T>
+shared_ptr(weak_ptr<T>) -> shared_ptr<T>;
+template<class T, class D>
+shared_ptr(unique_ptr<T, D>) -> shared_ptr<T>;
+
 // shared_ptr comparisons:
 template<class T, class U>
     bool operator==(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept;
@@ -541,6 +546,9 @@
     template<class U> bool owner_before(weak_ptr<U> const& b) const noexcept;
 };
 
+template<class T>
+weak_ptr(shared_ptr<T>) -> weak_ptr<T>;
+
 // weak_ptr specialized algorithms:
 template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
 
@@ -3923,6 +3931,12 @@
     template <class _Up> friend class _LIBCPP_TEMPLATE_VIS weak_ptr;
 };
 
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+template<class _Tp>
+shared_ptr(weak_ptr<_Tp>) -> shared_ptr<_Tp>;
+template<class _Tp, class _Dp>
+shared_ptr(unique_ptr<_Tp, _Dp>) -> shared_ptr<_Tp>;
+#endif
 
 template<class _Tp>
 inline
@@ -4731,6 +4745,11 @@
     template <class _Up> friend class _LIBCPP_TEMPLATE_VIS shared_ptr;
 };
 
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+template<class _Tp>
+weak_ptr(shared_ptr<_Tp>) -> weak_ptr<_Tp>;
+#endif
+
 template<class _Tp>
 inline
 _LIBCPP_CONSTEXPR
Index: libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/deduction.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/deduction.pass.cpp
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+// UNSUPPORTED: libcpp-no-deduction-guides
+
+// template<class T> class shared_ptr
+
+// shared_ptr(weak_ptr<T>) -> shared_ptr<T>
+// shared_ptr(unique_ptr<T>) -> shared_ptr<T>
+
+#include <memory>
+
+struct A {};
+
+int main(int, char**)
+{
+  {
+    auto s0 = std::shared_ptr<A>(new A);
+    auto w = std::weak_ptr<A>(s0);
+    auto s = std::shared_ptr(w);
+  }
+  {
+    auto u = std::unique_ptr<A>(new A);
+    auto s = std::shared_ptr(std::move(u));
+  }
+
+  return 0;
+}
Index: libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.weak/util.smartptr.weak.const/shared_ptr_deduction.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.weak/util.smartptr.weak.const/shared_ptr_deduction.pass.cpp
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+// UNSUPPORTED: libcpp-no-deduction-guides
+
+// template<class T> class weak_ptr
+
+// weak_ptr(shared_ptr<T>) -> weak_ptr<T>
+
+#include <memory>
+
+struct A {};
+
+int main(int, char**)
+{
+  auto s = std::shared_ptr<A>(new A);
+  auto w = std::weak_ptr(s);
+
+  return 0;
+}