diff --git a/llvm/include/llvm/ADT/STLFunctionalExtras.h b/llvm/include/llvm/ADT/STLFunctionalExtras.h
--- a/llvm/include/llvm/ADT/STLFunctionalExtras.h
+++ b/llvm/include/llvm/ADT/STLFunctionalExtras.h
@@ -56,6 +56,8 @@
       // This is not the copy-constructor.
       std::enable_if_t<!std::is_same<remove_cvref_t<Callable>,
                                      function_ref>::value> * = nullptr,
+      // callable is not convertible to bool
+      std::enable_if_t<!std::is_convertible<Callable, bool>::value> * = nullptr,
       // Functor must be callable and return a suitable type.
       std::enable_if_t<std::is_void<Ret>::value ||
                        std::is_convertible<decltype(std::declval<Callable>()(
@@ -64,6 +66,28 @@
       : callback(callback_fn<typename std::remove_reference<Callable>::type>),
         callable(reinterpret_cast<intptr_t>(&callable)) {}
 
+  template <typename Callable>
+  function_ref(
+      Callable &&callable,
+      // This is not the copy-constructor.
+      std::enable_if_t<!std::is_same<remove_cvref_t<Callable>,
+                                     function_ref>::value> * = nullptr,
+      // callable is convertible to bool
+      std::enable_if_t<std::is_convertible<Callable, bool>::value> * = nullptr,
+      // Functor must be callable and return a suitable type.
+      std::enable_if_t<std::is_void<Ret>::value ||
+                       std::is_convertible<decltype(std::declval<Callable>()(
+                                               std::declval<Params>()...)),
+                                           Ret>::value> * = nullptr)
+      : callback(callback_fn<typename std::remove_reference<Callable>::type>),
+        callable(reinterpret_cast<intptr_t>(&callable)) {
+    // Handles construction from an empty callable (for example a default
+    // constructed std::function) and ensure that bool conversion to `false` is
+    // propagated.
+    if (!callable)
+      callback = nullptr;
+  }
+
   Ret operator()(Params ...params) const {
     return callback(callable, std::forward<Params>(params)...);
   }
diff --git a/llvm/unittests/ADT/FunctionRefTest.cpp b/llvm/unittests/ADT/FunctionRefTest.cpp
--- a/llvm/unittests/ADT/FunctionRefTest.cpp
+++ b/llvm/unittests/ADT/FunctionRefTest.cpp
@@ -59,4 +59,11 @@
   EXPECT_EQ("string", returns([] { return "hello"; }));
 }
 
+TEST(FunctionRefTest, std_function) {
+  // Check that function_ref constructed from an empty std::function is empty.
+  std::function<void()> f;
+  function_ref<void()> f_ref(f);
+  ASSERT_FALSE((bool)f_ref);
+}
+
 } // namespace