Add support for move-only and &&-qualified actions in WillOnce.

This provides a type-safe way for an action to express that it wants to be
called only once, or to capture move-only objects. It is a generalization of
the type system-evading hack in ByMove, with the improvement that it works for
_any_ action (including user-defined ones), and correctly expresses that the
action can only be used with WillOnce. I'll make existing actions benefit in a
future commit.

PiperOrigin-RevId: 440496139
Change-Id: I4145d191cca5655995ef41360bb126c123cb41d3

docs/reference/mocking.md

@@ -248,7 +248,9 @@ EXPECT_CALL(my_mock, GetNumber())
     .WillOnce(Return(3));
 ```
 
-The `WillOnce` clause can be used any number of times on an expectation.
+The `WillOnce` clause can be used any number of times on an expectation. Unlike
+`WillRepeatedly`, the action fed to each `WillOnce` call will be called at most
+once, so may be a move-only type and/or have an `&&`-qualified call operator.
 
 #### WillRepeatedly {#EXPECT_CALL.WillRepeatedly}
 

googlemock/include/gmock/gmock-actions.h

@@ -262,9 +262,65 @@ GMOCK_DEFINE_DEFAULT_ACTION_FOR_RETURN_TYPE_(double, 0);
 
 #undef GMOCK_DEFINE_DEFAULT_ACTION_FOR_RETURN_TYPE_
 
-// Simple two-arg form of std::disjunction.
+// Partial implementations of metaprogramming types from the standard library
-template <typename P, typename Q>
+// not available in C++11.
-using disjunction = typename ::std::conditional<P::value, P, Q>::type;
+
+template <typename P>
+struct negation
+    // NOLINTNEXTLINE
+    : std::integral_constant<bool, bool(!P::value)> {};
+
+// Base case: with zero predicates the answer is always true.
+template <typename...>
+struct conjunction : std::true_type {};
+
+// With a single predicate, the answer is that predicate.
+template <typename P1>
+struct conjunction<P1> : P1 {};
+
+// With multiple predicates the answer is the first predicate if that is false,
+// and we recurse otherwise.
+template <typename P1, typename... Ps>
+struct conjunction<P1, Ps...>
+    : std::conditional<bool(P1::value), conjunction<Ps...>, P1>::type {};
+
+template <typename...>
+struct disjunction : std::false_type {};
+
+template <typename P1>
+struct disjunction<P1> : P1 {};
+
+template <typename P1, typename... Ps>
+struct disjunction<P1, Ps...>
+    // NOLINTNEXTLINE
+    : std::conditional<!bool(P1::value), disjunction<Ps...>, P1>::type {};
+
+template <typename...>
+using void_t = void;
+
+// Like std::invoke_result_t from C++17, but works only for objects with call
+// operators (not e.g. member function pointers, which we don't need specific
+// support for in OnceAction because std::function deals with them).
+template <typename F, typename... Args>
+using call_result_t = decltype(std::declval<F>()(std::declval<Args>()...));
+
+template <typename Void, typename R, typename F, typename... Args>
+struct is_callable_r_impl : std::false_type {};
+
+// Specialize the struct for those template arguments where call_result_t is
+// well-formed. When it's not, the generic template above is chosen, resulting
+// in std::false_type.
+template <typename R, typename F, typename... Args>
+struct is_callable_r_impl<void_t<call_result_t<F, Args...>>, R, F, Args...>
+    : std::conditional<
+          std::is_same<R, void>::value,  //
+          std::true_type,                //
+          std::is_convertible<call_result_t<F, Args...>, R>>::type {};
+
+// Like std::is_invocable_r from C++17, but works only for objects with call
+// operators. See the note on call_result_t.
+template <typename R, typename F, typename... Args>
+using is_callable_r = is_callable_r_impl<void, R, F, Args...>;
 
 }  // namespace internal
 
@@ -596,6 +652,213 @@ inline PolymorphicAction<Impl> MakePolymorphicAction(const Impl& impl) {
 
 namespace internal {
 
+template <typename F>
+class TypedExpectation;
+
+// Specialized for function types below.
+template <typename F>
+class OnceAction;
+
+// An action that can only be used once.
+//
+// This is what is accepted by WillOnce, which doesn't require the underlying
+// action to be copy-constructible (only move-constructible), and promises to
+// invoke it as an rvalue reference. This allows the action to work with
+// move-only types like std::move_only_function in a type-safe manner.
+//
+// For example:
+//
+//     // Assume we have some API that needs to accept a unique pointer to some
+//     // non-copyable object Foo.
+//     void AcceptUniquePointer(std::unique_ptr<Foo> foo);
+//
+//     // We can define an action that provides a Foo to that API. Because It
+//     // has to give away its unique pointer, it must not be called more than
+//     // once, so its call operator is &&-qualified.
+//     struct ProvideFoo {
+//       std::unique_ptr<Foo> foo;
+//
+//       void operator()() && {
+//         AcceptUniquePointer(std::move(Foo));
+//       }
+//     };
+//
+//     // This action can be used with WillOnce.
+//     EXPECT_CALL(mock, Call)
+//         .WillOnce(ProvideFoo{std::make_unique<Foo>(...)});
+//
+//     // But a call to WillRepeatedly will fail to compile. This is correct,
+//     // since the action cannot correctly be used repeatedly.
+//     EXPECT_CALL(mock, Call)
+//         .WillRepeatedly(ProvideFoo{std::make_unique<Foo>(...)});
+//
+// A less-contrived example would be an action that returns an arbitrary type,
+// whose &&-qualified call operator is capable of dealing with move-only types.
+template <typename Result, typename... Args>
+class OnceAction<Result(Args...)> final {
+ private:
+  // True iff we can use the given callable type (or lvalue reference) directly
+  // via ActionAdaptor.
+  template <typename Callable>
+  using IsDirectlyCompatible = internal::conjunction<
+      // It must be possible to capture the callable in ActionAdaptor.
+      std::is_constructible<typename std::decay<Callable>::type, Callable>,
+      // The callable must be compatible with our signature.
+      internal::is_callable_r<Result, typename std::decay<Callable>::type,
+                              Args...>>;
+
+  // True iff we can use the given callable type via ActionAdaptor once we
+  // ignore incoming arguments.
+  template <typename Callable>
+  using IsCompatibleAfterIgnoringArguments = internal::conjunction<
+      // It must be possible to capture the callable in a lambda.
+      std::is_constructible<typename std::decay<Callable>::type, Callable>,
+      // The callable must be invocable with zero arguments, returning something
+      // convertible to Result.
+      internal::is_callable_r<Result, typename std::decay<Callable>::type>>;
+
+ public:
+  // Construct from a callable that is directly compatible with our mocked
+  // signature: it accepts our function type's arguments and returns something
+  // convertible to our result type.
+  template <typename Callable,
+            typename std::enable_if<
+                internal::conjunction<
+                    // Teach clang on macOS that we're not talking about a
+                    // copy/move constructor here. Otherwise it gets confused
+                    // when checking the is_constructible requirement of our
+                    // traits above.
+                    internal::negation<std::is_same<
+                        OnceAction, typename std::decay<Callable>::type>>,
+                    IsDirectlyCompatible<Callable>>  //
+                ::value,
+                int>::type = 0>
+  OnceAction(Callable&& callable)  // NOLINT
+      : action_(ActionAdaptor<typename std::decay<Callable>::type>(
+            {}, std::forward<Callable>(callable))) {}
+
+  // As above, but for a callable that ignores the mocked function's arguments.
+  template <typename Callable,
+            typename std::enable_if<
+                internal::conjunction<
+                    // Teach clang on macOS that we're not talking about a
+                    // copy/move constructor here. Otherwise it gets confused
+                    // when checking the is_constructible requirement of our
+                    // traits above.
+                    internal::negation<std::is_same<
+                        OnceAction, typename std::decay<Callable>::type>>,
+                    // Exclude callables for which the overload above works.
+                    // We'd rather provide the arguments if possible.
+                    internal::negation<IsDirectlyCompatible<Callable>>,
+                    IsCompatibleAfterIgnoringArguments<Callable>>::value,
+                int>::type = 0>
+  OnceAction(Callable&& callable)  // NOLINT
+                                   // Call the constructor above with a callable
+                                   // that ignores the input arguments.
+      : OnceAction(IgnoreIncomingArguments<typename std::decay<Callable>::type>{
+            std::forward<Callable>(callable)}) {}
+
+  // A fallback constructor for anything that is convertible to Action, for use
+  // with legacy actions that uses older styles like implementing
+  // ActionInterface or a conversion operator to Action. Modern code should
+  // implement a call operator with appropriate restrictions.
+  template <typename T,
+            typename std::enable_if<
+                internal::conjunction<
+                    // Teach clang on macOS that we're not talking about a
+                    // copy/move constructor here. Otherwise it gets confused
+                    // when checking the is_constructible requirement of our
+                    // traits above.
+                    internal::negation<
+                        std::is_same<OnceAction, typename std::decay<T>::type>>,
+                    // Exclude the overloads above, which we want to take
+                    // precedence.
+                    internal::negation<IsDirectlyCompatible<T>>,
+                    internal::negation<IsCompatibleAfterIgnoringArguments<T>>,
+                    // It must be possible to turn the object into an action of
+                    // the appropriate type.
+                    std::is_convertible<T, Action<Result(Args...)>>  //
+                    >::value,
+                int>::type = 0>
+  OnceAction(T&& action) : action_(std::forward<T>(action)) {}  // NOLINT
+
+  // We are naturally copyable because we store only an Action, but semantically
+  // we should not be copyable.
+  OnceAction(const OnceAction&) = delete;
+  OnceAction& operator=(const OnceAction&) = delete;
+  OnceAction(OnceAction&&) = default;
+
+ private:
+  // Allow TypedExpectation::WillOnce to use our type-unsafe API below.
+  friend class TypedExpectation<Result(Args...)>;
+
+  // An adaptor that wraps a callable that is compatible with our signature and
+  // being invoked as an rvalue reference so that it can be used as an
+  // Action. This throws away type safety, but that's fine because this is only
+  // used by WillOnce, which we know calls at most once.
+  template <typename Callable>
+  class ActionAdaptor final {
+   public:
+    // A tag indicating that the (otherwise universal) constructor is accepting
+    // the callable itself, instead of e.g. stealing calls for the move
+    // constructor.
+    struct CallableTag final {};
+
+    template <typename F>
+    explicit ActionAdaptor(CallableTag, F&& callable)
+        : callable_(std::make_shared<Callable>(std::forward<F>(callable))) {}
+
+    // Rather than explicitly returning Result, we return whatever the wrapped
+    // callable returns. This allows for compatibility with existing uses like
+    // the following, when the mocked function returns void:
+    //
+    //     EXPECT_CALL(mock_fn_, Call)
+    //         .WillOnce([&] {
+    //            [...]
+    //            return 0;
+    //         });
+    //
+    // This works with Action since such a callable can be turned into
+    // std::function<void()>. If we use an explicit return type of Result here
+    // then it *doesn't* work with OnceAction, because we'll get a "void
+    // function should not return a value" error.
+    //
+    // We need not worry about incompatible result types because the SFINAE on
+    // OnceAction already checks this for us. std::is_invocable_r_v itself makes
+    // the same allowance for void result types.
+    template <typename... ArgRefs>
+    internal::call_result_t<Callable, ArgRefs...> operator()(
+        ArgRefs&&... args) const {
+      return std::move(*callable_)(std::forward<ArgRefs>(args)...);
+    }
+
+   private:
+    // We must put the callable on the heap so that we are copyable, which
+    // Action needs.
+    std::shared_ptr<Callable> callable_;
+  };
+
+  // An adaptor that makes a callable that accepts zero arguments callable with
+  // our mocked arguments.
+  template <typename Callable>
+  struct IgnoreIncomingArguments {
+    internal::call_result_t<Callable> operator()(Args&&...) {
+      return std::move(callable)();
+    }
+
+    Callable callable;
+  };
+
+  // Return an Action that calls the underlying callable in a type-safe manner.
+  // The action's Perform method must be called at most once.
+  //
+  // This is the transition from a type-safe API to a type-unsafe one, since
+  // "must be called at most once" is no longer reflecting in the type system.
+  Action<Result(Args...)> ReleaseAction() && { return std::move(action_); }
+
+  Action<Result(Args...)> action_;
+};
+
 // Helper struct to specialize ReturnAction to execute a move instead of a copy
 // on return. Useful for move-only types, but could be used on any type.
 template <typename T>

googlemock/include/gmock/gmock-spec-builders.h

@@ -992,14 +992,16 @@ class TypedExpectation : public ExpectationBase {
     return After(s1, s2, s3, s4).After(s5);
   }
 
-  // Implements the .WillOnce() clause.
+  // Implements the .WillOnce() clause for copyable actions.
-  TypedExpectation& WillOnce(const Action<F>& action) {
+  TypedExpectation& WillOnce(OnceAction<F> once_action) {
     ExpectSpecProperty(last_clause_ <= kWillOnce,
                        ".WillOnce() cannot appear after "
                        ".WillRepeatedly() or .RetiresOnSaturation().");
     last_clause_ = kWillOnce;
 
-    untyped_actions_.push_back(new Action<F>(action));
+    untyped_actions_.push_back(
+        new Action<F>(std::move(once_action).ReleaseAction()));
+
     if (!cardinality_specified()) {
       set_cardinality(Exactly(static_cast<int>(untyped_actions_.size())));
     }

googlemock/test/gmock-actions_test.cc

@@ -55,37 +55,170 @@
 #include "gtest/gtest-spi.h"
 #include "gtest/gtest.h"
 
+namespace testing {
 namespace {
 
-using ::testing::_;
-using ::testing::Action;
-using ::testing::ActionInterface;
-using ::testing::Assign;
-using ::testing::ByMove;
-using ::testing::ByRef;
-using ::testing::DefaultValue;
-using ::testing::DoAll;
-using ::testing::DoDefault;
-using ::testing::IgnoreResult;
-using ::testing::Invoke;
-using ::testing::InvokeWithoutArgs;
-using ::testing::MakePolymorphicAction;
-using ::testing::PolymorphicAction;
-using ::testing::Return;
-using ::testing::ReturnNew;
-using ::testing::ReturnNull;
-using ::testing::ReturnRef;
-using ::testing::ReturnRefOfCopy;
-using ::testing::ReturnRoundRobin;
-using ::testing::SetArgPointee;
-using ::testing::SetArgumentPointee;
-using ::testing::Unused;
-using ::testing::WithArgs;
 using ::testing::internal::BuiltInDefaultValue;
 
-#if !GTEST_OS_WINDOWS_MOBILE
+TEST(TypeTraits, Negation) {
-using ::testing::SetErrnoAndReturn;
+  // Direct use with std types.
-#endif
+  static_assert(std::is_base_of<std::false_type,
+                                internal::negation<std::true_type>>::value,
+                "");
+
+  static_assert(std::is_base_of<std::true_type,
+                                internal::negation<std::false_type>>::value,
+                "");
+
+  // With other types that fit the requirement of a value member that is
+  // convertible to bool.
+  static_assert(std::is_base_of<
+                    std::true_type,
+                    internal::negation<std::integral_constant<int, 0>>>::value,
+                "");
+
+  static_assert(std::is_base_of<
+                    std::false_type,
+                    internal::negation<std::integral_constant<int, 1>>>::value,
+                "");
+
+  static_assert(std::is_base_of<
+                    std::false_type,
+                    internal::negation<std::integral_constant<int, -1>>>::value,
+                "");
+}
+
+// Weird false/true types that aren't actually bool constants (but should still
+// be legal according to [meta.logical] because `bool(T::value)` is valid), are
+// distinct from std::false_type and std::true_type, and are distinct from other
+// instantiations of the same template.
+//
+// These let us check finicky details mandated by the standard like
+// "std::conjunction should evaluate to a type that inherits from the first
+// false-y input".
+template <int>
+struct MyFalse : std::integral_constant<int, 0> {};
+
+template <int>
+struct MyTrue : std::integral_constant<int, -1> {};
+
+TEST(TypeTraits, Conjunction) {
+  // Base case: always true.
+  static_assert(std::is_base_of<std::true_type, internal::conjunction<>>::value,
+                "");
+
+  // One predicate: inherits from that predicate, regardless of value.
+  static_assert(
+      std::is_base_of<MyFalse<0>, internal::conjunction<MyFalse<0>>>::value,
+      "");
+
+  static_assert(
+      std::is_base_of<MyTrue<0>, internal::conjunction<MyTrue<0>>>::value, "");
+
+  // Multiple predicates, with at least one false: inherits from that one.
+  static_assert(
+      std::is_base_of<MyFalse<1>, internal::conjunction<MyTrue<0>, MyFalse<1>,
+                                                        MyTrue<2>>>::value,
+      "");
+
+  static_assert(
+      std::is_base_of<MyFalse<1>, internal::conjunction<MyTrue<0>, MyFalse<1>,
+                                                        MyFalse<2>>>::value,
+      "");
+
+  // Short circuiting: in the case above, additional predicates need not even
+  // define a value member.
+  struct Empty {};
+  static_assert(
+      std::is_base_of<MyFalse<1>, internal::conjunction<MyTrue<0>, MyFalse<1>,
+                                                        Empty>>::value,
+      "");
+
+  // All predicates true: inherits from the last.
+  static_assert(
+      std::is_base_of<MyTrue<2>, internal::conjunction<MyTrue<0>, MyTrue<1>,
+                                                       MyTrue<2>>>::value,
+      "");
+}
+
+TEST(TypeTraits, Disjunction) {
+  // Base case: always false.
+  static_assert(
+      std::is_base_of<std::false_type, internal::disjunction<>>::value, "");
+
+  // One predicate: inherits from that predicate, regardless of value.
+  static_assert(
+      std::is_base_of<MyFalse<0>, internal::disjunction<MyFalse<0>>>::value,
+      "");
+
+  static_assert(
+      std::is_base_of<MyTrue<0>, internal::disjunction<MyTrue<0>>>::value, "");
+
+  // Multiple predicates, with at least one true: inherits from that one.
+  static_assert(
+      std::is_base_of<MyTrue<1>, internal::disjunction<MyFalse<0>, MyTrue<1>,
+                                                       MyFalse<2>>>::value,
+      "");
+
+  static_assert(
+      std::is_base_of<MyTrue<1>, internal::disjunction<MyFalse<0>, MyTrue<1>,
+                                                       MyTrue<2>>>::value,
+      "");
+
+  // Short circuiting: in the case above, additional predicates need not even
+  // define a value member.
+  struct Empty {};
+  static_assert(
+      std::is_base_of<MyTrue<1>, internal::disjunction<MyFalse<0>, MyTrue<1>,
+                                                       Empty>>::value,
+      "");
+
+  // All predicates false: inherits from the last.
+  static_assert(
+      std::is_base_of<MyFalse<2>, internal::disjunction<MyFalse<0>, MyFalse<1>,
+                                                        MyFalse<2>>>::value,
+      "");
+}
+
+TEST(TypeTraits, IsInvocableRV) {
+  struct C {
+    int operator()() const { return 0; }
+    void operator()(int) & {}
+    std::string operator()(int) && { return ""; };
+  };
+
+  // The first overload is callable for const and non-const rvalues and lvalues.
+  // It can be used to obtain an int, void, or anything int is convertible too.
+  static_assert(internal::is_callable_r<int, C>::value, "");
+  static_assert(internal::is_callable_r<int, C&>::value, "");
+  static_assert(internal::is_callable_r<int, const C>::value, "");
+  static_assert(internal::is_callable_r<int, const C&>::value, "");
+
+  static_assert(internal::is_callable_r<void, C>::value, "");
+  static_assert(internal::is_callable_r<char, C>::value, "");
+
+  // It's possible to provide an int. If it's given to an lvalue, the result is
+  // void. Otherwise it is std::string (which is also treated as allowed for a
+  // void result type).
+  static_assert(internal::is_callable_r<void, C&, int>::value, "");
+  static_assert(!internal::is_callable_r<int, C&, int>::value, "");
+  static_assert(!internal::is_callable_r<std::string, C&, int>::value, "");
+  static_assert(!internal::is_callable_r<void, const C&, int>::value, "");
+
+  static_assert(internal::is_callable_r<std::string, C, int>::value, "");
+  static_assert(internal::is_callable_r<void, C, int>::value, "");
+  static_assert(!internal::is_callable_r<int, C, int>::value, "");
+
+  // It's not possible to provide other arguments.
+  static_assert(!internal::is_callable_r<void, C, std::string>::value, "");
+  static_assert(!internal::is_callable_r<void, C, int, int>::value, "");
+
+  // Nothing should choke when we try to call other arguments besides directly
+  // callable objects, but they should not show up as callable.
+  static_assert(!internal::is_callable_r<void, int>::value, "");
+  static_assert(!internal::is_callable_r<void, void (C::*)()>::value, "");
+  static_assert(!internal::is_callable_r<void, void (C::*)(), C*>::value, "");
+}
 
 // Tests that BuiltInDefaultValue<T*>::Get() returns NULL.
 TEST(BuiltInDefaultValueTest, IsNullForPointerTypes) {
@@ -1428,6 +1561,154 @@ TEST(MockMethodTest, CanTakeMoveOnlyValue) {
   EXPECT_EQ(42, *saved);
 }
 
+// It should be possible to use callables with an &&-qualified call operator
+// with WillOnce, since they will be called only once. This allows actions to
+// contain and manipulate move-only types.
+TEST(MockMethodTest, ActionHasRvalueRefQualifiedCallOperator) {
+  struct Return17 {
+    int operator()() && { return 17; }
+  };
+
+  // Action is directly compatible with mocked function type.
+  {
+    MockFunction<int()> mock;
+    EXPECT_CALL(mock, Call).WillOnce(Return17());
+
+    EXPECT_EQ(17, mock.AsStdFunction()());
+  }
+
+  // Action doesn't want mocked function arguments.
+  {
+    MockFunction<int(int)> mock;
+    EXPECT_CALL(mock, Call).WillOnce(Return17());
+
+    EXPECT_EQ(17, mock.AsStdFunction()(0));
+  }
+}
+
+// Edge case: if an action has both a const-qualified and an &&-qualified call
+// operator, there should be no "ambiguous call" errors. The &&-qualified
+// operator should be used by WillOnce (since it doesn't need to retain the
+// action beyond one call), and the const-qualified one by WillRepeatedly.
+TEST(MockMethodTest, ActionHasMultipleCallOperators) {
+  struct ReturnInt {
+    int operator()() && { return 17; }
+    int operator()() const& { return 19; }
+  };
+
+  // Directly compatible with mocked function type.
+  {
+    MockFunction<int()> mock;
+    EXPECT_CALL(mock, Call).WillOnce(ReturnInt()).WillRepeatedly(ReturnInt());
+
+    EXPECT_EQ(17, mock.AsStdFunction()());
+    EXPECT_EQ(19, mock.AsStdFunction()());
+    EXPECT_EQ(19, mock.AsStdFunction()());
+  }
+
+  // Ignores function arguments.
+  {
+    MockFunction<int(int)> mock;
+    EXPECT_CALL(mock, Call).WillOnce(ReturnInt()).WillRepeatedly(ReturnInt());
+
+    EXPECT_EQ(17, mock.AsStdFunction()(0));
+    EXPECT_EQ(19, mock.AsStdFunction()(0));
+    EXPECT_EQ(19, mock.AsStdFunction()(0));
+  }
+}
+
+// WillOnce should have no problem coping with a move-only action, whether it is
+// &&-qualified or not.
+TEST(MockMethodTest, MoveOnlyAction) {
+  // &&-qualified
+  {
+    struct Return17 {
+      Return17() = default;
+      Return17(Return17&&) = default;
+
+      Return17(const Return17&) = delete;
+      Return17 operator=(const Return17&) = delete;
+
+      int operator()() && { return 17; }
+    };
+
+    MockFunction<int()> mock;
+    EXPECT_CALL(mock, Call).WillOnce(Return17());
+    EXPECT_EQ(17, mock.AsStdFunction()());
+  }
+
+  // Not &&-qualified
+  {
+    struct Return17 {
+      Return17() = default;
+      Return17(Return17&&) = default;
+
+      Return17(const Return17&) = delete;
+      Return17 operator=(const Return17&) = delete;
+
+      int operator()() const { return 17; }
+    };
+
+    MockFunction<int()> mock;
+    EXPECT_CALL(mock, Call).WillOnce(Return17());
+    EXPECT_EQ(17, mock.AsStdFunction()());
+  }
+}
+
+// It should be possible to use an action that returns a value with a mock
+// function that doesn't, both through WillOnce and WillRepeatedly.
+TEST(MockMethodTest, ActionReturnsIgnoredValue) {
+  struct ReturnInt {
+    int operator()() const { return 0; }
+  };
+
+  MockFunction<void()> mock;
+  EXPECT_CALL(mock, Call).WillOnce(ReturnInt()).WillRepeatedly(ReturnInt());
+
+  mock.AsStdFunction()();
+  mock.AsStdFunction()();
+}
+
+// Despite the fanciness around move-only actions and so on, it should still be
+// possible to hand an lvalue reference to a copyable action to WillOnce.
+TEST(MockMethodTest, WillOnceCanAcceptLvalueReference) {
+  MockFunction<int()> mock;
+
+  const auto action = [] { return 17; };
+  EXPECT_CALL(mock, Call).WillOnce(action);
+
+  EXPECT_EQ(17, mock.AsStdFunction()());
+}
+
+// A callable that doesn't use SFINAE to restrict its call operator's overload
+// set, but is still picky about which arguments it will accept.
+struct StaticAssertSingleArgument {
+  template <typename... Args>
+  static constexpr bool CheckArgs() {
+    static_assert(sizeof...(Args) == 1, "");
+    return true;
+  }
+
+  template <typename... Args, bool = CheckArgs<Args...>()>
+  int operator()(Args...) const {
+    return 17;
+  }
+};
+
+// WillOnce and WillRepeatedly should both work fine with naïve implementations
+// of actions that don't use SFINAE to limit the overload set for their call
+// operator. If they are compatible with the actual mocked signature, we
+// shouldn't probe them with no arguments and trip a static_assert.
+TEST(MockMethodTest, ActionSwallowsAllArguments) {
+  MockFunction<int(int)> mock;
+  EXPECT_CALL(mock, Call)
+      .WillOnce(StaticAssertSingleArgument{})
+      .WillRepeatedly(StaticAssertSingleArgument{});
+
+  EXPECT_EQ(17, mock.AsStdFunction()(0));
+  EXPECT_EQ(17, mock.AsStdFunction()(0));
+}
+
 // Tests for std::function based action.
 
 int Add(int val, int& ref, int* ptr) {  // NOLINT
@@ -1552,7 +1833,8 @@ TEST(ActionMacro, LargeArity) {
                                    14, 15, 16, 17, 18, 19)));
 }
 
-}  // Unnamed namespace
+}  // namespace
+}  // namespace testing
 
 #ifdef _MSC_VER
 #if _MSC_VER == 1900