[base] Add ResultType to base::Callback

This change adds a public ResultType alias to base::Callback, specifying
the return type of `Run()`. This is analogous to std::function's nested
result_type.

Bug: 554299
Change-Id: Ie478345b68051b03cd5b064b0833d528d47990b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436536
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812087}

base/callback.h

@@ -57,6 +57,7 @@ namespace base {
 template <typename R, typename... Args>
 class OnceCallback<R(Args...)> : public internal::CallbackBase {
  public:
+  using ResultType = R;
   using RunType = R(Args...);
   using PolymorphicInvoke = R (*)(internal::BindStateBase*,
                                   internal::PassingType<Args>...);
@@ -103,6 +104,7 @@ class OnceCallback<R(Args...)> : public internal::CallbackBase {
 template <typename R, typename... Args>
 class RepeatingCallback<R(Args...)> : public internal::CallbackBaseCopyable {
  public:
+  using ResultType = R;
   using RunType = R(Args...);
   using PolymorphicInvoke = R (*)(internal::BindStateBase*,
                                   internal::PassingType<Args>...);

base/callback_unittest.cc

@@ -58,6 +58,24 @@ class CallbackTest : public ::testing::Test {
   RepeatingCallback<void()> null_callback_;
 };
 
+TEST_F(CallbackTest, Types) {
+  static_assert(std::is_same<void, OnceClosure::ResultType>::value, "");
+  static_assert(std::is_same<void(), OnceClosure::RunType>::value, "");
+
+  using OnceCallbackT = OnceCallback<double(int, char)>;
+  static_assert(std::is_same<double, OnceCallbackT::ResultType>::value, "");
+  static_assert(std::is_same<double(int, char), OnceCallbackT::RunType>::value,
+                "");
+
+  static_assert(std::is_same<void, RepeatingClosure::ResultType>::value, "");
+  static_assert(std::is_same<void(), RepeatingClosure::RunType>::value, "");
+
+  using RepeatingCallbackT = RepeatingCallback<bool(float, short)>;
+  static_assert(std::is_same<bool, RepeatingCallbackT::ResultType>::value, "");
+  static_assert(
+      std::is_same<bool(float, short), RepeatingCallbackT::RunType>::value, "");
+}
+
 // Ensure we can create unbound callbacks. We need this to be able to store
 // them in class members that can be initialized later.
 TEST_F(CallbackTest, DefaultConstruction) {