Allow base::IgnoreArgs to return a non-void type

Bug: None
Change-Id: Idac3691c9635d31ee54b91bb3193a16f091751be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5756150
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1336833}

base/functional/callback_helpers.h

@@ -122,41 +122,45 @@ SplitOnceCallback(OnceCallback<void(Args...)> callback) {
 // additional parameters. Returns a null callback if `callback` is null.
 //
 // Usage:
-//   void LogError(char* error_message) {
+//   bool LogError(char* error_message) {
 //     if (error_message) {
 //       cout << "Log: " << error_message << endl;
+//       return false;
 //     }
+//     return true;
 //   }
-//   base::RepeatingCallback<void(int, char*)> cb =
+//   base::RepeatingCallback<bool(int, char*)> cb =
 //      base::IgnoreArgs<int>(base::BindRepeating(&LogError));
-//   cb.Run(42, nullptr);
+//   CHECK_EQ(true, cb.Run(42, nullptr));
 //
 // Note in the example above that the type(s) passed to `IgnoreArgs`
 // represent the additional prepended parameters (those which will be
 // "ignored").
-template <typename... Preargs, typename... Args>
+template <typename... Preargs, typename... Args, typename R>
-RepeatingCallback<void(Preargs..., Args...)> IgnoreArgs(
+RepeatingCallback<R(Preargs..., Args...)> IgnoreArgs(
-    RepeatingCallback<void(Args...)> callback) {
+    RepeatingCallback<R(Args...)> callback) {
   return callback ? ::base::BindRepeating(
-                        [](RepeatingCallback<void(Args...)> callback,
+                        [](RepeatingCallback<R(Args...)> callback, Preargs...,
-                           Preargs..., Args... args) {
+                           Args... args) {
-                          std::move(callback).Run(std::forward<Args>(args)...);
+                          return std::move(callback).Run(
+                              std::forward<Args>(args)...);
                         },
                         std::move(callback))
-                  : RepeatingCallback<void(Preargs..., Args...)>();
+                  : RepeatingCallback<R(Preargs..., Args...)>();
 }
 
 // As above, but for OnceCallback.
-template <typename... Preargs, typename... Args>
+template <typename... Preargs, typename... Args, typename R>
-OnceCallback<void(Preargs..., Args...)> IgnoreArgs(
+OnceCallback<R(Preargs..., Args...)> IgnoreArgs(
-    OnceCallback<void(Args...)> callback) {
+    OnceCallback<R(Args...)> callback) {
   return callback ? ::base::BindOnce(
-                        [](OnceCallback<void(Args...)> callback, Preargs...,
+                        [](OnceCallback<R(Args...)> callback, Preargs...,
                            Args... args) {
-                          std::move(callback).Run(std::forward<Args>(args)...);
+                          return std::move(callback).Run(
+                              std::forward<Args>(args)...);
                         },
                         std::move(callback))
-                  : OnceCallback<void(Preargs..., Args...)>();
+                  : OnceCallback<R(Preargs..., Args...)>();
 }
 
 // ScopedClosureRunner is akin to std::unique_ptr<> for Closures. It ensures

base/functional/callback_helpers_unittest.cc

@@ -137,6 +137,10 @@ void IncrementWithRef(int& value) {
   value++;
 }
 
+int IncrementAndReturn(int* value) {
+  return ++(*value);
+}
+
 TEST(CallbackHelpersTest, ScopedClosureRunnerHasClosure) {
   base::ScopedClosureRunner runner1;
   EXPECT_FALSE(runner1);
@@ -362,6 +366,41 @@ TEST(CallbackHelpersTest, IgnoreArgs_EmptyCallback) {
   EXPECT_FALSE(once_int_cb);
 }
 
+TEST(CallbackHelpersTest, IgnoreArgs_NonVoidReturn) {
+  int count = 0;
+  base::RepeatingCallback<int(void)> repeating_no_param_cb =
+      base::BindRepeating(&IncrementAndReturn, &count);
+  base::OnceCallback<int(void)> once_no_param_cb =
+      base::BindOnce(&IncrementAndReturn, &count);
+
+  base::RepeatingCallback<int(int)> repeating_int_cb =
+      base::IgnoreArgs<int>(repeating_no_param_cb);
+  EXPECT_EQ(0, count);
+  EXPECT_EQ(1, repeating_int_cb.Run(42));
+  EXPECT_EQ(1, count);
+  EXPECT_EQ(2, repeating_int_cb.Run(42));
+  EXPECT_EQ(2, count);
+
+  base::OnceCallback<int(int)> once_int_cb =
+      base::IgnoreArgs<int>(std::move(once_no_param_cb));
+  EXPECT_EQ(2, count);
+  EXPECT_EQ(3, std::move(once_int_cb).Run(42));
+  EXPECT_EQ(3, count);
+
+  // Ignore only some (one) argument and forward the rest.
+  auto repeating_cb = base::BindRepeating(&IncrementAndReturn);
+  auto repeating_cb_with_extra_arg = base::IgnoreArgs<bool>(repeating_cb);
+  EXPECT_EQ(4, repeating_cb_with_extra_arg.Run(false, &count));
+  EXPECT_EQ(4, count);
+
+  // Ignore two arguments and forward the rest.
+  auto once_cb = base::BindOnce(&IncrementAndReturn);
+  auto once_cb_with_extra_arg =
+      base::IgnoreArgs<char, bool>(std::move(once_cb));
+  EXPECT_EQ(5, std::move(once_cb_with_extra_arg).Run('d', false, &count));
+  EXPECT_EQ(5, count);
+}
+
 TEST(CallbackHelpersTest, ForwardRepeatingCallbacks) {
   int count = 0;
   auto tie_cb =

docs/callback.md

@@ -716,13 +716,16 @@ designated callback type expects. The extra arguments can be ignored as long
 as they are leading.
 
 ```cpp
-void LogError(char* error_message) {
+bool LogError(char* error_message) {
-  if (error_message)
+  if (error_message) {
     cout << "Log: " << error_message << endl;
+    return false;
+  }
+  return true;
 }
-base::RepeatingCallback<void(int, char*)> cb =
+base::RepeatingCallback<bool(int, char*)> cb =
     base::IgnoreArgs<int>(base::BindRepeating(&LogError));
-cb.Run(42, nullptr);
+CHECK_EQ(true, cb.Run(42, nullptr));
 ```
 
 Note in the example above that the type(s) passed to `IgnoreArgs` represent