[Proposal] Remove CheckReturnValue on subscribe() overloads

src/main/java/io/reactivex/Completable.java

@@ -2318,7 +2318,7 @@ public final <E extends CompletableObserver> E subscribeWith(E observer) {
      * @return the Disposable that can be used for disposing the subscription asynchronously
      * @throws NullPointerException if either callback is null
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(final Action onComplete, final Consumer<? super Throwable> onError) {
         ObjectHelper.requireNonNull(onError, "onError is null");
@@ -2345,7 +2345,7 @@ public final Disposable subscribe(final Action onComplete, final Consumer<? supe
      * @param onComplete the runnable called when this Completable completes normally
      * @return the Disposable that allows disposing the subscription
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(final Action onComplete) {
         ObjectHelper.requireNonNull(onComplete, "onComplete is null");

src/main/java/io/reactivex/Flowable.java

@@ -14455,7 +14455,7 @@ public final Disposable subscribe() {
      *             if {@code onNext} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext) {
@@ -14486,7 +14486,7 @@ public final Disposable subscribe(Consumer<? super T> onNext) {
      *             if {@code onNext} is null, or
      *             if {@code onError} is null
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError) {
@@ -14520,7 +14520,7 @@ public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super T
      *             if {@code onComplete} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
@@ -14558,7 +14558,7 @@ public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super T
      *             if {@code onSubscribe} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @BackpressureSupport(BackpressureKind.SPECIAL)
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,

src/main/java/io/reactivex/Maybe.java

@@ -4095,7 +4095,7 @@ public final Disposable subscribe() {
      *             if {@code onSuccess} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onSuccess) {
         return subscribe(onSuccess, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION);
@@ -4121,7 +4121,7 @@ public final Disposable subscribe(Consumer<? super T> onSuccess) {
      *             if {@code onSuccess} is null, or
      *             if {@code onError} is null
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onSuccess, Consumer<? super Throwable> onError) {
         return subscribe(onSuccess, onError, Functions.EMPTY_ACTION);
@@ -4151,7 +4151,7 @@ public final Disposable subscribe(Consumer<? super T> onSuccess, Consumer<? supe
      *             if {@code onComplete} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onSuccess, Consumer<? super Throwable> onError,
             Action onComplete) {

src/main/java/io/reactivex/Observable.java

@@ -12079,7 +12079,7 @@ public final Disposable subscribe() {
      *             if {@code onNext} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext) {
         return subscribe(onNext, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION, Functions.emptyConsumer());
@@ -12105,7 +12105,7 @@ public final Disposable subscribe(Consumer<? super T> onNext) {
      *             if {@code onNext} is null, or
      *             if {@code onError} is null
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError) {
         return subscribe(onNext, onError, Functions.EMPTY_ACTION, Functions.emptyConsumer());
@@ -12135,7 +12135,7 @@ public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super T
      *             if {@code onComplete} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
             Action onComplete) {
@@ -12169,7 +12169,7 @@ public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super T
      *             if {@code onSubscribe} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
             Action onComplete, Consumer<? super Disposable> onSubscribe) {

src/main/java/io/reactivex/Single.java

@@ -3418,7 +3418,7 @@ public final Disposable subscribe() {
      * @throws NullPointerException
      *             if {@code onCallback} is null
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(final BiConsumer<? super T, ? super Throwable> onCallback) {
         ObjectHelper.requireNonNull(onCallback, "onCallback is null");
@@ -3446,7 +3446,7 @@ public final Disposable subscribe(final BiConsumer<? super T, ? super Throwable>
      *             if {@code onSuccess} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(Consumer<? super T> onSuccess) {
         return subscribe(onSuccess, Functions.ON_ERROR_MISSING);
@@ -3471,7 +3471,7 @@ public final Disposable subscribe(Consumer<? super T> onSuccess) {
      *             if {@code onSuccess} is null, or
      *             if {@code onError} is null
      */
-    @CheckReturnValue
+    @OptionalCheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(final Consumer<? super T> onSuccess, final Consumer<? super Throwable> onError) {
         ObjectHelper.requireNonNull(onSuccess, "onSuccess is null");

src/main/java/io/reactivex/annotations/OptionalCheckReturnValue.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.annotations;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Marks methods whose return values should be optionally checked by separate tooling.
+ * <p>History: 2.2.4 - experimental
+ * @since 2.2.4
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Documented
+@Target(ElementType.METHOD)
+public @interface OptionalCheckReturnValue {
+
+}

src/test/java/io/reactivex/validators/BaseTypeAnnotations.java

@@ -42,12 +42,18 @@ static void checkCheckReturnValueSupport(Class<?> clazz) {
                 continue;
             }
             if (m.getDeclaringClass() == clazz) {
-                boolean isSubscribeMethod = "subscribe".equals(m.getName()) && m.getParameterTypes().length == 0;
+                boolean isSubscribeMethod = "subscribe".equals(m.getName());
+                boolean isNoArgSubscribeMethod = isSubscribeMethod && m.getParameterTypes().length == 0;
                 boolean isAnnotationPresent = m.isAnnotationPresent(CheckReturnValue.class);
+                boolean isVoid = m.getReturnType().equals(Void.TYPE);
 
                 if (isSubscribeMethod) {
-                    if (isAnnotationPresent) {
-                        b.append("subscribe() method has @CheckReturnValue: ").append(m).append("\r\n");
+                    if (isNoArgSubscribeMethod) {
+                        if (isAnnotationPresent) {
+                            b.append("subscribe() method has @CheckReturnValue: ").append(m).append("\r\n");
+                        }
+                    } else if (!isVoid && !m.isAnnotationPresent(OptionalCheckReturnValue.class)) {
+                            b.append("subscribe() method missing @OptionalCheckReturnValue: ").append(m).append("\r\n");
                     }
                     continue;
                 }
@@ -57,7 +63,7 @@ static void checkCheckReturnValueSupport(Class<?> clazz) {
                     continue;
                 }
 
-                if (m.getReturnType().equals(Void.TYPE)) {
+                if (isVoid) {
                     if (isAnnotationPresent) {
                         b.append("Void method has @CheckReturnValue: ").append(m).append("\r\n");
                     }