Make CacheLoaderNull check for more kinds of null returns.

core/src/main/java/com/google/errorprone/bugpatterns/ImpossibleNullComparison.java

@@ -241,7 +241,7 @@ private void handleSwitch(ExpressionTree expression, List<? extends CaseTree> ca
             .filter(unused -> getFixer(withoutParens, subState).isPresent())
             .ifPresent(
                 e ->
-                    // NOTE: This fix is possibly too big: you can write `case null, default ->`.
+                    // NOTE:  user ->`.
                     state.reportMatch(describeMatch(caseTree, SuggestedFix.delete(caseTree))));
       }
     }

core/src/main/java/com/google/errorprone/bugpatterns/TypeParameterNaming.java

@@ -225,7 +225,7 @@ private static String suggestedSingleLetter(String id, Tree tree) {
   // T -> T2
   // T2 -> T3
   // T -> T4 (if T2 and T3 already exist)
-  // TODO(user) : combine this method with TypeParameterShadowing.replacementTypeVarName
+  // TODO(siyuanl) : combine this method with TypeParameterShadowing.replacementTypeVarName
   private static String firstLetterReplacementName(String name, List<String> superTypeVars) {
     String firstLetterOfBase = Character.toString(name.charAt(0));
     int typeVarNum = 2;

core/src/main/java/com/google/errorprone/bugpatterns/nullness/CacheLoaderNull.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2019 The Error Prone Authors.
+ *
+ * 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 com.google.errorprone.bugpatterns.nullness;
+
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.fixes.SuggestedFix.emptyFix;
+
+import com.google.common.cache.CacheLoader;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.sun.source.tree.ExpressionTree;
+
+/** A {@link BugChecker}; see the associated {@link BugPattern} annotation for details. */
+@BugPattern(summary = "The result of CacheLoader#load must be non-null.", severity = WARNING)
+public final class CacheLoaderNull extends AbstractAsyncTypeReturnsNull {
+  public CacheLoaderNull() {
+    super(CacheLoader.class);
+  }
+
+  @Override
+  protected SuggestedFix provideFix(ExpressionTree tree) {
+    // The default suggestion is immediateFuture(null), which doesn't make sense for CacheLoader.
+    return emptyFix();
+  }
+}

core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java

@@ -65,7 +65,6 @@
 import com.google.errorprone.bugpatterns.BugChecker;
 import com.google.errorprone.bugpatterns.BugPatternNaming;
 import com.google.errorprone.bugpatterns.ByteBufferBackingArray;
-import com.google.errorprone.bugpatterns.CacheLoaderNull;
 import com.google.errorprone.bugpatterns.CannotMockFinalClass;
 import com.google.errorprone.bugpatterns.CannotMockMethod;
 import com.google.errorprone.bugpatterns.CanonicalDuration;
@@ -579,6 +578,7 @@
 import com.google.errorprone.bugpatterns.nullness.AddNullMarkedToPackageInfo;
 import com.google.errorprone.bugpatterns.nullness.AsyncCallableReturnsNull;
 import com.google.errorprone.bugpatterns.nullness.AsyncFunctionReturnsNull;
+import com.google.errorprone.bugpatterns.nullness.CacheLoaderNull;
 import com.google.errorprone.bugpatterns.nullness.DereferenceWithNullBranch;
 import com.google.errorprone.bugpatterns.nullness.EqualsBrokenForNull;
 import com.google.errorprone.bugpatterns.nullness.EqualsMissingNullable;

core/src/test/java/com/google/errorprone/bugpatterns/CacheLoaderNullTest.java → core/src/test/java/com/google/errorprone/bugpatterns/nullness/CacheLoaderNullTest.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package com.google.errorprone.bugpatterns;
+package com.google.errorprone.bugpatterns.nullness;
 
 import com.google.errorprone.CompilationTestHelper;
 import org.junit.Test;
@@ -45,6 +45,13 @@ public String load(String key) {
                     return null;
                   }
                 };
+                new CacheLoader<String, String>() {
+                  @Override
+                  public String load(String key) {
+                    // BUG: Diagnostic contains:
+                    return key.equals("") ? null : key;
+                  }
+                };
                 abstract class MyCacheLoader extends CacheLoader<String, String> {}
                 new MyCacheLoader() {
                   @Override
@@ -92,7 +99,7 @@ public String get() {
                         return null;
                       }
                     }
-                    ;
+
                     return "";
                   }
                 };