TypeParameterUnusedInFormals: ignore methods that always return null …

…or throw.

It's OK for a method to declare that it can return any type as long as it never
returns anything, or always returns null. We check for casts to the return type
as a heuristic.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=84139059

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

@@ -39,6 +39,7 @@
 import com.sun.tools.javac.code.Types;
 import com.sun.tools.javac.tree.JCTree;
 import com.sun.tools.javac.tree.JCTree.JCIdent;
+import com.sun.tools.javac.tree.JCTree.JCTypeCast;
 import com.sun.tools.javac.tree.JCTree.JCTypeParameter;
 import com.sun.tools.javac.tree.TreeScanner;
 
@@ -110,6 +111,12 @@ public Description matchMethod(MethodTree tree, VisitorState state) {
       }
     }
 
+    // Ignore cases where the type is never used in the method body. Methods that claim to return
+    // 'any' type are OK if they only return null, or always throw.
+    if (!CastFinder.find(tree.getBody(), retType)) {
+      return Description.NO_MATCH;
+    }
+
     return attemptFix(retType, tree, state);
   }
 
@@ -261,4 +268,26 @@ public Void visitType(Type type, Void unused) {
       return null;
     }
   }
+
+  private static class CastFinder extends TreeScanner {
+
+    static boolean find(Tree tree, Type retType) {
+      CastFinder finder = new CastFinder(retType);
+      ((JCTree) tree).accept(finder);
+      return finder.found;
+    }
+
+    Type retType;
+    boolean found = false;
+
+    private CastFinder(Type retType) {
+      this.retType = retType;
+    }
+
+    @Override
+    public void visitTypeCast(JCTypeCast tree) {
+      found |= retType.tsym.equals(ASTHelpers.getSymbol(tree.clazz));
+      scan(tree.expr);
+    }
+  }
 }

core/src/test/java/com/google/errorprone/bugpatterns/TypeParameterUnusedInFormalsTest.java

@@ -23,7 +23,6 @@
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
-
 @RunWith(JUnit4.class)
 public class TypeParameterUnusedInFormalsTest {
 
@@ -157,4 +156,35 @@ public void okNotMyParam()  throws Exception {
             "}")
     );
   }
+
+  @Test
+  public void okAlwaysThrows() throws Exception {
+      compilationHelper.assertCompileSucceeds(
+        compilationHelper.fileManager().forSourceLines("Test.java",
+            "class Test {",
+            "  <T> T logAndThrow() { throw new RuntimeException(); }",
+            "}")
+    );
+  }
+
+  @Test
+  public void okAlwaysReturnsNull() throws Exception {
+      compilationHelper.assertCompileSucceeds(
+        compilationHelper.fileManager().forSourceLines("Test.java",
+            "class Test {",
+            "  <T> T nullFactory() { return null; }",
+            "}")
+    );
+  }
+
+  @Test
+  public void badCast() throws Exception {
+      compilationHelper.assertCompileSucceedsWithMessages(
+        compilationHelper.fileManager().forSourceLines("Test.java",
+            "class Test {",
+            "  // BUG: Diagnostic contains:",
+            "  <T> T nullFactory() { return (T) null; }",
+            "}")
+    );
+  }
 }