EqualsIncompatibleType: ignore the Enum supertype when considering if…

… two

arguments share a non-Object supertype for warning about incompatible argument
types.

RELNOTES: EqualsIncompatibleType will report when enums of incompatible types
are passed to .equals()

MOE_MIGRATED_REVID=142612571

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

@@ -123,8 +123,10 @@ public boolean apply(MethodSymbol methodSymbol) {
     for (MethodSymbol method : overridesOfEquals) {
       ClassSymbol methodClass = method.enclClass();
       if (argumentClass.isSubClass(methodClass, types)
-          && !methodClass.equals(state.getSymtab().objectType.tsym)) {
-        // The type of the argument shares a superclass (other then java.lang.Object) or interface
+          && !methodClass.equals(state.getSymtab().objectType.tsym)
+          && !methodClass.equals(state.getSymtab().enumSym)) {
+        // The type of the argument shares a superclass
+        // (other then java.lang.Object or java.lang.Enum) or interface
         // with the receiver that implements an override of java.lang.Object.equals().
         return Description.NO_MATCH;
       }

core/src/test/java/com/google/errorprone/bugpatterns/testdata/EqualsIncompatibleTypePositiveCases.java

@@ -88,4 +88,20 @@ void checkEqualsDD1D2(D d, D1 d1, D2 d2) {
     // BUG: Diagnostic contains: incompatible types
     d2.equals(d1);
   }
+
+  enum MyEnum {}
+
+  enum MyOtherEnum {}
+
+  void enumEquals(MyEnum m, MyOtherEnum mm) {
+    // BUG: Diagnostic contains: incompatible types
+    m.equals(mm);
+    // BUG: Diagnostic contains: incompatible types
+    mm.equals(m);
+
+    // BUG: Diagnostic contains: incompatible types
+    com.google.common.base.Objects.equal(m, mm);
+    // BUG: Diagnostic contains: incompatible types
+    com.google.common.base.Objects.equal(mm, m);
+  }
 }