Fix $Gson$Types equals method for TypeVariable when its generic decla…

…ration is not a Class (#2599)

* Fix $Gson$Types equals method for TypeVariable when its generic declaration is not a Class

* Test $Gson$Types equals method with TypeVariable when its generic declaration is not a Class

* Add @SuppressWarnings in GsonTypesTest.java

gson/src/main/java/com/google/gson/internal/$Gson$Types.java

@@ -217,7 +217,7 @@ public static boolean equals(Type a, Type b) {
       }
       TypeVariable<?> va = (TypeVariable<?>) a;
       TypeVariable<?> vb = (TypeVariable<?>) b;
-      return va.getGenericDeclaration() == vb.getGenericDeclaration()
+      return Objects.equals(va.getGenericDeclaration(), vb.getGenericDeclaration())
           && va.getName().equals(vb.getName());
 
     } else {

gson/src/test/java/com/google/gson/internal/GsonTypesTest.java

@@ -19,6 +19,8 @@
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.assertThrows;
 
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Method;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.List;
@@ -98,4 +100,37 @@ public static Type getFirstTypeArgument(Type type) throws Exception {
     }
     return $Gson$Types.canonicalize(actualTypeArguments[0]);
   }
+
+  @Test
+  public void testEqualsOnMethodTypeVariables() throws Exception {
+    Method m1 = TypeVariableTest.class.getMethod("method");
+    Method m2 = TypeVariableTest.class.getMethod("method");
+
+    Type rt1 = m1.getGenericReturnType();
+    Type rt2 = m2.getGenericReturnType();
+
+    assertThat($Gson$Types.equals(rt1, rt2)).isTrue();
+  }
+
+  @Test
+  public void testEqualsOnConstructorParameterTypeVariables() throws Exception {
+    Constructor<TypeVariableTest> c1 = TypeVariableTest.class.getConstructor(Object.class);
+    Constructor<TypeVariableTest> c2 = TypeVariableTest.class.getConstructor(Object.class);
+
+    Type rt1 = c1.getGenericParameterTypes()[0];
+    Type rt2 = c2.getGenericParameterTypes()[0];
+
+    assertThat($Gson$Types.equals(rt1, rt2)).isTrue();
+  }
+
+  private static final class TypeVariableTest {
+
+    @SuppressWarnings({"UnusedMethod", "UnusedVariable", "TypeParameterUnusedInFormals"})
+    public <T> TypeVariableTest(T parameter) {}
+
+    @SuppressWarnings({"UnusedMethod", "UnusedVariable", "TypeParameterUnusedInFormals"})
+    public <T> T method() {
+      return null;
+    }
+  }
 }