SI-5663: Tweak warnings on case class equals

Re-enable testing the sensibility of comparing instances
of two case classes.  In particular, warn if we detect that
the two objects inherit from different case classes.  In
addition, avoid emitting misleading warnings when comparing "new C".

3 files changed, 99 insertions, 0 deletions

diff --git a/test/files/neg/t5663-badwarneq.check b/test/files/neg/t5663-badwarneq.check
new file mode 100644
index 0000000000..00c2234e9d
--- /dev/null
+++ b/test/files/neg/t5663-badwarneq.check
@@ -0,0 +1,22 @@
+t5663-badwarneq.scala:42: error: comparing case class values of types Some[Int] and None.type using `==' will always yield false
+    println(new Some(1) == None) // Should complain on type, was: spuriously complains on fresh object
+                        ^
+t5663-badwarneq.scala:43: error: comparing case class values of types Some[Int] and Thing using `==' will always yield false
+    println(Some(1) == new Thing(1)) // Should complain on type, was: spuriously complains on fresh object
+                    ^
+t5663-badwarneq.scala:51: error: ThingOne and Thingy are unrelated: they will most likely never compare equal
+    println(t1 == t2) // true, but apparently unrelated, a compromise warning
+               ^
+t5663-badwarneq.scala:52: error: ThingThree and Thingy are unrelated: they will most likely never compare equal
+    println(t4 == t2) // true, complains because ThingThree is final and Thingy not a subclass, stronger claim than unrelated
+               ^
+t5663-badwarneq.scala:55: error: comparing case class values of types ThingTwo and Some[Int] using `==' will always yield false
+    println(t3 == Some(1)) // false, warn on different cases
+               ^
+t5663-badwarneq.scala:56: error: comparing values of types ThingOne and Cousin using `==' will always yield false
+    println(t1 == c) // should warn
+               ^
+t5663-badwarneq.scala:64: error: comparing case class values of types Simple and SimpleSibling.type using `==' will always yield false
+    println(new Simple() == SimpleSibling) // like Some(1) == None, but needn't be final case
+                         ^
+7 errors found
diff --git a/test/files/neg/t5663-badwarneq.flags b/test/files/neg/t5663-badwarneq.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/neg/t5663-badwarneq.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/neg/t5663-badwarneq.scala b/test/files/neg/t5663-badwarneq.scala
new file mode 100644
index 0000000000..56ec389c03
--- /dev/null
+++ b/test/files/neg/t5663-badwarneq.scala
@@ -0,0 +1,76 @@
+
+// alias
+trait Thingy
+
+class Gramps
+
+// sibling classes that extend a case class
+case class Thing(i: Int) extends Gramps
+class ThingOne(x:Int) extends Thing(x)
+class ThingTwo(y:Int) extends Thing(y) with Thingy
+final class ThingThree(z:Int) extends Thing(z)
+
+// not case cousin
+class Cousin extends Gramps
+
+class SimpleParent
+case class Simple() extends SimpleParent
+case object SimpleSibling extends SimpleParent
+
+/* It's not possible to run partest without -deprecation.
+ * Since detecting the warnings requires a neg test with
+ * -Xfatal-warnings, and deprecation terminates the compile,
+ * we'll just comment out the nasty part.  The point was
+ * just to show there's nothing special about a trait
+ * that extends a case class, which is only permitted
+ * (deprecatingly) by omitting the parens.
+ *
+// common ancestor is something else
+class AnyThing
+case class SomeThing extends AnyThing // deprecation
+class OtherThing extends AnyThing
+
+// how you inherit caseness doesn't matter
+trait InThing extends SomeThing
+class MyThing extends InThing
+*/
+
+object Test {
+  def main(a: Array[String]) {
+    // nothing to do with Gavin
+    println(new Some(1) == new Some(1)) // OK, true
+    println(new Some(1) == None) // Should complain on type, was: spuriously complains on fresh object
+    println(Some(1) == new Thing(1)) // Should complain on type, was: spuriously complains on fresh object
+
+    val t1 = new ThingOne(11)
+    val t2: Thingy = new ThingTwo(11)
+    val t3 = new ThingTwo(11)
+    val t4 = new ThingThree(11)
+    val c = new Cousin
+
+    println(t1 == t2) // true, but apparently unrelated, a compromise warning
+    println(t4 == t2) // true, complains because ThingThree is final and Thingy not a subclass, stronger claim than unrelated
+    println(t2 == t3) // OK, two Thingy
+    println(t3 == t2) // ditto with case receiver
+    println(t3 == Some(1)) // false, warn on different cases
+    println(t1 == c) // should warn
+
+    // don't warn on fresh cases
+    println(new ThingOne(11) == t1) // OK, was: two cases not warnable on trunk
+    println(new ThingTwo(11) == t2) // true, was: spuriously complains on fresh object
+    println(new ThingOne(11) == t3) // two cases not warnable on trunk
+    println(new ThingTwo(11) == t3) // ditto
+
+    println(new Simple() == SimpleSibling) // like Some(1) == None, but needn't be final case
+
+    /*
+    val mine = new MyThing
+    val some = new SomeThing
+    val other = new OtherThing
+    println(mine == some) // OK, two Something
+    println(some == mine)
+    println(mine == other) // OK, two Anything?
+    println(mine == t1) // false
+    */
+  }
+}