Fix scala#1463: no type cast in singelton equality test

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -839,7 +839,12 @@ object PatternMatcher {
           else
             If(emitCondition(plan).withPos(plan.pos), emit(plan.onSuccess), emit(plan.onFailure))
         case LetPlan(sym, body) =>
-          seq(ValDef(sym, initializer(sym).ensureConforms(sym.info)) :: Nil, emit(body))
+          sym.info match {
+            case tf: TermRef =>                            // check #1463, no type cast needed
+              seq(ValDef(sym, ref(tf)) :: Nil, emit(body))
+            case _ =>
+              seq(ValDef(sym, initializer(sym).ensureConforms(sym.info)) :: Nil, emit(body))
+          }
         case LabelledPlan(label, body, params) =>
           label.info = MethodType.fromSymbols(params, resultType)
           val labelDef = DefDef(label, Nil, params :: Nil, resultType, emit(labelled(label)))

tests/run/i1463.scala

@@ -0,0 +1,26 @@
+object Test {
+  case object Bob { override def equals(other: Any) = true }
+
+  class Bob2 {
+    override def equals(other: Any) = true
+  }
+  val Bob2 = new Bob2
+
+  def f0(x: Any) = x match { case Bob2 => Bob2 } // class cast exception at runtime, dotc only
+  def f1(x: Any) = x match { case Bob => Bob } // class cast exception at runtime, dotc only
+  def f2(x: Any): Bob.type = x match { case x @ Bob => x } // class cast exception at runtime, dotc and javac.
+
+  def main(args: Array[String]): Unit = {
+    assert(f0(Bob2) eq Bob2)
+    assert(f0(0) eq Bob2)  // only dotty fails here
+    assert(f0(Nil) eq Bob2)
+
+    assert(f1(Bob) eq Bob)
+    assert(f1(0) eq Bob)  // only dotty fails here
+    assert(f1(Nil) eq Bob)
+
+    assert(f2(Bob) eq Bob)
+    assert(f2(0) eq Bob) // both dotty and scalac fail here
+    assert(f2(Nil) eq Bob)
+  }
+}