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

@@ -20,7 +20,7 @@ object Test {
     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)
+    assert(f2(0) eq Bob) // both dotty and scalac fail here
+    assert(f2(Nil) eq Bob)
   }
 }