handle ConstantType in TypeComparer

Author: Reto Hablützel

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -260,6 +260,12 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           secondTry(tp1, tp2)
       }
       compareErasedValueType
+    case ConstantType(v2) =>
+      def sameValue = tp1 match {
+        case ConstantType(v1) => v1.value == v2.value
+        case _ => secondTry(tp1, tp2)
+      }
+      sameValue
     case ErrorType =>
       true
     case _ =>

tests/repl/def.check

@@ -6,4 +6,8 @@ scala> def baz() = 2
 def baz(): Int
 scala> def qux(): Int = 2
 def qux(): Int
+scala> def id(x: 4): 4 = x
+def id(x: 4.type): Int(4)
+scala> id(4)
+val res0: Int = 4
 scala> :quit