Revert support for t11603

- it's not a real use case and it's hard to imagine one
- it does not work if the scrutinee and case are both singletons

Author: liufengyun

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

@@ -1020,12 +1020,6 @@ object Types {
       case _ => this
     }
 
-    /** Widen singleton type modulo constant types */
-    final def widenNonConstant(implicit ctx: Context): Type = this match {
-      case _: ConstantType => this
-      case _ => widen
-    }
-
     /** Widen type if it is unstable (i.e. an ExprType, or TermRef to unstable symbol */
     final def widenIfUnstable(implicit ctx: Context): Type = stripTypeVar match {
       case tp: ExprType => tp.resultType.widenIfUnstable

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -725,7 +725,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   def checkExhaustivity(_match: Match): Unit = {
     val Match(sel, cases) = _match
-    val selTyp = sel.tpe.widenNonConstant.dealias
+    val selTyp = sel.tpe.widen.dealias
 
     if (!exhaustivityCheckable(sel)) return
 
@@ -756,7 +756,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   def checkRedundancy(_match: Match): Unit = {
     val Match(sel, cases) = _match
-    val selTyp = sel.tpe.widenNonConstant.dealias
+    val selTyp = sel.tpe.widen.dealias
 
     if (!redundancyCheckable(sel)) return