exhaust unit: consider Unit as sealed

one of the most boring sealed types imaginable, but a sealed type all the same

Author: Adriaan Moors

src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala

@@ -2350,16 +2350,19 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
 
     // exhaustivity
 
-    // make sure it's not a primitive, else (5: Byte) match { case 5 => ... } sees no Byte
-    // TODO: domain of feasibly enumerable built-in types (enums, char?)
+    // TODO: domain of other feasibly enumerable built-in types (char?)
     def enumerateSubtypes(tp: Type): Option[List[Type]] =
       tp.typeSymbol match {
+        // TODO case _ if tp.isTupleType => // recurse into component types?
+        case UnitClass =>
+          Some(List(UnitClass.tpe))
         case BooleanClass =>
           // patmatDebug("enum bool "+ tp)
           Some(List(ConstantType(Constant(true)), ConstantType(Constant(false))))
         // TODO case _ if tp.isTupleType => // recurse into component types
         case modSym: ModuleClassSymbol =>
           Some(List(tp))
+        // make sure it's not a primitive, else (5: Byte) match { case 5 => ... } sees no Byte
         case sym if !sym.isSealed || isPrimitiveValueClass(sym) =>
           // patmatDebug("enum unsealed "+ (tp, sym, sym.isSealed, isPrimitiveValueClass(sym)))
           None