Parser simple expression error recovery change from null to ??? (#…

…19103)

Previously, simpleExpr was recovered as `Literal(Constant(null))` which
led to some errors in interactive.

Type inference in Scala 3 works on whole chain, thus type vars were
inferred as union type of `Null` because of this very reason. Recovering
such errors as `unimplementedExpr` which has a type of `Nothing`, solves
the issue.

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

@@ -947,6 +947,7 @@ class Definitions {
   def TupleXXLModule(using Context): Symbol = TupleXXLClass.companionModule
 
     def TupleXXL_fromIterator(using Context): Symbol = TupleXXLModule.requiredMethod("fromIterator")
+    def TupleXXL_unapplySeq(using Context): Symbol = TupleXXLModule.requiredMethod(nme.unapplySeq)
 
   @tu lazy val RuntimeTupleMirrorTypeRef: TypeRef = requiredClassRef("scala.runtime.TupleMirror")
 

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

@@ -838,6 +838,7 @@ object SpaceEngine {
       val tpw = tp.widen.dealias
       val classSym = tpw.classSymbol
       classSym.is(Sealed) && !tpw.isLargeGenericTuple || // exclude large generic tuples from exhaustivity
+                                                         // requires an unknown number of changes to make work
       tpw.isInstanceOf[OrType] ||
       (tpw.isInstanceOf[AndType] && {
         val and = tpw.asInstanceOf[AndType]