Fix #21914: Don't project nested wildcard patterns to nullable (#21934)

Fix #21914

The wildcard patterns are projected to nullable #21850, which increases
the running time exponentially for complex patterns, due to the way we
simplify and minus spaces.

However, the current space analyse setup is only able to track nullable
value at top level, so we should not project nested wildcard patterns
(not at top level) to nullable.

The warnings in `tests/warn/i20121.scala`, `tests/warn/i20122.scala`,
and `tests/warn/i20123.scala` will become wrong again, but there is no
simple solution to fix them quickly.

I couldn't create a minimised test for #21914, but I have verified
locally the compile time becomes normal again with this fix.

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

@@ -337,13 +337,6 @@ object SpaceEngine {
     case pat: Ident if isBackquoted(pat) =>
       Typ(pat.tpe, decomposed = false)
 
-    case Ident(nme.WILDCARD) =>
-      val tp = pat.tpe.stripAnnots.widenSkolem
-      val isNullable = tp.isInstanceOf[FlexibleType] || tp.classSymbol.isNullableClass
-      val tpSpace = Typ(erase(tp, isValue = true), decomposed = false)
-      if isNullable then Or(tpSpace :: nullSpace :: Nil)
-      else tpSpace
-
     case Ident(_) | Select(_, _) =>
       Typ(erase(pat.tpe.stripAnnots.widenSkolem, isValue = true), decomposed = false)
 
@@ -716,7 +709,6 @@ object SpaceEngine {
           else NoType
         }.filter(_.exists)
         parts
-      case tp: FlexibleType => List(tp.underlying, ConstantType(Constant(null)))
       case _ => ListOfNoType
     end rec
 
@@ -939,11 +931,15 @@ object SpaceEngine {
       then project(OrType(selTyp, ConstantType(Constant(null)), soft = false))
       else project(selTyp)
     var hadNullOnly = false
+    def projectPat(pat: Tree): Space =
+      // Project toplevel wildcard pattern to nullable
+      if isNullable && isWildcardArg(pat) then Or(project(pat) :: nullSpace :: Nil)
+      else project(pat)
     @tailrec def recur(cases: List[CaseDef], prevs: List[Space], deferred: List[Tree]): Unit =
       cases match
         case Nil =>
         case CaseDef(pat, guard, _) :: rest =>
-          val curr = trace(i"project($pat)")(project(pat))
+          val curr = trace(i"project($pat)")(projectPat(pat))
           val covered = trace("covered")(simplify(intersect(curr, targetSpace)))
           val prev = trace("prev")(simplify(Or(prevs)))
           if prev == Empty && covered == Empty then // defer until a case is reachable

tests/patmat/i12530.check

@@ -1 +1,2 @@
 6: Match case Unreachable
+14: Match case Unreachable

tests/patmat/null.check

@@ -1,4 +1,4 @@
 6: Match case Unreachable
 13: Pattern Match
-18: Match case Unreachable
 20: Pattern Match
+21: Match case Unreachable

tests/patmat/null.scala

@@ -18,5 +18,6 @@ class Test {
     case Some(null)     =>
     case None           =>
     case y              =>
+    case _              =>
   }
 }

tests/warn/i20121.scala

@@ -5,8 +5,8 @@ case class CC_B[A](a: A) extends T_A[A, X]
 
 val v_a: T_A[X, X] = CC_B(null)
 val v_b = v_a match
-  case CC_B(_) => 0
-  case _       => 1 // warn: null only
+  case CC_B(_) => 0 // warn: unreachable
+  case _       => 1
     // for CC_B[A] to match T_A[X, X]
     // A := X
     // so require X, aka T_A[Byte, Byte]

tests/warn/i20122.scala

@@ -7,7 +7,7 @@ case class CC_E(a: CC_C[Char, Byte])
 
 val v_a: T_B[Int, CC_A] = CC_B(CC_E(CC_C(null)))
 val v_b = v_a match
-  case CC_B(CC_E(CC_C(_))) => 0
+  case CC_B(CC_E(CC_C(_))) => 0 // warn: unreachable
   case _                   => 1
     // for CC_B[A, C] to match T_B[C, CC_A]
     // C <: Int, ok

tests/warn/i20123.scala

@@ -8,7 +8,7 @@ case class CC_G[A, C](c: C) extends T_A[A, C]
 val v_a: T_A[Boolean, T_B[Boolean]] = CC_G(null)
 val v_b = v_a match {
   case CC_D()  => 0
-  case CC_G(_) => 1
+  case CC_G(_) => 1 // warn: unreachable
     // for CC_G[A, C] to match T_A[Boolean, T_B[Boolean]]
     // A := Boolean, which is ok
     // C := T_B[Boolean],