Don't peoject nested wildcard pattern to nullable

Author: noti0na1

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

@@ -690,7 +690,6 @@ object SpaceEngine {
           else NoType
         }.filter(_.exists)
         parts
-
       case _ => ListOfNoType
     end rec
 

tests/warn/i20121.scala

@@ -0,0 +1,13 @@
+sealed trait T_A[A, B]
+type X = T_A[Byte, Byte]
+
+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 // warn: unreachable
+  case _       => 1
+    // for CC_B[A] to match T_A[X, X]
+    // A := X
+    // so require X, aka T_A[Byte, Byte]
+    // which isn't instantiable, outside of null

tests/warn/i20122.scala

@@ -0,0 +1,17 @@
+sealed trait T_B[C, D]
+
+case class CC_A()
+case class CC_B[A, C](a: A) extends T_B[C, CC_A]
+case class CC_C[C, D](a: T_B[C, D]) extends T_B[Int, CC_A]
+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 // warn: unreachable
+  case _                   => 1
+    // for CC_B[A, C] to match T_B[C, CC_A]
+    // C <: Int, ok
+    // A <: CC_E, ok
+    // but you need a CC_C[Char, Byte]
+    // which requires a T_B[Char, Byte]
+    // which isn't instantiable, outside of null

tests/warn/i20123.scala

@@ -0,0 +1,16 @@
+sealed trait T_A[A, B]
+sealed trait T_B[C]
+
+case class CC_D[A, C]() extends T_A[A, C]
+case class CC_E() extends T_B[Nothing]
+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 // warn: unreachable
+    // for CC_G[A, C] to match T_A[Boolean, T_B[Boolean]]
+    // A := Boolean, which is ok
+    // C := T_B[Boolean],
+    // which isn't instantiable, outside of null
+}