Fix a bundle of patmat issues

Some of the issues are legitimate

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

@@ -528,8 +528,7 @@ object SpaceEngine {
           // force type inference to infer a narrower type: could be singleton
           // see tests/patmat/i4227.scala
           mt.paramInfos(0) <:< scrutineeTp
-          instantiateSelected(mt, tvars)
-          isFullyDefined(mt, ForceDegree.all)
+          maximizeType(mt.paramInfos(0), Spans.NoSpan)
           mt
     }
 
@@ -543,7 +542,7 @@ object SpaceEngine {
     // Case unapplySeq:
     // 1. return the type `List[T]` where `T` is the element type of the unapplySeq return type `Seq[T]`
 
-    val resTp = ctx.typeAssigner.safeSubstMethodParams(mt, scrutineeTp :: Nil).finalResultType
+    val resTp = wildApprox(ctx.typeAssigner.safeSubstMethodParams(mt, scrutineeTp :: Nil).finalResultType)
 
     val sig =
       if (resTp.isRef(defn.BooleanClass))
@@ -564,20 +563,14 @@ object SpaceEngine {
           if (arity > 0)
             productSelectorTypes(resTp, unappSym.srcPos)
           else {
-            val getTp = resTp.select(nme.get).finalResultType match
-              case tp: TermRef if !tp.isOverloaded =>
-                // Like widenTermRefExpr, except not recursively.
-                // For example, in i17184 widen Option[foo.type]#get
-                // to Option[foo.type] instead of Option[Int].
-                tp.underlying.widenExpr
-              case tp => tp
+            val getTp = extractorMemberType(resTp, nme.get, unappSym.srcPos)
             if (argLen == 1) getTp :: Nil
             else productSelectorTypes(getTp, unappSym.srcPos)
           }
         }
       }
 
-    sig.map(_.annotatedToRepeated)
+    sig.map { case tp: WildcardType => tp.bounds.hi case tp => tp }
   }
 
   /** Whether the extractor covers the given type */
@@ -623,7 +616,21 @@ object SpaceEngine {
         // For instance, from i15029, `decompose((X | Y).Field[T]) = [X.Field[T], Y.Field[T]]`.
         parts.map(tp.derivedAppliedType(_, targs))
 
-      case tp if tp.isDecomposableToChildren =>
+      case tpOriginal if tpOriginal.isDecomposableToChildren =>
+        // isDecomposableToChildren uses .classSymbol.is(Sealed)
+        // But that classSymbol could be from an AppliedType
+        // where the type constructor is a non-class type
+        // E.g. t11620 where `?1.AA[X]` returns as "sealed"
+        // but using that we're not going to infer A1[X] and A2[X]
+        // but end up with A1[<?>] and A2[<?>].
+        // So we widen (like AppliedType superType does) away
+        // non-class type constructors.
+        def getAppliedClass(tp: Type): Type = tp match
+          case tp @ AppliedType(_: HKTypeLambda, _)                        => tp
+          case tp @ AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass => tp
+          case tp @ AppliedType(tycon: TypeProxy, _)                       => getAppliedClass(tycon.superType.applyIfParameterized(tp.args))
+          case tp                                                          => tp
+        val tp = getAppliedClass(tpOriginal)
         def getChildren(sym: Symbol): List[Symbol] =
           sym.children.flatMap { child =>
             if child eq sym then List(sym) // i3145: sealed trait Baz, val x = new Baz {}, Baz.children returns Baz...

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
+}

tests/warn/i20128.scala

@@ -0,0 +1,9 @@
+sealed trait T_A[A]
+case class CC_B[A](a: T_A[A]) extends T_A[Byte]
+case class CC_E[A](b: T_A[A]) extends T_A[Byte]
+
+val v_a: T_A[Byte] = CC_E(CC_B(null))
+val v_b: Int = v_a match { // warn: not exhaustive
+  case CC_E(CC_E(_)) => 0
+  case CC_B(_)       => 1
+}

tests/warn/i20129.scala

@@ -0,0 +1,14 @@
+sealed trait T_A[A]
+case class CC_B[A](a: T_A[A], c: T_A[A]) extends T_A[Char]
+case class CC_C[A]() extends T_A[A]
+case class CC_G() extends T_A[Char]
+
+val v_a: T_A[Char] = CC_B(CC_G(), CC_C())
+val v_b: Int = v_a match { // warn: not exhaustive
+  case CC_C()                   => 0
+  case CC_G()                   => 1
+  case CC_B(CC_B(_, _), CC_C()) => 2
+  case CC_B(CC_C(), CC_C())     => 3
+  case CC_B(_, CC_G())          => 4
+  case CC_B(_, CC_B(_, _))      => 5
+}

tests/warn/i20130.scala

@@ -0,0 +1,11 @@
+sealed trait T_A[B]
+sealed trait T_B[C]
+case class CC_B[C]() extends T_A[T_B[C]]
+case class CC_C[B, C](c: T_A[B], d: T_B[C]) extends T_B[C]
+case class CC_E[C]() extends T_B[C]
+
+val v_a: T_B[Int] = CC_C(null, CC_E())
+val v_b: Int = v_a match { // warn: not exhaustive
+  case CC_C(_, CC_C(_, _)) => 0
+  case CC_E()              => 5
+}

tests/warn/i20131.scala

@@ -0,0 +1,17 @@
+sealed trait Foo
+case class Foo1() extends Foo
+case class Foo2[A, B]() extends Foo
+
+sealed trait Bar[A, B]
+case class Bar1[A, C, D](a: Bar[C, D])         extends Bar[A, Bar[C, D]]
+case class Bar2[   C, D](b: Bar[C, D], c: Foo) extends Bar[Bar1[Int, Byte, Int], Bar[C, D]]
+
+class Test:
+  def m1(bar: Bar[Bar1[Int, Byte, Int], Bar[Char, Char]]): Int = bar match
+    case Bar1(_)         => 0
+    case Bar2(_, Foo2()) => 1
+  def t1 = m1(Bar2(null, Foo1()))
+    // for Bar2[C, D] to match the scrutinee
+    // C := Char and D := Char
+    // which requires a Bar[Char, Char]
+    // which isn't instantiable, outside of null

tests/warn/i20132.alt.scala

@@ -0,0 +1,8 @@
+sealed trait Foo[A]
+case class Bar[C](x: Foo[C]) extends Foo[C]
+case class End[B]()          extends Foo[B]
+class Test:
+  def m1[M](foo: Foo[M]): Int = foo match // warn: not exhaustive
+    case End()      => 0
+    case Bar(End()) => 1
+  def t1 = m1[Int](Bar[Int](Bar[Int](End[Int]())))

tests/warn/i20132.scala

@@ -0,0 +1,8 @@
+sealed trait Foo[A]
+case class Bar[C](x: Foo[C]) extends Foo[Int]
+case class End[B]()          extends Foo[B]
+class Test:
+  def m1[M](foo: Foo[M]): Int = foo match // warn: not exhaustive
+    case End()      => 0
+    case Bar(End()) => 1
+  def t1 = m1[Int](Bar[Int](Bar[Int](End[Int]())))

tests/warn/i20132.wo.scala

@@ -0,0 +1,8 @@
+sealed trait Foo[A]
+case class Bar[C](x: Foo[C]) extends Foo[Int]
+case class End[B]()          extends Foo[B]
+class Test:
+  def m1[M](foo: Foo[M]): Int = foo match
+    case End()  => 0
+    case Bar(_) => 1
+  def t1 = m1[Int](Bar[Int](Bar[Int](End[Int]())))

tests/warn/i5422.scala

@@ -0,0 +1,9 @@
+sealed trait Foo[A[_]]
+
+case class Bar[C[_], X](x: C[X]) extends Foo[C]
+case class End[B[_]]()           extends Foo[B]
+
+class Test:
+  def foo[M[_]](foo: Foo[M]): Int = foo match
+    case End()  => 0
+    case Bar(_) => 1

tests/warn/t11620.scala

@@ -0,0 +1,9 @@
+sealed trait A[+T0]
+case class A1[+T1](t1: T1) extends A[T1]
+case class A2[+T2](t2: T2) extends A[T2]
+sealed trait B[+T3] { type AA[+U] <: A[U] ; def a: AA[T3] }
+object B { def unapply[T4](b: B[T4]): Some[b.AA[T4]] = Some(b.a) }
+class Test:
+  def m1[X](b: B[X]): X = b match
+    case B(A1(v1)) => v1
+    case B(A2(v2)) => v2