Backport "Fix a bundle of patmat issues" to 3.3.4 (#21549)

Backports #21000 and #21532 to 3.3.4-RC2. Fixes regression introduced in
RC1

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -35,13 +35,6 @@ class PatternMatcher extends MiniPhase {
 
   override def runsAfter: Set[String] = Set(ElimRepeated.name)
 
-  private val InInlinedCode = new util.Property.Key[Boolean]
-  private def inInlinedCode(using Context) = ctx.property(InInlinedCode).getOrElse(false)
-
-  override def prepareForInlined(tree: Inlined)(using Context): Context =
-    if inInlinedCode then ctx
-    else ctx.fresh.setProperty(InInlinedCode, true)
-
   override def transformMatch(tree: Match)(using Context): Tree =
     if (tree.isInstanceOf[InlineMatch]) tree
     else {
@@ -53,13 +46,10 @@ class PatternMatcher extends MiniPhase {
         case rt => tree.tpe
       val translated = new Translator(matchType, this).translateMatch(tree)
 
-      if !inInlinedCode then
+      // Skip analysis on inlined code (eg pos/i19157)
+      if !tpd.enclosingInlineds.nonEmpty then
         // check exhaustivity and unreachability
         SpaceEngine.checkMatch(tree)
-      else
-        // only check exhaustivity, as inlining may generate unreachable code 
-        // like in i19157.scala
-        SpaceEngine.checkMatchExhaustivityOnly(tree)
 
       translated.ensureConforms(matchType)
     }

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

@@ -3,7 +3,9 @@ package dotc
 package transform
 package patmat
 
-import core.*, Constants.*, Contexts.*, Decorators.*, Flags.*, Names.*, NameOps.*, StdNames.*, Symbols.*, Types.*
+import core.*
+import Constants.*, Contexts.*, Decorators.*, Flags.*, NullOpsDecorator.*, Symbols.*, Types.*
+import Names.*, NameOps.*, StdNames.*
 import ast.*, tpd.*
 import config.Printers.*
 import printing.{ Printer, * }, Texts.*
@@ -359,7 +361,7 @@ object SpaceEngine {
       val funRef = fun1.tpe.asInstanceOf[TermRef]
       if (fun.symbol.name == nme.unapplySeq)
         val (arity, elemTp, resultTp) = unapplySeqInfo(fun.tpe.widen.finalResultType, fun.srcPos)
-        if (fun.symbol.owner == defn.SeqFactoryClass && defn.ListType.appliedTo(elemTp) <:< pat.tpe)
+        if fun.symbol.owner == defn.SeqFactoryClass && pat.tpe.hasClassSymbol(defn.ListClass) then
           // The exhaustivity and reachability logic already handles decomposing sum types (into its subclasses)
           // and product types (into its components).  To get better counter-examples for patterns that are of type
           // List (or a super-type of list, like LinearSeq) we project them into spaces that use `::` and Nil.
@@ -531,14 +533,26 @@ object SpaceEngine {
     val mt: MethodType = unapp.widen match {
       case mt: MethodType => mt
       case pt: PolyType   =>
+          val locked = ctx.typerState.ownedVars
           val tvars = constrained(pt)
           val mt = pt.instantiate(tvars).asInstanceOf[MethodType]
           scrutineeTp <:< mt.paramInfos(0)
           // 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)
+          if !(ctx.typerState.ownedVars -- locked).isEmpty then
+            // constraining can create type vars out of wildcard types
+            // (in legalBound, by using a LevelAvoidMap)
+            // maximise will only do one pass at maximising the type vars in the target type
+            // which means we can maximise to types that include other type vars
+            // this fails TreeChecker's "non-empty constraint at end of $fusedPhase" check
+            // e.g. run-macros/string-context-implicits
+            // I can't prove that a second call won't also create type vars,
+            // but I'd rather have an unassigned new-new type var, than an infinite loop.
+            // After all, there's nothing strictly "wrong" with unassigned type vars,
+            // it just fails TreeChecker's linting.
+            maximizeType(mt.paramInfos(0), Spans.NoSpan)
           mt
     }
 
@@ -552,7 +566,7 @@ 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))
@@ -573,14 +587,14 @@ object SpaceEngine {
           if (arity > 0)
             productSelectorTypes(resTp, unappSym.srcPos)
           else {
-            val getTp = resTp.select(nme.get).finalResultType.widenTermRefExpr
+            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 */
@@ -618,14 +632,36 @@ object SpaceEngine {
       case tp if tp.classSymbol.isAllOf(JavaEnumTrait) => tp.classSymbol.children.map(_.termRef)
         // the class of a java enum value is the enum class, so this must follow SingletonType to not loop infinitely
 
-      case tp @ AppliedType(Parts(parts), targs) if tp.classSymbol.children.isEmpty =>
+      case Childless(tp @ AppliedType(Parts(parts), targs)) =>
         // It might not obvious that it's OK to apply the type arguments of a parent type to child types.
         // But this is guarded by `tp.classSymbol.children.isEmpty`,
         // meaning we'll decompose to the same class, just not the same type.
         // 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.
+        //
+        // Can't use `tpOriginal.baseType(cls)` because it causes
+        // i15893 to return exhaustivity warnings, because instead of:
+        //    <== refineUsingParent(N, class Succ, []) = Succ[<? <: NatT>]
+        //    <== isSub(Succ[<? <: NatT>] <:< Succ[Succ[<?>]]) = true
+        // we get
+        //    <== refineUsingParent(NatT, class Succ, []) = Succ[NatT]
+        //    <== isSub(Succ[NatT] <:< Succ[Succ[<?>]]) = false
+        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...
@@ -678,6 +714,12 @@ object SpaceEngine {
   final class PartsExtractor(val get: List[Type]) extends AnyVal:
     def isEmpty: Boolean = get == ListOfNoType
 
+  object Childless:
+    def unapply(tp: Type)(using Context): Result =
+      Result(if tp.classSymbol.children.isEmpty then tp else NoType)
+    class Result(val get: Type) extends AnyVal:
+      def isEmpty: Boolean = !get.exists
+
   /** Show friendly type name with current scope in mind
    *
    *  E.g.    C.this.B     -->  B     if current owner is C
@@ -774,12 +816,15 @@ object SpaceEngine {
     doShow(s)
   }
 
-  private def exhaustivityCheckable(sel: Tree)(using Context): Boolean = {
+  extension (self: Type) private def stripUnsafeNulls()(using Context): Type =
+    if Nullables.unsafeNullsEnabled then self.stripNull else self
+
+  private def exhaustivityCheckable(sel: Tree)(using Context): Boolean = trace(i"exhaustivityCheckable($sel ${sel.className})") {
     val seen = collection.mutable.Set.empty[Symbol]
 
     // Possible to check everything, but be compatible with scalac by default
-    def isCheckable(tp: Type): Boolean =
-      val tpw = tp.widen.dealias
+    def isCheckable(tp: Type): Boolean = trace(i"isCheckable($tp ${tp.className})"):
+      val tpw = tp.widen.dealias.stripUnsafeNulls()
       val classSym = tpw.classSymbol
       classSym.is(Sealed) && !tpw.isLargeGenericTuple || // exclude large generic tuples from exhaustivity
                                                          // requires an unknown number of changes to make work
@@ -814,7 +859,7 @@ object SpaceEngine {
   /** Return the underlying type of non-module, non-constant, non-enum case singleton types.
    *  Also widen ExprType to its result type, and rewrap any annotation wrappers.
    *  For example, with `val opt = None`, widen `opt.type` to `None.type`. */
-  def toUnderlying(tp: Type)(using Context): Type = trace(i"toUnderlying($tp)")(tp match {
+  def toUnderlying(tp: Type)(using Context): Type = trace(i"toUnderlying($tp ${tp.className})")(tp match {
     case _: ConstantType                            => tp
     case tp: TermRef if tp.symbol.is(Module)        => tp
     case tp: TermRef if tp.symbol.isAllOf(EnumCase) => tp
@@ -825,7 +870,7 @@ object SpaceEngine {
   })
 
   def checkExhaustivity(m: Match)(using Context): Unit = trace(i"checkExhaustivity($m)") {
-    val selTyp = toUnderlying(m.selector.tpe).dealias
+    val selTyp = toUnderlying(m.selector.tpe.stripUnsafeNulls()).dealias
     val targetSpace = trace(i"targetSpace($selTyp)")(project(selTyp))
 
     val patternSpace = Or(m.cases.foldLeft(List.empty[Space]) { (acc, x) =>
@@ -903,9 +948,6 @@ object SpaceEngine {
   }
 
   def checkMatch(m: Match)(using Context): Unit =
-    checkMatchExhaustivityOnly(m)
-    if reachabilityCheckable(m.selector) then checkReachability(m)
-
-  def checkMatchExhaustivityOnly(m: Match)(using Context): Unit =
     if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
+    if reachabilityCheckable(m.selector) then checkReachability(m)
 }

tests/warn/i15893.min.scala

@@ -0,0 +1,13 @@
+sealed trait NatT
+case class Zero()                 extends NatT
+case class Succ[+N <: NatT](n: N) extends NatT
+
+type Mod2[N <: NatT] <: NatT = N match
+  case Zero                  => Zero
+  case Succ[Zero]            => Succ[Zero]
+  case Succ[Succ[predPredN]] => Mod2[predPredN]
+
+def dependentlyTypedMod2[N <: NatT](n: N): Mod2[N] = n match
+  case Zero(): Zero                         => Zero() // warn
+  case Succ(Zero()): Succ[Zero]             => Succ(Zero()) // warn
+  case Succ(Succ(predPredN)): Succ[Succ[?]] => dependentlyTypedMod2(predPredN) // warn

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.list-Seq.scala

@@ -0,0 +1,10 @@
+class D1
+class D2
+
+class Test1:
+  type Ds = List[D1] | List[D2]
+  def m1(dss: List[Ds]) =
+    dss.flatMap:
+      case Seq(d)           => Some(1)
+      case Seq(head, tail*) => Some(2)
+      case Seq()            => None

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/i21218.scala

@@ -0,0 +1,10 @@
+def Test[U, A](thisElem: A, thatElem: U) = {
+  case object passedEnd
+  val any: Seq[Any] = ???
+  any.zip(any)
+    .map {
+      case (`passedEnd`, r: U @unchecked) => (thisElem, r)
+      case (l: A @unchecked, `passedEnd`) => (l, thatElem)
+      case t: (A, U) @unchecked           => t // false-positive warning
+    }
+}

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