fix false warning about Nil

scala.Nil refines scala.collection.immutable.Nil, this commit
follows redefinition of case objects.

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

@@ -51,13 +51,11 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
   private var _id = 0 // left for debuging
 
   override def transformMatch(tree: Match)(implicit ctx: Context, info: TransformerInfo): Tree = {
-    val Match(sel, cases) = tree
-
     val translated = new Translator()(ctx).translator.translateMatch(tree)
 
     // check exhaustivity and unreachability
     val engine = new SpaceEngine
-    if (engine.checkable(sel.tpe.widen.elimAnonymousClass)) {
+    if (engine.checkable(tree)) {
       engine.checkExhaustivity(tree)
       engine.checkRedundancy(tree)
     }

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

@@ -116,7 +116,7 @@ trait SpaceLogic {
       ss.exists(subspace(a, _)) ||
       (canDecompose(tp1) && subspace(Or(partitions(tp1)), b))
     case (Typ(tp1, _), Kon(tp2, ss)) =>
-      isSubType(tp1, tp2) && subspace(Kon(tp2, signature(tp2).map(tp => Typ(tp, false))), b)
+      isSubType(tp1, tp2) && subspace(Kon(tp2, signature(tp2).map(Typ(_, false))), b)
     case (Kon(tp1, ss), Typ(tp2, _)) =>
       isSubType(tp1, tp2) ||
       simplify(a) == Empty ||
@@ -188,7 +188,7 @@ trait SpaceLogic {
         minus(Or(partitions(tp1)), b)
       else a
     case (Typ(tp1, _), Kon(tp2, ss)) =>
-      if (isSubType(tp1, tp2)) minus(Kon(tp2, signature(tp2).map(tp => Typ(tp, false))), b)
+      if (isSubType(tp1, tp2)) minus(Kon(tp2, signature(tp2).map(Typ(_, false))), b)
       else if (isSubType(tp2, tp1) && canDecompose(tp1))
         minus(Or(partitions(tp1)), b)
       else a
@@ -242,15 +242,17 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
   def project(pat: Tree, roundUp: Boolean = true)(implicit ctx: Context): Space = pat match {
     case Literal(c) => Const(c, c.tpe)
     case _: BackquotedIdent => Var(pat.symbol, pat.tpe)
-    case Ident(_) =>
-      Typ(pat.tpe.stripAnnots, false)
-    case Select(_, _)  =>
-      if (pat.symbol.is(Module))
-        Typ(pat.tpe.stripAnnots, false)
-      else if (pat.symbol.is(Enum))
-        Const(Constant(pat.symbol), pat.tpe)
-      else
-        Var(pat.symbol, pat.tpe)
+    case Ident(_) | Select(_, _) =>
+      pat.tpe.stripAnnots match {
+        case tp: TermRef =>
+          if (pat.symbol.is(Enum))
+            Const(Constant(pat.symbol), tp)
+          else if (tp.underlyingIterator.exists(_.classSymbol.is(Module)))
+            Typ(tp.widenTermRefExpr.stripAnnots, false)
+          else
+            Var(pat.symbol, tp)
+        case tp => Typ(tp, false)
+      }
     case Alternative(trees) => Or(trees.map(project(_, roundUp)))
     case Bind(_, pat) => project(pat)
     case UnApply(_, _, pats) =>
@@ -295,6 +297,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           ktor
       }
 
+    // refine path-dependent type in params. refer to t9672
     meth.firstParamTypes.map(_.stripTypeVar).map(refine(tp, _))
   }
 
@@ -415,37 +418,42 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           if (mergeList) "_" else "List(_)"
         else if (tp.classSymbol.is(CaseClass))
         // use constructor syntax for case class
-          showType(tp) + signature(tp).map(_ => "_").mkString("(", ",", ")")
+          showType(tp) + signature(tp).map(_ => "_").mkString("(", ", ", ")")
         else if (signature(tp).nonEmpty)
-          tp.classSymbol.name + signature(tp).map(_ => "_").mkString("(", ",", ")")
+          tp.classSymbol.name + signature(tp).map(_ => "_").mkString("(", ", ", ")")
         else if (decomposed) "_: " + showType(tp)
         else "_"
       case Kon(tp, params) =>
         if (ctx.definitions.isTupleType(tp))
-          "(" + params.map(p => doShow(p)).mkString(", ") + ")"
+          "(" + params.map(doShow(_)).mkString(", ") + ")"
         else if (tp.widen.classSymbol.showFullName == "scala.collection.immutable.::")
-          if (mergeList) params.map(p => doShow(p, mergeList)).mkString(", ")
-          else params.map(p => doShow(p, true)).mkString("List(", ", ", ")")
+          if (mergeList) params.map(doShow(_, mergeList)).mkString(", ")
+          else params.map(doShow(_, true)).filter(_ != "Nil").mkString("List(", ", ", ")")
         else
-          showType(tp) + params.map(p => doShow(p)).mkString("(", ", ", ")")
+          showType(tp) + params.map(doShow(_)).mkString("(", ", ", ")")
       case Or(_) =>
         throw new Exception("incorrect flatten result " + s)
     }
 
     flatten(s).map(doShow(_, false)).distinct.mkString(", ")
   }
 
-  def checkable(tp: Type): Boolean = tp match {
-    case AnnotatedType(tp, annot) =>
-      (ctx.definitions.UncheckedAnnot != annot.symbol) && checkable(tp)
-    case _ => true  // actually everything is checkable unless @unchecked
+  def checkable(tree: Match): Boolean = {
+    def isCheckable(tp: Type): Boolean = tp match {
+      case AnnotatedType(tp, annot) =>
+        (ctx.definitions.UncheckedAnnot != annot.symbol) && isCheckable(tp)
+      case _ => true // actually everything is checkable unless @unchecked
 
       // tp.classSymbol.is(Sealed) ||
       //   tp.isInstanceOf[OrType] ||
       //   tp.classSymbol.is(Enum) ||
       //   Boolean
       //   Int
       //   ...
+    }
+
+    val Match(sel, cases) = tree
+    isCheckable(sel.tpe.widen.elimAnonymousClass)
   }
 
   def checkExhaustivity(_match: Match): Unit = {

tests/patmat/enum/expected.check

@@ -2,4 +2,8 @@
 It would fail on the following input: SATURDAY, FRIDAY, THURSDAY, SUNDAY
   day match {
       ^
-one warning found
+./tests/patmat/enum/patmat-enum.scala:15: warning: match may not be exhaustive.
+It would fail on the following input: SATURDAY, FRIDAY, THURSDAY
+  day match {
+      ^
+two warnings found

tests/patmat/enum/patmat-enum.scala

@@ -1,9 +1,21 @@
-object Test {
+object Test1 {
   val day: Day = ???
 
   day match {
     case Day.MONDAY => true
     case Day.TUESDAY => true
     case Day.WEDNESDAY => true
   }
+}
+
+object Test2 {
+  import Day._
+  val day: Day = ???
+
+  day match {
+    case MONDAY => true
+    case TUESDAY => true
+    case WEDNESDAY => true
+    case SUNDAY => true
+  }
 }

tests/patmat/patmat-adt.scala

@@ -50,4 +50,9 @@ object PatmatADT {
   def foo5(tree: Tree) : Any = tree match {
     case LetL(CharLit) =>
   }
+
+  def foo6[T](l: List[T]): Boolean = l match {
+    case x::xs => true
+    case Nil => false
+  }
 }

tests/patmat/patmat-indent.check

@@ -0,0 +1,13 @@
+./tests/patmat/patmat-indent.scala:9: warning: match may not be exhaustive.
+It would fail on the following input: Nil
+  def foo1a[T](l: List[T]) = l match {
+                               ^
+./tests/patmat/patmat-indent.scala:23: warning: match may not be exhaustive.
+It would fail on the following input: _: Boolean
+  def foo2(b: Boolean) = b match {
+                           ^
+./tests/patmat/patmat-indent.scala:27: warning: match may not be exhaustive.
+It would fail on the following input: _: Int
+  def foo3(x: Int) = x match {
+                       ^
+three warnings found

tests/patmat/patmat-indent.scala

@@ -0,0 +1,30 @@
+object Test {
+  val Nil = scala.Nil
+  val X = 5
+
+  object Inner {
+    val Y = false
+  }
+
+  def foo1a[T](l: List[T]) = l match {
+    case x::xs => false
+  }
+
+  def foo1b[T](l: List[T]) = l match {
+    case Nil => true
+    case x::xs => false
+  }
+
+  def foo1c[T](l: List[T]) = l match {
+    case Test.Nil => true
+    case x::xs => false
+  }
+
+  def foo2(b: Boolean) = b match {
+    case Inner.Y => false
+  }
+
+  def foo3(x: Int) = x match {
+    case X => 0
+  }
+}

tests/patmat/t7466.check

@@ -1,5 +1,5 @@
 ./tests/patmat/t7466.scala:8: warning: match may not be exhaustive.
-It would fail on the following input: (true, _), (false, _), (_, true), (_, false)
+It would fail on the following input: (_, _)
     (b1, b2) match {
              ^
 one warning found

tests/patmat/t9411a.scala

@@ -0,0 +1,27 @@
+object OhNoes {
+
+  sealed trait F
+  sealed abstract class FA extends F
+  sealed abstract class FB extends F
+
+  case object FA1 extends FA
+  case object FB1 extends FB
+  case object FB2 extends FB
+
+  sealed trait G
+  case object G1 extends G
+  case object G2 extends G
+
+  sealed trait H
+  case class H1(a: FB, b: G) extends H
+  case class H2(a: F)        extends H
+
+  val demo: H => Unit = {
+    case H1(FB1, G1) =>
+    case H1(FB2, G2) =>
+    case H2(_: FB) =>
+    case H2(_: FA) =>
+    case H1(FB1, G2) =>
+    case H1(FB2, G1) =>
+  }
+}

tests/patmat/t9411b.scala

@@ -0,0 +1,36 @@
+object OhNoes {
+
+  sealed trait F
+  sealed abstract class FA extends F
+  sealed abstract class FB extends F
+
+  case object FA1 extends FA
+  case object FB1 extends FB
+  case object FB2 extends FB
+
+  sealed trait G
+  case object G1 extends G
+  case object G2 extends G
+
+  sealed trait H
+  case class H1(a: FB, b: G) extends H
+  case class H2(b: F)        extends H
+
+  val demo: H => Unit = {
+    case H1(FB1, G1) =>
+    case H1(FB2, G2) =>
+    case H2(_: FB) =>
+    case H2(_: FA) =>
+    case H1(FB1, G2) =>
+    case H1(FB2, G1) =>
+  }
+
+  val demo2: H => Unit = {
+    case H2(_: FA) =>
+    case H2(_: FB) =>
+    case H1(FB1, G1) =>
+    case H1(FB2, G1) =>
+    case H1(FB1, G2) =>
+    case H1(FB2, G2) =>
+  }
+}

tests/patmat/virtpatmat_apply.check

@@ -0,0 +1,5 @@
+./tests/patmat/virtpatmat_apply.scala:2: warning: match may not be exhaustive.
+It would fail on the following input: List(_)
+  List(1, 2, 3) match {
+                ^
+one warning found

tests/patmat/virtpatmat_apply.scala

@@ -0,0 +1,7 @@
+object Test {
+  List(1, 2, 3) match {
+    case Nil => println("FAIL")
+    case x :: y :: xs if xs.length == 2 => println("FAIL")
+    case x :: y :: xs if xs.length == 1 => println("OK "+ y)
+  }
+}