Fix #3145: treat sealed classes and traits as inhabited

Sealed classes and traits are effectively inhabited in
practice. By treating them as inhabited by default,
we avoid spurious warnings that ignore anonymous classes.

When we have a mechanism to track anonymous children or
disallow anonymous children for sealed classes, we
could revert this change to make the check stronger.

Author: liufengyun

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

@@ -575,9 +575,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
       case tp: RefinedType =>
         recur(tp.parent)
       case tp: TypeRef =>
-        recur(tp.prefix) &&
-        !(tp.classSymbol.is(Sealed) && tp.classSymbol.is(AbstractOrTrait) && tp.classSymbol.children.isEmpty) &&
-        !(tp.classSymbol.is(AbstractFinal))
+        recur(tp.prefix) && !(tp.classSymbol.is(AbstractFinal))
       case _ =>
         true
     }
@@ -706,8 +704,10 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
   /** Abstract sealed types, or-types, Boolean and Java enums can be decomposed */
   def canDecompose(tp: Type): Boolean = {
     val dealiasedTp = tp.dealias
-    val res = tp.classSymbol.is(allOf(Abstract, Sealed)) ||
-      tp.classSymbol.is(allOf(Trait, Sealed)) ||
+    val res =
+      (tp.classSymbol.is(Sealed) &&
+        tp.classSymbol.is(AbstractOrTrait) &&
+        tp.classSymbol.children.nonEmpty ) ||
       dealiasedTp.isInstanceOf[OrType] ||
       (dealiasedTp.isInstanceOf[AndType] && {
         val and = dealiasedTp.asInstanceOf[AndType]

tests/patmat/3144.check

@@ -1 +1 @@
-7: Match case Unreachable
+6: Pattern Match Exhaustivity: _: Foo

tests/patmat/3144c.check

@@ -1 +1 @@
-7: Match case Unreachable
+6: Pattern Match Exhaustivity: _: Foo

tests/patmat/3145.scala

@@ -0,0 +1,43 @@
+object Test {
+  sealed trait Foo
+  class Bar(val s: String) extends Foo
+  sealed abstract class Baz(val s: String) extends Foo
+
+  val f: Foo => String = {
+    case bar: Bar => bar.s
+    case baz: Baz => baz.s
+  }
+}
+
+object Test2 {
+  sealed trait Foo
+  class Bar extends Foo
+  sealed trait Baz extends Foo
+
+  def f(x: Foo) = x match {
+    case bar: Bar => 1
+    case baz: Baz => 2
+  }
+}
+
+object Test3 {
+  sealed trait Foo
+  class Bar extends Foo
+  sealed trait Baz extends Foo
+
+  def foo = {
+    val x: Foo = new Baz {}
+    x match {
+      case bar: Bar => 1
+      case baz: Baz => 2
+    }
+  }
+
+  def bar = {
+    val x: Baz  = new Baz {}
+    x match {
+      case bar: Bar => 1
+      case baz: Baz => 2
+    }
+  }
+}

tests/patmat/andtype-opentype-interaction.check

@@ -1,7 +1,7 @@
-23: Pattern Match Exhaustivity: _: SealedClass & OpenTrait, _: AbstractClass & OpenTrait, _: Clazz & OpenTrait, _: Trait & OpenTrait
-27: Pattern Match Exhaustivity: _: SealedClass & OpenTrait & OpenTrait2, _: AbstractClass & OpenTrait & OpenTrait2, _: Clazz & OpenTrait & OpenTrait2, _: Trait & OpenTrait & OpenTrait2
-31: Pattern Match Exhaustivity: _: Trait & OpenClass
-35: Pattern Match Exhaustivity: _: Trait & OpenTrait & OpenClass
+23: Pattern Match Exhaustivity: _: SealedAbstractClass & OpenTrait, _: SealedClass & OpenTrait, _: SealedTrait & OpenTrait, _: AbstractClass & OpenTrait, _: Clazz & OpenTrait, _: Trait & OpenTrait
+27: Pattern Match Exhaustivity: _: SealedAbstractClass & OpenTrait & OpenTrait2, _: SealedClass & OpenTrait & OpenTrait2, _: SealedTrait & OpenTrait & OpenTrait2, _: AbstractClass & OpenTrait & OpenTrait2, _: Clazz & OpenTrait & OpenTrait2, _: Trait & OpenTrait & OpenTrait2
+31: Pattern Match Exhaustivity: _: SealedTrait & OpenClass, _: Trait & OpenClass
+35: Pattern Match Exhaustivity: _: SealedTrait & OpenTrait & OpenClass, _: Trait & OpenTrait & OpenClass
 40: Match case Unreachable
-43: Pattern Match Exhaustivity: _: Trait & OpenAbstractClass
-47: Pattern Match Exhaustivity: _: Trait & OpenClass & OpenTrait & OpenClassSubclass
+43: Pattern Match Exhaustivity: _: SealedTrait & OpenAbstractClass, _: Trait & OpenAbstractClass
+47: Pattern Match Exhaustivity: _: SealedTrait & OpenClass & OpenTrait & OpenClassSubclass, _: Trait & OpenClass & OpenTrait & OpenClassSubclass

tests/patmat/patmatexhaust.check

@@ -4,6 +4,7 @@
 49: Pattern Match Exhaustivity: _: Gp
 59: Pattern Match Exhaustivity: Nil
 75: Pattern Match Exhaustivity: _: B
+87: Pattern Match Exhaustivity: _: C1
 100: Pattern Match Exhaustivity: _: C1
 114: Pattern Match Exhaustivity: D2(), D1
 126: Pattern Match Exhaustivity: _: C1

tests/patmat/patmatexhaust.scala

@@ -84,7 +84,7 @@ class TestSealedExhaustive { // compile only
     case class C3() extends C
     case object C4 extends C
 
-    def ma10(x: C) = x match {  // exhaustive: abstract sealed C1 is dead end.
+    def ma10(x: C) = x match {  // treat abstract sealed C1 is as inhabited.
       case C3()     => true
       case C2 | C4  => true
     }

tests/patmat/t8511.check

@@ -1 +1 @@
-18: Pattern Match Exhaustivity: Baz(), Bar(_)
+18: Pattern Match Exhaustivity: EatsExhaustiveWarning(_), Baz(), Bar(_)