Fix #13455: Also consider sealed classes closed sums

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2526,17 +2526,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         case _ => false
       })
 
-    /** Can we enumerate all instantiations of this type? */
-    def isClosedSum(tp: Symbol): Boolean =
-      tp.is(Sealed) && tp.isOneOf(AbstractOrTrait) && !tp.hasAnonymousChild
-
-    /** Splits a closed type into a disjunction of smaller types.
-     *  It should hold that `tp` and `decompose(tp).reduce(_ or _)`
-     *  denote the same set of values.
-     */
-    def decompose(sym: Symbol, tp: Type): List[Type] =
-      sym.children.map(x => refineUsingParent(tp, x)).filter(_.exists)
-
     def fullyInstantiated(tp: Type): Boolean = new TypeAccumulator[Boolean] {
       override def apply(x: Boolean, t: Type) =
         x && {
@@ -2558,6 +2547,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case (tp1: TypeRef, tp2: TypeRef) if tp1.symbol.isClass && tp2.symbol.isClass =>
         val cls1 = tp1.classSymbol
         val cls2 = tp2.classSymbol
+        def isDecomposable(tp: Symbol): Boolean =
+           tp.is(Sealed) && !tp.hasAnonymousChild
+        def decompose(sym: Symbol, tp: Type): List[Type] =
+          sym.children.map(x => refineUsingParent(tp, x)).filter(_.exists)
         if (cls1.derivesFrom(cls2) || cls2.derivesFrom(cls1))
           false
         else
@@ -2570,9 +2563,13 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             // subtype, so they must be unrelated by single inheritance
             // of classes.
             true
-          else if (isClosedSum(cls1))
+          else if (isDecomposable(cls1))
+            // At this point, !cls1.derivesFrom(cls2): we know that direct
+            // instantiations of `cls1` (terms of the form `new cls1`) are not
+            // of type `tp2`. Therefore, we can safely decompose `cls1` using
+            // `.children`, even if `cls1` is non abstract.
             decompose(cls1, tp1).forall(x => provablyDisjoint(x, tp2))
-          else if (isClosedSum(cls2))
+          else if (isDecomposable(cls2))
             decompose(cls2, tp2).forall(x => provablyDisjoint(x, tp1))
           else
             false

tests/patmat/andtype-opentype-interaction.check

@@ -1,5 +1,5 @@
-23: Pattern Match Exhaustivity: _: Trait & OpenTrait, _: Clazz & OpenTrait, _: AbstractClass & OpenTrait, _: SealedClass & OpenTrait
-27: Pattern Match Exhaustivity: _: Trait & OpenTrait & OpenTrait2, _: Clazz & OpenTrait & OpenTrait2, _: AbstractClass & OpenTrait & OpenTrait2, _: SealedClass & OpenTrait & OpenTrait2
+23: Pattern Match Exhaustivity: _: Trait & OpenTrait, _: Clazz & OpenTrait, _: AbstractClass & OpenTrait
+27: Pattern Match Exhaustivity: _: Trait & OpenTrait & OpenTrait2, _: Clazz & OpenTrait & OpenTrait2, _: AbstractClass & OpenTrait & OpenTrait2
 31: Pattern Match Exhaustivity: _: Trait & OpenClass
 35: Pattern Match Exhaustivity: _: Trait & OpenTrait & OpenClass
 43: Pattern Match Exhaustivity: _: Trait & OpenAbstractClass

tests/pos/13455.scala

@@ -0,0 +1,10 @@
+sealed class R
+
+type X[T] = T match {
+  case R        => String
+  case (z => r) => Int
+}
+def x[T]: X[T] = ???
+
+def i(i0: Int): Unit = ???
+val a = i(x[Int => String])