Dealias an applied match alias

Author: dwijnand

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

@@ -1441,9 +1441,12 @@ object Types extends TypeUtils {
     private def dealias1(keep: AnnotatedType => Context ?=> Boolean, keepOpaques: Boolean)(using Context): Type = this match {
       case tp: TypeRef =>
         if (tp.symbol.isClass) tp
+        else if keepOpaques && tp.symbol.is(Opaque) then tp
         else tp.info match {
-          case TypeAlias(alias) if !(keepOpaques && tp.symbol.is(Opaque)) =>
+          case TypeAlias(alias) =>
             alias.dealias1(keep, keepOpaques)
+          case MatchAlias(alias: AppliedType) =>
+            (alias: Type).dealias1(keep, keepOpaques)
           case _ => tp
         }
       case app @ AppliedType(tycon, _) =>

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -446,16 +446,13 @@ object Inlines:
                 evidence
           }
 
-        def unrollTupleTypes(tpe: Type): Option[List[Type]] = tpe.dealias match
+        def unrollTupleTypes(tpe: Type): Option[List[Type]] = tpe.dealias.normalized match
           case AppliedType(tycon, args) if defn.isTupleClass(tycon.typeSymbol) =>
             Some(args)
           case AppliedType(tycon, head :: tail :: Nil) if tycon.isRef(defn.PairClass) =>
             unrollTupleTypes(tail).map(head :: _)
           case tpe: TermRef if tpe.symbol == defn.EmptyTupleModule =>
             Some(Nil)
-          case tpRef: TypeRef => tpRef.info match
-            case MatchAlias(alias) => unrollTupleTypes(alias.tryNormalize)
-            case _ => None
           case _ =>
             None
 

tests/pos/i19821.scala

@@ -0,0 +1,23 @@
+type F[X] = X match { case String    => Option[Int] }
+type G[X] = X match { case Option[x] => Int         }
+
+trait T:
+  type S
+  val opt1: F[S]
+  val opt2: O1
+  type O1 = F[S]
+
+class Test:
+  def test: Unit =
+    val t: T { type S = String } = ???
+
+    val i1: G[t.opt1.type] = ???
+    val j1: Int = i1
+
+    val i2: G[t.opt2.type] = ???
+    val j2: Int = i2 // was:
+//[E007] Type Mismatch Error: tests/pos/i19821.scala:17:18 --------------------
+//   val j2: Int = i2
+//                 ^^
+//                 Found:    (i2 : G[(t.bar : t.O)])
+//                 Required: Int