fix #3645: handle type alias in child instantiation

Author: liufengyun

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

@@ -594,8 +594,8 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
     // replace type parameter references with bounds
     val typeParamMap = new TypeMap {
-      def apply(t: Type): Type = t match {
-        case tp: TypeRef if tp.symbol.is(TypeParam) && tp.underlying.isInstanceOf[TypeBounds] =>
+      def apply(t: Type): Type = t.dealias match {
+        case tp: TypeRef if tp.underlying.isInstanceOf[TypeBounds] =>
           // See tests/patmat/gadt.scala  tests/patmat/exhausting.scala  tests/patmat/t9657.scala
           val exposed =
             if (variance == 0) newTypeVar(tp.underlying.bounds)

tests/patmat/3543.check

@@ -0,0 +1 @@
+20: Pattern Match Exhaustivity: KInt

tests/patmat/i3645.check

@@ -0,0 +1,26 @@
+object App {
+  def main(args: Array[String]): Unit = {
+    trait AgeT {
+      type T
+      def subst[F[_]](fa: F[Int]): F[T]
+    }
+
+    type Age = Age.T
+
+    val Age: AgeT = new AgeT {
+      type T = Int
+      def subst[F[_]](fa: F[Int]): F[T] = fa
+    }
+
+    sealed abstract class K[A]
+    final case object KAge extends K[Age]
+    final case object KInt extends K[Int]
+
+    val kint: K[Age] = Age.subst[K](KInt)
+    def get(k: K[Age]): String = k match {
+      case KAge => "Age"
+    }
+
+    get(kint)
+  }
+}