Merge pull request #7533 from dotty-staging/fix-#7532

Fix #7532: Do not search for ClassTag if abstract types are equal

Author: odersky

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -680,7 +680,7 @@ class Typer extends Namer
    *  @pre We are in pattern-matching mode (Mode.Pattern)
    */
   def tryWithClassTag(tree: Typed, pt: Type)(implicit ctx: Context): Tree = tree.tpt.tpe.dealias match {
-    case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper =>
+    case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper && !(tref =:= pt) =>
       require(ctx.mode.is(Mode.Pattern))
       inferImplicit(defn.ClassTagClass.typeRef.appliedTo(tref),
                     EmptyTree, tree.tpt.span)(ctx.retractMode(Mode.Pattern)) match {

tests/pos/i7532.scala

@@ -0,0 +1,18 @@
+
+class Tasty {
+  type Term
+  type Select <: Term
+
+  given scala.reflect.ClassTag[Term] = ???
+  given scala.reflect.ClassTag[Select] = ???
+  object Select {
+    def unapply(x: Select): Boolean = ???
+  }
+}
+
+object Foo {
+  def impl(given tasty: Tasty): Unit = {
+    import tasty.{_, given}
+    val Select() = (??? : Term)
+  }
+}