Fix isAliasType

Symbols that had the TypeParam flag set were classified as alias types unless they also had
the Deferred flag set. Maybe this did not break that much since Namer always added the Deferred
for type parameters. But export forwarders use synthesized parameters which did not have Deferred
set.

Author: odersky

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

@@ -689,7 +689,8 @@ object SymDenotations {
     final def isAbstractType(using Context): Boolean = this.is(DeferredType)
 
     /** Is this symbol an alias type? */
-    final def isAliasType(using Context): Boolean = isAbstractOrAliasType && !this.is(Deferred)
+    final def isAliasType(using Context): Boolean =
+      isAbstractOrAliasType && !isAbstractOrParamType
 
     /** Is this symbol an abstract or alias type? */
     final def isAbstractOrAliasType: Boolean = isType & !isClass

tests/neg/i20079/Lib_1.scala

@@ -0,0 +1,5 @@
+object Foo:
+  def xyz[A, CC[X] <: Iterable[X]](coll: CC[A]): Unit = ()
+
+object Bar:
+  export Foo.xyz

tests/neg/i20079/Test_2.scala

@@ -0,0 +1,6 @@
+object Test:
+  val ints = List(1)
+  Foo.xyz[Int, List](ints)
+  Foo.xyz[Int, scala.collection.View](ints) // error
+  Bar.xyz[Int, List](ints)
+  Bar.xyz[Int, scala.collection.View](ints) // error