Follow type aliases in widenAbstractTypes

SeklomType are already mapped over correctly, that is, as .info.

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

@@ -2150,11 +2150,20 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
  case _ =>
  cas
  }
- def widenAbstractTypes(tp: Type) = new TypeMap {
+ def widenAbstractTypes(tp: Type): Type = new TypeMap {
  def apply(tp: Type) = tp match {
- case tp: TypeRef if tp.symbol.isAbstractOrParamType | tp.symbol.isOpaqueAlias => WildcardType
+ case tp: TypeRef =>
+ if (tp.symbol.isAbstractOrParamType | tp.symbol.isOpaqueAlias)
+ WildcardType
+ else tp.info match {
+ case TypeAlias(alias) =>
+ val alias1 = widenAbstractTypes(alias)
+ if (alias1 ne alias) alias1 else tp
+ case _ => mapOver(tp)
+ }
+
  case tp: TypeVar if !tp.isInstantiated => WildcardType
- case _: SkolemType | _: TypeParamRef => WildcardType
+ case _: TypeParamRef => WildcardType
  case _ => mapOver(tp)
  }
  }.apply(tp)

tests/neg/6314.scala

@@ -71,3 +71,28 @@ object Test3 {
  def apply(fa: Bar[X & Foo]): Bar[Y & Foo] = fa
  }
 }
+
+object Test4 {
+ type Bar[A] = A match {
+ case X => String
+ case Y => Int
+ }
+
+ trait XX {
+ type Foo
+ type FooAlias = Foo
+
+ val a: Bar[X & FooAlias] = "hello"
+ val b: Bar[Y & FooAlias] = 1 // error
+
+ def apply(fa: Bar[X & FooAlias]): Bar[Y & FooAlias]
+
+ def boom: Int = apply(a) // error
+ }
+
+ trait YY extends XX {
+ type Foo = X & Y
+
+ def apply(fa: Bar[X & FooAlias]): Bar[Y & FooAlias] = fa
+ }
+}