Fix variance computation for parameterized type aliases

smarter · smarter · commit 983cd8568c68 · 2020-01-21T17:26:50.000+01:00
The variance of a type alias was taken to be the variance of its right-hand side, but that doesn't make sense for a parameterized type alias which is free to appear in any position: variance checking should only kick in when it is applied to something. It was possible to work around this by using a type lambda instead of a type alias, cats just had to do that: typelevel/cats#3264
diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -3076,7 +3076,15 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
   class AliasTypeSymbol protected[Symbols] (initOwner: Symbol, initPos: Position, initName: TypeName)
   extends TypeSymbol(initOwner, initPos, initName) {
     type TypeOfClonedSymbol = TypeSymbol
-    override def variance = if (isLocalToThis) Bivariant else info.typeSymbol.variance
+    override def variance =
+      // A non-applied parameterized type alias can appear in any variance position
+      if (typeParams.nonEmpty)
+        Invariant
+      else if (isLocalToThis)
+        Bivariant
+      else
+        info.typeSymbol.variance
+
     override def isContravariant = variance.isContravariant
     override def isCovariant     = variance.isCovariant
     final override def isAliasType = true
diff --git a/test/files/neg/variance-alias.scala b/test/files/neg/variance-alias.scala
@@ -0,0 +1,7 @@
+trait Inv[A]
+
+class X[+T] {
+  type Id[+A] = A
+  val a: Inv[({type L[+X] = X})#L[T]] = new Inv[({type L[+X] = X})#L[T]] {} // error
+  val b: Inv[Id[T]] = new Inv[Id[T]] {} // error
+}
diff --git a/test/files/pos/variance-alias.scala b/test/files/pos/variance-alias.scala
@@ -0,0 +1,6 @@
+trait Tc[F[_]]
+object X {
+  type Id[+A] = A
+  val a: Tc[({type L[+X] = X})#L] = new Tc[({type L[+X] = X})#L] {} // ok, therefore the following should be to:
+  val b: Tc[Id] = new Tc[Id] {} // ok
+}
