Merge pull request #14058 from dotty-staging/fix-13997

Check variance of RHS of opaque type

Author: prolativ

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -378,6 +378,8 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           else (tree.rhs, sym.info) match
             case (rhs: LambdaTypeTree, bounds: TypeBounds) =>
               VarianceChecker.checkLambda(rhs, bounds)
+              if sym.isOpaqueAlias then
+                VarianceChecker.checkLambda(rhs, TypeBounds.upper(sym.opaqueAlias))
             case _ =>
           processMemberDef(super.transform(tree))
         case tree: New if isCheckable(tree) =>

tests/neg/i13997.scala

@@ -0,0 +1,10 @@
+  opaque type CovariantArray[+A] = Array[A] // error
+
+  object CovariantArray:
+    def crash() =
+      val stringArray: CovariantArray[String] = Array("foo", "bar")
+      val anyArray: CovariantArray[Any] = stringArray
+      anyArray(0) = 42
+      stringArray(0).length
+
+  @main def Test = CovariantArray.crash()