Don't expand match aliases in variance checking

They might be recursive, so we can't blindly expand them for variance checking.
Instead use normalize to follow possible reductions.

Author: odersky

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

@@ -34,7 +34,7 @@ object VarianceChecker {
           val paramVarianceStr = if (v == 0) "contra" else "co"
           val occursStr = variance match {
             case -1 => "contra"
-            case 0 => "non"
+            case 0 => "in"
             case 1 => "co"
           }
           val pos = tree.tparams
@@ -123,18 +123,19 @@ class VarianceChecker()(implicit ctx: Context) {
     def apply(status: Option[VarianceError], tp: Type): Option[VarianceError] = trace(s"variance checking $tp of $base at $variance", variances) {
       try
         if (status.isDefined) status
-        else tp match {
+        else tp.normalized match {
           case tp: TypeRef =>
             val sym = tp.symbol
             if (sym.variance != 0 && base.isContainedIn(sym.owner)) checkVarianceOfSymbol(sym)
-            else if (sym.isAliasType) this(status, sym.info.bounds.hi)
-            else foldOver(status, tp)
+            else sym.info match {
+              case MatchAlias(_) => foldOver(status, tp)
+              case TypeAlias(alias) => this(status, alias)
+              case _ => foldOver(status, tp)
+            }
           case tp: MethodOrPoly =>
             this(status, tp.resultType) // params will be checked in their TypeDef or ValDef nodes.
           case AnnotatedType(_, annot) if annot.symbol == defn.UncheckedVarianceAnnot =>
             status
-          case tp: MatchType =>
-            apply(status, tp.bound)
           case tp: ClassInfo =>
             foldOver(status, tp.classParents)
           case _ =>

tests/neg/i6047.scala

@@ -1,43 +1,36 @@
-type Id[+X] = X match { // error: covariant type X appears in nonvariant position
+type Id[+X] = X match { // error: covariant type X appears in invariant position
   case Int => Int
   case String => String
 }
-type T1[+X, Y] = Y match {  // error: covariant type X appears in nonvariant position
+type T1[+X, Y] = Y match {  // error: covariant type X appears in invariant position
   case List[X] => Int
   case _ => String
 }
-type T2[+X, Y] = Y match { // error: covariant type X appears in nonvariant position
+type T2[+X, Y] = Y match { // error: covariant type X appears in invariant position
   case List[_ >: X] => Int
   case _ => String
 }
-type T3[+X, Y] = Y match { // error: covariant type X appears in nonvariant position
+type T3[+X, Y] = Y match { // error: covariant type X appears in invariant position
   case List[_ <: X] => Int
   case _ => String
 }
-type Id2[-X] = X match { // error: contravariant type X appears in nonvariant position
+type Id2[-X] = X match { // error: contravariant type X appears in invariant position
   case Int => Int
   case String => String
 }
-type T4[+X, Y] = Y match {  // error: contravariant type X appears in nonvariant position
+type T4[-X, Y] = Y match {  // error: contravariant type X appears in invariant position
   case List[X] => Int
   case _ => String
 }
-type T5[+X, Y] = Y match { // error: contravariant type X appears in nonvariant position
+type T5[-X, Y] = Y match { // error: contravariant type X appears in invariant position
   case List[_ >: X] => Int
   case _ => String
 }
-type T6[+X, Y] = Y match { // error: contravariant type X appears in nonvariant position
+type T6[-X, Y] = Y match { // error: contravariant type X appears in invariant position
   case List[_ <: X] => Int
   case _ => String
 }
 type T7[-X, Y] = Y match { // error: contravariant type X appears in covariant position
   case List[_] => X
   case _ => String
 }
-
-class C[+X] {
-  type Id = X match {
-    case Int => Int
-    case String => String
-  }
-}

tests/neg/i6047a.scala

@@ -0,0 +1,45 @@
+class C[+X] {
+  type Id = X match {   // error: covariant type X appears in invariant position
+    case Int => Int
+    case String => String
+  }
+
+  type T1[Y] = Y match {  // error: covariant type X appears in invariant position
+    case List[X] => Int
+    case _ => String
+  }
+  type T2[Y] = Y match { // error: covariant type X appears in invariant position
+    case List[_ >: X] => Int
+    case _ => String
+  }
+  type T3[Y] = Y match { // error: covariant type X appears in invariant position
+    case List[_ <: X] => Int
+    case _ => String
+  }
+
+  def foo[Y <: X](): Unit = ??? // error: covariant type X appears in contravariant position
+  type Foo[Y <: X] <: Y  // error: covariant type X appears in contravariant position
+}
+class D[-X] {
+
+  type Id2 = X match { // error: contravariant type X appears in invariant position
+    case Int => Int
+    case String => String
+  }
+  type T4[Y] = Y match {  // error: contravariant type X appears in invariant position
+    case List[X] => Int
+    case _ => String
+  }
+  type T5[Y] = Y match { // error: contravariant type X appears in invariant position
+    case List[_ >: X] => Int
+    case _ => String
+  }
+  type T6[Y] = Y match { // error: contravariant type X appears in invariant position
+    case List[_ <: X] => Int
+    case _ => String
+  }
+  type T7[Y] = Y match { // error: contravariant type X appears in covariant position
+    case List[_] => X
+    case _ => String
+  }
+}