Fix #8702: Update outdated paragraph on MT variance

docs/docs/reference/new-types/match-types.md

@@ -214,14 +214,10 @@ type errors. If there is a stack overflow during subtyping, the exception will
 be caught and turned into a compile-time error that indicates a trace of the
 subtype tests that caused the overflow without showing a full stack trace.
 
-## Variance Laws for Match Types
 
-**Note:** This section does not reflect the current implementation.
+## Match Types Variance
 
-Within a match type `Match(S, Cs) <: B`, all occurrences of type variables count
-as covariant. By the nature of the cases `Ci` this means that occurrences in
-pattern position are contravariant (since patterns are represented as function
-type arguments).
+All type positions in a match type (scrutinee, patterns, bodies) are considered invariant.
 
 ## Related Work
 

tests/neg/8702.scala

@@ -0,0 +1,63 @@
+// All three All type positions in a match type (scrutinee, patterns, bodies)
+// are considered invariant, as showed by the following examples:
+
+trait TA[+Plus] { type M[X] = Plus match { case Int => String } } // error
+//               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//               covariant type Plus occurs in invariant position in type [X] =
+//                    Plus match {
+//                      case Int => String
+//                    } of type M
+
+trait TB[+Plus] { type M[X] = X match { case Plus => String } } // error
+//               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//               covariant type Plus occurs in invariant position in type [X] =
+//                 X match {
+//                   case Plus => String
+//                 } of type M
+
+trait TC[+Plus] { type M[X] = X match { case Int => Plus } } // error
+//               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//               covariant type Plus occurs in invariant position in type [X] =
+//                 X match {
+//                   case Int => Plus
+//                 } of type M
+
+trait TD[-Minus] { type M[X] = Minus match { case Int => String } } // error
+//                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//                contravariant type Minus occurs in invariant position in type [X] =
+//                  Minus match {
+//                    case Int => String
+//                  } of type M
+
+trait TE[-Minus] { type M[X] = X match { case Minus => String } } // error
+//                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//                contravariant type Minus occurs in invariant position in type [X] =
+//                  X match {
+//                    case Minus => String
+//                  } of type M
+
+trait TF[-Minus] { type M[X] = X match { case Int => Minus } } // error
+//                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//                contravariant type Minus occurs in invariant position in type [X] =
+//                  X match {
+//                    case Int => Minus
+//                  } of type M
+
+// Furthermore, both unannotated type parameters and unannotated type bindings
+// in patterns are invariant, as showed by the following examples:
+
+trait Cov[+X]
+trait Contra[-X]
+
+// As usual:
+type Test0[X] = Cov[X]     // OK
+type Test1[X] = Contra[X]  // OK
+type Test2[+X] = Contra[X] // error: covariant type parameter X occurs in
+                           // in contravariant position in Contra[X]
+type Test3[-X] = Cov[X]    // error: contravariant type parameter X occurs in
+                           // covariant position in Cov[X]
+
+type M0[X] = X match { case Int => Cov[X] }
+type M1[X] = X match { case Int => Contra[X] }
+type M2[X] = X match { case Cov[x] => Contra[x] }
+type M3[X] = X match { case Contra[x] => Cov[x] }