Use explicit bound in recursive match type

Author: OlivierBlanvillain

library/src/scala/Tuple.scala

@@ -75,7 +75,7 @@ sealed trait Tuple extends Any {
 object Tuple {
 
   /** Type of the head of a tuple */
-  type Head[X <: NonEmptyTuple] = X match {
+  type Head[X <: NonEmptyTuple] <: Any = X match {
     case x *: _ => x
   }
 
@@ -91,12 +91,14 @@ object Tuple {
   }
 
   /** Type of the element a position N in the tuple X */
-  type Elem[X <: Tuple, N <: Int] = X match {
+  type Elem[X <: Tuple, N <: Int] <: Any = X match {
     case x *: xs =>
-      N match {
-        case 0 => x
-        case S[n1] => Elem[xs, n1]
-      }
+      Elem0[x, xs, N]
+  }
+
+  type Elem0[x, xs, N] <: Any = N match {
+    case 0 => x
+    case S[n1] => Elem[xs, n1]
   }
 
   /** Literal constant Int size of a tuple */
@@ -139,19 +141,23 @@ object Tuple {
   /** Transforms a tuple `(T1, ..., Tn)` into `(T1, ..., Ti)`. */
   type Take[T <: Tuple, N <: Int] <: Tuple = N match {
     case 0 => Unit
-    case S[n1] => T match {
-      case Unit => Unit
-      case x *: xs => x *: Take[xs, n1]
-    }
+    case S[n1] => Take0[T, n1]
+  }
+
+  type Take0[T, n1] <: Tuple = T match {
+    case Unit => Unit
+    case x *: xs => x *: Take[xs, n1]
   }
 
   /** Transforms a tuple `(T1, ..., Tn)` into `(Ti+1, ..., Tn)`. */
   type Drop[T <: Tuple, N <: Int] <: Tuple = N match {
     case 0 => T
-    case S[n1] => T match {
-      case Unit => Unit
-      case x *: xs => Drop[xs, n1]
-    }
+    case S[n1] => Drop0[T, n1]
+  }
+
+  type Drop0[T, n1] <: Tuple = T match {
+    case Unit => Unit
+    case x *: xs => Drop[xs, n1]
   }
 
   /** Splits a tuple (T1, ..., Tn) into a pair of two tuples `(T1, ..., Ti)` and

tests/neg-custom-args/matchtype-loop.scala

@@ -1,8 +1,8 @@
 object Test {
-  type L[X] = X match {
+  type L[X] <: Any = X match {
     case Int => L[X]
   }
-  type LL[X] = X match {
+  type LL[X] <: Any = X match {
     case Int => LL[LL[X]]
   }
   def a: L[Boolean] = ???

tests/neg/matchtype-loop2.scala

@@ -1,8 +1,8 @@
 object Test {
-  type L[X] = X match {
+  type L[X] <: Any = X match {
     case Int => L[X]
   }
-  type LL[X] = X match {   // error: recursion limit exceeded
+  type LL[X] <: Any = X match {   // error: recursion limit exceeded
     case Int => LL[LL[X]]
   }
 }

tests/pos/6322.scala

@@ -7,7 +7,7 @@ object Test {
     def apply(x: T1): Unit
   }
 
-  type F[N] = N match {
+  type F[N] <: Any = N match {
     case A => F1[String]
     case B => F[A]
     case C => F[B]

tests/pos/6362.scala

@@ -1,5 +1,5 @@
 object Test {
-  type LeafElem[X] = X match {
+  type LeafElem[X] <: Any = X match {
     case String => Char
     case Array[t] => LeafElem[t]
     case Iterable[t] => LeafElem[t]

tests/pos/i5625.scala

@@ -1,6 +1,6 @@
 object Test {
 
-  type LeafElem[X] = X match {
+  type LeafElem[X] <: Any = X match {
     case String => Char
     case Array[t] => LeafElem[t]
     case Iterable[t] => LeafElem[t]
@@ -11,4 +11,4 @@ object Test {
   summon[LeafElem[Array[Int]] =:= Int]
   summon[LeafElem[Iterable[Int]] =:= Int]
   summon[LeafElem[Int] =:= Int]
-}
+}

tests/pos/i5625b.scala

@@ -2,7 +2,7 @@ object Test {
 
   type AV[t <: AnyVal] = t
 
-  type LeafElem[X] = X match {
+  type LeafElem[X] <: Any = X match {
     case String => Char
     case Array[t] => LeafElem[t]
     case Iterable[t] => LeafElem[t]
@@ -13,4 +13,4 @@ object Test {
   summon[LeafElem[Array[Int]] =:= Int]
   summon[LeafElem[Iterable[Int]] =:= Int]
   summon[LeafElem[Int] =:= Int]
-}
+}