Allow Tuple.Head and Tuple.Tail to accept any Tuple

Author: Decel

compiler/test/dotc/pos-test-pickling.blacklist

@@ -21,6 +21,8 @@ i15181.scala
 i15922.scala
 t5031_2.scala
 i16997.scala
+i17186.scala
+i11982a.scala
 
 # Tree is huge and blows stack for printing Text
 i7034.scala

library/src/scala/Tuple.scala

@@ -89,7 +89,11 @@ object Tuple {
   }
 
   /** Type of the head of a tuple */
-  type Head[X <: NonEmptyTuple] = X match {
+  // Only bounded by `<: Tuple` not `<: NonEmptyTuple`
+  // even though it only matches non-empty tuples.
+  // This allows an irreducible type like `Tuple.Head[Tuple.Tail[X]]`
+  // to avoid running into type bounds violation checks.
+  type Head[X <: Tuple] = X match {
     case x *: _ => x
   }
 
@@ -101,7 +105,8 @@ object Tuple {
   }
 
   /** Type of the tail of a tuple */
-  type Tail[X <: NonEmptyTuple] <: Tuple = X match {
+  // See Head for why not `<: NonEmptyTuple`
+  type Tail[X <: Tuple] <: Tuple = X match {
     case _ *: xs => xs
   }
 

tests/neg/i11982a.scala renamed to tests/pos/i11982a.scala

@@ -0,0 +1,5 @@
+type SecondOfTwo[X <: Tuple2[Any, Any]] = Tuple.Head[Tuple.Tail[X]]
+val a = implicitly[SecondOfTwo[Tuple2[Int, String]] =:= String]
+
+type LastOfThree[X <: Tuple3[Any, Any, Any]] = Tuple.Tail[Tuple.Tail[X]]
+val b = implicitly[LastOfThree[Tuple3[Int, String, Boolean]] =:= Tuple1[Boolean]]