Fix tests and add check files

Author: odersky

tests/neg/i12049.check

@@ -48,3 +48,58 @@ longer explanation available when compiling with `-explain`
    |                        case EmptyTuple => EmptyTuple
 
 longer explanation available when compiling with `-explain`
+-- Error: tests/neg/i12049.scala:24:20 ---------------------------------------------------------------------------------
+24 |val _ = summon[M[B]]  // error
+   |                    ^
+   |                   no implicit argument of type M[B] was found for parameter x of method summon in object Predef
+   |
+   |                   Note: a match type could not be fully reduced:
+   |
+   |                     trying to reduce  M[B]
+   |                     failed since selector  B
+   |                     does not match  case A => Int
+   |                     and cannot be shown to be disjoint from it either.
+   |                     Therefore, reduction cannot advance to the remaining case
+   |
+   |                       case B => String
+-- Error: tests/neg/i12049.scala:25:78 ---------------------------------------------------------------------------------
+25 |val _ = summon[String =:= Last[Int *: Int *: Boolean *: String *: EmptyTuple]] // error
+   |                                                                              ^
+   |                                                             Cannot prove that String =:= Last[EmptyTuple.type].
+   |
+   |                                                             Note: a match type could not be fully reduced:
+   |
+   |                                                               trying to reduce  Last[EmptyTuple.type]
+   |                                                               failed since selector  EmptyTuple.type
+   |                                                               matches none of the cases
+   |
+   |                                                                 case _ *: _ *: t => Last[t]
+   |                                                                 case t *: EmptyTuple => t
+-- Error: tests/neg/i12049.scala:26:48 ---------------------------------------------------------------------------------
+26 |val _ = summon[(A, B, A) =:= Reverse[(A, B, A)]] // error
+   |                                                ^
+   |                            Cannot prove that (A, B, A) =:= Tuple.Concat[Reverse[A *: EmptyTuple.type], (B, A)].
+   |
+   |                            Note: a match type could not be fully reduced:
+   |
+   |                              trying to reduce  Tuple.Concat[Reverse[A *: EmptyTuple.type], (B, A)]
+   |                              trying to reduce  Reverse[A *: EmptyTuple.type]
+   |                              failed since selector  A *: EmptyTuple.type
+   |                              matches none of the cases
+   |
+   |                                case t1 *: t2 *: ts => Tuple.Concat[Reverse[ts], (t2, t1)]
+   |                                case EmptyTuple => EmptyTuple
+-- [E008] Not Found Error: tests/neg/i12049.scala:28:21 ----------------------------------------------------------------
+28 |val _ = (??? : M[B]).length // error
+   |        ^^^^^^^^^^^^^^^^^^^
+   |        value length is not a member of M[B]
+   |
+   |        Note: a match type could not be fully reduced:
+   |
+   |          trying to reduce  M[B]
+   |          failed since selector  B
+   |          does not match  case A => Int
+   |          and cannot be shown to be disjoint from it either.
+   |          Therefore, reduction cannot advance to the remaining case
+   |
+   |            case B => String

tests/neg/i12049.scala

@@ -21,9 +21,9 @@ val z1: (B, A) = ??? : Reverse[(A, B)]
 val z2: (B, A, B, A) = ??? : Reverse[(A, B, A, B)]
 val z3: (A, B, A) = ??? : Reverse[(A, B, A)] // error
 
-val _ = summon[M[B]]
-val _ = summon[String =:= Last[Int *: Int *: Boolean *: String *: EmptyTuple]]
-val _ = summon[(A, B, A) =:= Reverse[(A, B, A)]]
+val _ = summon[M[B]]  // error
+val _ = summon[String =:= Last[Int *: Int *: Boolean *: String *: EmptyTuple]] // error
+val _ = summon[(A, B, A) =:= Reverse[(A, B, A)]] // error
 
-val _ = (??? : M[B]).length
+val _ = (??? : M[B]).length // error