Fix #1959: infix type operators in the REPL

Infix type operators were broken in the REPL.
The REPL uses fold methods from the untpd package,
but those were skipping the operator subtree when folding
over an InfixOp.

Fix the issue by taking the operator into account.

Tested:
1) Verified that only the REPL code uses the modified
fold method.
2) Added repl test.

Author: Abel Nieto

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -526,7 +526,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       case Function(args, body) =>
         this(this(x, args), body)
       case InfixOp(left, op, right) =>
-        this(this(x, left), right)
+        this(this(this(x, left), op), right)
       case PostfixOp(od, op) =>
         this(x, od)
       case PrefixOp(op, od) =>

tests/repl/infixTypeOp.check

@@ -0,0 +1,5 @@
+scala> trait +[A, B]
+defined trait +
+scala> type IntAndString = Int + String
+defined type alias IntAndString
+scala> :quit