Always generate a tuple in Desugar.smallTuple

Previously, the case where arity = 1 was treated specially.

Author: odersky

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

@@ -1013,8 +1013,7 @@ object desugar {
     val arity = ts.length
     assert(arity <= Definitions.MaxTupleArity)
     def tupleTypeRef = defn.TupleType(arity)
-    if (arity == 1) ts.head
-    else if (arity == 0)
+    if (arity == 0)
       if (ctx.mode is Mode.Type) TypeTree(defn.UnitType) else unitLiteral
     else if (ctx.mode is Mode.Type) AppliedTypeTree(ref(tupleTypeRef), ts)
     else Apply(ref(tupleTypeRef.classSymbol.companionModule.termRef), ts)