TypeDecoder: Unwrap unlabeled one-element tuples when expanding pack expansions

Author: slavapestov

include/swift/Demangling/TypeDecoder.h

@@ -1052,12 +1052,12 @@ class TypeDecoder {
           return *optError;
       }
 
-      // Unwrap one-element tuples.
+      // Unwrap unlabeled one-element tuples.
       //
       // FIXME: The behavior of one-element labeled tuples is inconsistent throughout
       // the different re-implementations of type substitution and pack expansion.
-      // if (elements.size() == 1)
-      //  return elements[0];
+      if (elements.size() == 1 && labels[0].empty())
+        return elements[0];
 
       return Builder.createTupleType(elements, labels);
     }

test/Interpreter/variadic_generic_type_witnesses.swift

@@ -63,13 +63,11 @@ conformances.test("tupleWitnesses") {
 
 conformances.test("singletonTupleWitnesses") {
   let g1 = SingletonTupleWitnesses<Bool>.self
-  // FIXME: Unwrap one-element tuples
-  // expectEqual(Bool.self, getA(g1))
+  expectEqual(Bool.self, getA(g1))
 
   let g2 = SingletonTupleWitnesses< >.self
-  // FIXME: Unwrap one-element tuples
-  // expectEqual(Int.self, getB(g2))
-  // expectEqual(Int.self, getC(g2))
+  expectEqual(Int.self, getB(g2))
+  expectEqual(Int.self, getC(g2))
 }
 
 conformances.test("functionWitnesses") {