[CS] Dont wrap packexp in extra layer of expansion

Arguments that were already pack expansions were being wrapped in a
second layer--preventing some would-be unambiguous overloads from
resolving. This adds a check to avoid doing that.

Author: jameesbrown

lib/Sema/CSRanking.cpp

@@ -703,11 +703,16 @@ bool CompareDeclSpecializationRequest::evaluate(
            paramIdx != numParams; ++paramIdx) {
         const auto &param = params2[paramIdx];
         auto paramTy = param.getOldType();
+        auto argIndices = matching->parameterBindings[paramIdx];
+        if (argIndices.empty())
+          continue;
 
         if (paramListInfo.isVariadicGenericParameter(paramIdx) &&
-            isPackExpansionType(paramTy)) {
+            isPackExpansionType(paramTy) &&
+            (argIndices.size() > 1 ||
+             !isPackExpansionType(args[argIndices.front()].getOldType()))) {
           SmallVector<Type, 2> argTypes;
-          for (auto argIdx : matching->parameterBindings[paramIdx]) {
+          for (auto argIdx : argIndices) {
             // Don't prefer `T...` over `repeat each T`.
             if (args[argIdx].isVariadic())
               return completeResult(false);
@@ -721,7 +726,7 @@ bool CompareDeclSpecializationRequest::evaluate(
           continue;
         }
 
-        for (auto argIdx : matching->parameterBindings[paramIdx]) {
+        for (auto argIdx : argIndices) {
           const auto &arg = args[argIdx];
           // Always prefer non-variadic version when possible.
           if (arg.isVariadic())

test/Constraints/variadic_generic_overload_ranking.swift

@@ -85,3 +85,13 @@ func test_ranking_with_multiple_expansions() {
   // CHECK-NEXT: {{.*}} = function_ref @$s33variadic_generic_overload_ranking05test_D25_with_multiple_expansionsyyF7BuilderL_V5buildyAaByyF5TupleL_VyxxQp_tGxxQpRvzAA1PRzlFZ : $@convention(method) <each τ_0_0 where repeat each τ_0_0 : P> (@pack_guaranteed Pack{repeat each τ_0_0}, @thin Builder.Type) -> Tuple<(repeat each τ_0_0)>
   _ = Builder.build(Empty(), Tuple<(Int, String)>((42, "")))
 }
+protocol Q: P {}
+
+func test_ranking_with_protocol_refinement() {
+  func f<each T: Q>(_ a: repeat each T) {}
+  func f<each T: P>(_ a: repeat each T) {}
+  struct S: Q {}
+  // CHECK: // function_ref f #1 <each A>(_:) in test_ranking_with_protocol_refinement()
+  // CHECK-NEXT: {{.*}} = function_ref @$s33variadic_generic_overload_ranking05test_D25_with_protocol_refinementyyF1fL_yyxxQpRvzAA1QRzlF : $@convention(thin) <each τ_0_0 where repeat each τ_0_0 : Q> (@pack_guaranteed Pack{repeat each τ_0_0}) -> ()
+  f(S())
+}