Sema: Fix matchCallArguments() to allow empty pack type bindings

Thanks to @hborla for suggesting this fix.

Author: slavapestov

lib/Sema/CSSimplify.cpp

@@ -1423,10 +1423,14 @@ class OpenParameterPackElements {
 
   void intoPackTypes(llvm::function_ref<void(TypeVariableType *, Type)> fn) && {
     if (argIdx == -1) {
-      (void)patternTy.transform(*this);
-      for (auto &entry : PackElementCache) {
-        entry.second.clear();
-      }
+      // No arguments. Each pack in the pattern will have no elements.
+      patternTy.visit([&](Type type) {
+        auto *typeVar = type->getAs<TypeVariableType>();
+        if (!typeVar || !typeVar->getImpl().getGenericParameter()->isParameterPack())
+          return;
+
+        this->PackElementCache[typeVar].clear();
+      });
     }
 
     for (const auto &entry : PackElementCache) {

test/Constraints/variadic_generic_functions.swift

@@ -26,20 +26,16 @@ func invalidPacks() {
 
 func call() {
   func multipleParameters<T...>(xs: T..., ys: T...) -> (T...) { return (_: xs) }
-  // expected-note@-1 {{in call to function 'multipleParameters(xs:ys:)'}}
-  _ = multipleParameters()
-  // expected-error@-1 2 {{generic parameter 'T' could not be inferred}}
+  multipleParameters()
+
   let x: (_: String) = multipleParameters(xs: "", ys: "")
   let (one, two) = multipleParameters(xs: "", 5.0, ys: "", 5.0)
   multipleParameters(xs: "", 5.0, ys: 5.0, "") // expected-error {{type of expression is ambiguous without more context}}
 
   func multipleSequences<T..., U...>(xs: T..., ys: U...) -> (T...) { return (_: ys) }
-  // expected-note@-1 {{in call to function 'multipleSequences(xs:ys:)'}}
-  // expected-error@-2 {{cannot convert return expression of type '(U...)' to return type '(T...)'}}
+  // expected-error@-1 {{cannot convert return expression of type '(U...)' to return type '(T...)'}}
 
-  _ = multipleSequences()
-  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
-  // expected-error@-2 {{generic parameter 'U' could not be inferred}}
+  multipleSequences()
   _ = multipleSequences(xs: "", ys: "")
   _ = multipleSequences(xs: "", 5.0, ys: 5.0, "")
 }