[ConstraintSystem] Use fixes to diagnose missing argument labels

Let the solver disregard missing argument labels and record correct
ones, so such problem could be diagnosed later on iff there were no
other more serious failures.

lib/Sema/CSSimplify.cpp

@@ -499,6 +499,15 @@ matchCallArguments(ArrayRef<AnyFunctionType::Param> args,
         auto fromArgIdx = boundArgIdx;
         auto toArgIdx = argIdx;
 
+        // If there is no re-ordering going on, and index is past
+        // the number of parameters, it could only mean that this
+        // is variadic parameter, so let's just move on.
+        if (fromArgIdx == toArgIdx && toArgIdx >= params.size()) {
+          assert(args[fromArgIdx].getLabel().empty());
+          argIdx++;
+          continue;
+        }
+
         // First let's double check if out-of-order argument is nothing
         // more than a simple label mismatch, because in situation where
         // one argument requires label and another one doesn't, but caller
@@ -692,6 +701,32 @@ getCalleeDeclAndArgs(ConstraintSystem &cs,
   return std::make_tuple(nullptr, 0, argLabels, hasTrailingClosure);
 }
 
+class ArgumentFailureTracker : public MatchCallArgumentListener {
+  ConstraintSystem &CS;
+  ConstraintLocatorBuilder Locator;
+
+public:
+  ArgumentFailureTracker(ConstraintSystem &cs, ConstraintLocatorBuilder locator)
+    : CS(cs), Locator(locator) {}
+
+  bool missingLabel(unsigned paramIndex) override {
+    return !CS.shouldAttemptFixes();
+  }
+
+  bool relabelArguments(ArrayRef<Identifier> newLabels) override {
+    if (!CS.shouldAttemptFixes())
+      return true;
+
+    auto *anchor = Locator.getBaseLocator()->getAnchor();
+    if (!anchor || !isa<CallExpr>(anchor))
+      return true;
+
+    CS.recordFix(Fix::fixArgumentLabels(CS, newLabels),
+                 CS.getConstraintLocator(anchor));
+    return false;
+  }
+};
+
 // Match the argument of a call to the parameter.
 static ConstraintSystem::TypeMatchResult
 matchCallArguments(ConstraintSystem &cs, ConstraintKind kind,
@@ -747,7 +782,7 @@ matchCallArguments(ConstraintSystem &cs, ConstraintKind kind,
   auto args = decomposeArgType(argType, argLabels);
   
   // Match up the call arguments to the parameters.
-  MatchCallArgumentListener listener;
+  ArgumentFailureTracker listener(cs, locator);
   SmallVector<ParamBinding, 4> parameterBindings;
   if (constraints::matchCallArguments(args, params,
                                       defaultMap,
@@ -4910,6 +4945,7 @@ ConstraintSystem::simplifyFixConstraint(Fix fix, Type type1, Type type2,
   case FixKind::ExplicitlyEscaping:
   case FixKind::ExplicitlyEscapingToAny:
   case FixKind::CoerceToCheckedCast:
+  case FixKind::RelabelArguments:
     llvm_unreachable("handled elsewhere");
   }