[Diagnostics] Correctly diagnose misplaced missing argument

Due to the fact that `matchCallArgument` can't and
doesn't take types into consideration while matching
arguments to parameters, when both arguments are
un-labeled, it's impossible to say which one is missing:

func foo(_: Int, _: String) {}
foo("")

In this case first argument is missing, but we end up with
two fixes - argument mismatch (for #1) and missing argument
(for #2), which is incorrect so it has to be handled specially.

lib/Sema/CSDiagnostics.cpp

@@ -3565,6 +3565,7 @@ bool ImplicitInitOnNonConstMetatypeFailure::diagnoseAsError() {
 }
 
 bool MissingArgumentsFailure::diagnoseAsError() {
+  auto &cs = getConstraintSystem();
   auto *locator = getLocator();
   auto path = locator->getPath();
 
@@ -3574,6 +3575,11 @@ bool MissingArgumentsFailure::diagnoseAsError() {
         path.back().getKind() == ConstraintLocator::ApplyArgument))
     return false;
 
+  // If this is a misplaced `missng argument` situation, it would be
+  // diagnosed by invalid conversion fix.
+  if (isMisplacedMissingArgument(cs, locator))
+    return false;
+
   auto *anchor = getAnchor();
   if (auto *captureList = dyn_cast<CaptureListExpr>(anchor))
     anchor = captureList->getClosureBody();
@@ -3901,6 +3907,68 @@ bool MissingArgumentsFailure::isPropertyWrapperInitialization() const {
   return NTD && NTD->getAttrs().hasAttribute<PropertyWrapperAttr>();
 }
 
+bool MissingArgumentsFailure::isMisplacedMissingArgument(
+    ConstraintSystem &cs, ConstraintLocator *locator) {
+  auto *calleeLocator = cs.getCalleeLocator(locator);
+  auto overloadChoice = cs.findSelectedOverloadFor(calleeLocator);
+  if (!overloadChoice)
+    return false;
+
+  auto *fnType =
+      cs.simplifyType(overloadChoice->ImpliedType)->getAs<FunctionType>();
+  if (!(fnType && fnType->getNumParams() == 2))
+    return false;
+
+  auto *anchor = locator->getAnchor();
+
+  auto hasFixFor = [&](FixKind kind, ConstraintLocator *locator) -> bool {
+    auto fix = llvm::find_if(cs.getFixes(), [&](const ConstraintFix *fix) {
+      return fix->getLocator() == locator;
+    });
+
+    if (fix == cs.getFixes().end())
+      return false;
+
+    return (*fix)->getKind() == kind;
+  };
+
+  auto *callLocator =
+      cs.getConstraintLocator(anchor, ConstraintLocator::ApplyArgument);
+
+  auto argFlags = fnType->getParams()[0].getParameterFlags();
+  auto *argLoc = cs.getConstraintLocator(
+      callLocator, LocatorPathElt::ApplyArgToParam(0, 0, argFlags));
+
+  if (!(hasFixFor(FixKind::AllowArgumentTypeMismatch, argLoc) &&
+        hasFixFor(FixKind::AddMissingArguments, callLocator)))
+    return false;
+
+  Expr *argExpr = nullptr;
+  if (auto *call = dyn_cast<CallExpr>(anchor)) {
+    argExpr = call->getArg();
+  } else if (auto *subscript = dyn_cast<SubscriptExpr>(anchor)) {
+    argExpr = subscript->getIndex();
+  } else {
+    return false;
+  }
+
+  Expr *argument = nullptr;
+  if (auto *PE = dyn_cast<ParenExpr>(argExpr)) {
+    argument = PE->getSubExpr();
+  } else {
+    auto *tuple = cast<TupleExpr>(argExpr);
+    if (tuple->getNumElements() != 1)
+      return false;
+    argument = tuple->getElement(0);
+  }
+
+  auto argType = cs.simplifyType(cs.getType(argument));
+  auto paramType = fnType->getParams()[1].getPlainType();
+
+  auto &TC = cs.getTypeChecker();
+  return TC.isConvertibleTo(argType, paramType, cs.DC);
+}
+
 bool ClosureParamDestructuringFailure::diagnoseAsError() {
   auto *closure = cast<ClosureExpr>(getAnchor());
   auto params = closure->getParameters();
@@ -4845,6 +4913,9 @@ void InOutConversionFailure::fixItChangeArgumentType() const {
 }
 
 bool ArgumentMismatchFailure::diagnoseAsError() {
+  if (diagnoseMisplacedMissingArgument())
+    return true;
+
   if (diagnoseConversionToBool())
     return true;
 
@@ -5074,6 +5145,33 @@ bool ArgumentMismatchFailure::diagnoseArchetypeMismatch() const {
   return true;
 }
 
+bool ArgumentMismatchFailure::diagnoseMisplacedMissingArgument() const {
+  auto &cs = getConstraintSystem();
+  auto *locator = getLocator();
+
+  if (!MissingArgumentsFailure::isMisplacedMissingArgument(cs, locator))
+    return false;
+
+  auto info = *getFunctionArgApplyInfo(locator);
+
+  auto *argType = cs.createTypeVariable(
+      cs.getConstraintLocator(locator, LocatorPathElt::SynthesizedArgument(1)),
+      /*flags=*/0);
+
+  // Assign new type variable to a type of a parameter.
+  auto *fnType = info.getFnType();
+  const auto &param = fnType->getParams()[0];
+  cs.assignFixedType(argType, param.getOldType());
+
+  auto *anchor = getRawAnchor();
+
+  MissingArgumentsFailure failure(
+      getParentExpr(), cs, {param.withType(argType)},
+      cs.getConstraintLocator(anchor, ConstraintLocator::ApplyArgument));
+
+  return failure.diagnoseSingleMissingArgument();
+}
+
 void ExpandArrayIntoVarargsFailure::tryDropArrayBracketsFixIt(
     Expr *anchor) const {
   // If this is an array literal, offer to remove the brackets and pass the

lib/Sema/CSDiagnostics.h

@@ -1197,9 +1197,9 @@ class MissingArgumentsFailure final : public FailureDiagnostic {
 
   bool diagnoseAsError() override;
 
-private:
   bool diagnoseSingleMissingArgument() const;
 
+private:
   /// If missing arguments come from a closure,
   /// let's produce tailored diagnostics.
   bool diagnoseClosure(ClosureExpr *closure);
@@ -1212,6 +1212,21 @@ class MissingArgumentsFailure final : public FailureDiagnostic {
   /// an implicit call to a property wrapper initializer e.g.
   /// `@Foo(answer: 42) var question = "ultimate question"`
   bool isPropertyWrapperInitialization() const;
+
+public:
+  /// Due to the fact that `matchCallArgument` can't and
+  /// doesn't take types into consideration while matching
+  /// arguments to parameters, for cases where both arguments
+  /// are un-labeled, it's impossible to say which one is missing:
+  ///
+  /// func foo(_: Int, _: String) {}
+  /// foo("")
+  ///
+  /// In this case first argument is missing, but we end up with
+  /// two fixes - argument mismatch (for #1) and missing argument
+  /// (for #2), which is incorrect so it has to be handled specially.
+  static bool isMisplacedMissingArgument(ConstraintSystem &cs,
+                                         ConstraintLocator *locator);
 };
 
 class OutOfOrderArgumentFailure final : public FailureDiagnostic {
@@ -1648,6 +1663,16 @@ class ArgumentMismatchFailure : public ContextualFailure {
   bool diagnoseUseOfReferenceEqualityOperator() const;
 
 protected:
+
+  /// Situations like this:
+  ///
+  /// func foo(_: Int, _: String) {}
+  /// foo("")
+  ///
+  /// Are currently impossible to fix correctly,
+  /// so we have to attend to that in diagnostics.
+  bool diagnoseMisplacedMissingArgument() const;
+
   SourceLoc getLoc() const { return getAnchor()->getLoc(); }
 
   ValueDecl *getDecl() const {