[CSOptimizer] Use matchCallArguments to establish argument-to-parameter relationships

This helps to find defaulted parameter positions as well.

lib/Sema/CSOptimizer.cpp

@@ -116,6 +116,13 @@ static void determineBestChoicesInContext(
     if (!argumentList || cs.containsIDEInspectionTarget(argumentList))
       return;
 
+    SmallVector<FunctionType::Param, 8> argsWithLabels;
+    {
+      argsWithLabels.append(argFuncType->getParams().begin(),
+                            argFuncType->getParams().end());
+      FunctionType::relabelParams(argsWithLabels, argumentList);
+    }
+
     SmallVector<SmallVector<std::pair<Type, /*fromLiteral=*/bool>, 2>, 2>
         candidateArgumentTypes;
     candidateArgumentTypes.resize(argFuncType->getNumParams());
@@ -159,6 +166,27 @@ static void determineBestChoicesInContext(
       resultTypes.push_back(resultType);
     }
 
+    // Match arguments to the given overload choice.
+    auto matchArguments =
+        [&](OverloadChoice choice,
+            FunctionType *overloadType) -> Optional<MatchCallArgumentResult> {
+      auto *decl = choice.getDeclOrNull();
+      assert(decl);
+
+      auto hasAppliedSelf =
+          decl->hasCurriedSelf() &&
+          doesMemberRefApplyCurriedSelf(choice.getBaseType(), decl);
+
+      ParameterListInfo paramListInfo(overloadType->getParams(), decl,
+                                      hasAppliedSelf);
+
+      MatchCallArgumentListener listener;
+      return matchCallArguments(argsWithLabels, overloadType->getParams(),
+                                paramListInfo,
+                                argumentList->getFirstTrailingClosureIndex(),
+                                /*allow fixes*/ false, listener, None);
+    };
+
     // The choice with the best score.
     double bestScore = 0.0;
     SmallVector<std::pair<Constraint *, double>, 2> favoredChoices;
@@ -182,27 +210,35 @@ static void determineBestChoicesInContext(
               return;
           }
 
-          ParameterListInfo paramListInfo(
-              overloadType->getParams(), decl,
-              hasAppliedSelf(cs, choice->getOverloadChoice()));
+          auto matchings =
+              matchArguments(choice->getOverloadChoice(), overloadType);
+          if (!matchings)
+            return;
 
           double score = 0.0;
-          for (unsigned i = 0, n = overloadType->getNumParams(); i != n; ++i) {
-            const auto &param = overloadType->getParams()[i];
+          for (unsigned paramIdx = 0, n = overloadType->getNumParams();
+               paramIdx != n; ++paramIdx) {
+            const auto &param = overloadType->getParams()[paramIdx];
 
-            if (i >= candidateArgumentTypes.size()) {
-              // If parameter has a default - continue matching,
-              // all of the subsequence parameters should have defaults
-              // as well, if they don't the overload choice in not viable.
-              if (paramListInfo.hasDefaultArgument(i))
-                continue;
+            auto argIndices = matchings->parameterBindings[paramIdx];
+            switch (argIndices.size()) {
+            case 0:
+              // Current parameter is defaulted.
+              continue;
 
-              // Early return because this overload choice is not viable
-              // without default value for the current parameter.
+            case 1:
+              // One-to-one match between argument and parameter.
+              break;
+
+            default:
+              // Cannot deal with multiple possible matchings at the moment.
               return;
             }
 
-            if (candidateArgumentTypes[i].empty())
+            auto argIdx = argIndices.front();
+
+            // Looks like there is nothing know about the argument.
+            if (candidateArgumentTypes[argIdx].empty())
               continue;
 
             const auto paramFlags = param.getParameterFlags();
@@ -220,9 +256,6 @@ static void determineBestChoicesInContext(
             if (paramType->is<FunctionType>())
               continue;
 
-            if (candidateArgumentTypes[i].empty())
-              continue;
-
             // Check protocol requirement(s) if this parameter is a
             // generic parameter type.
             GenericSignature::RequiredProtocols protocolRequirements;
@@ -248,11 +281,11 @@ static void determineBestChoicesInContext(
             // all bound concrete types, we consider this is mismatch
             // at this parameter position and remove the overload choice
             // from consideration.
-
             double bestCandidateScore = 0;
-            llvm::BitVector mismatches(candidateArgumentTypes[i].size());
+            llvm::BitVector mismatches(candidateArgumentTypes[argIdx].size());
 
-            for (unsigned candidateIdx : indices(candidateArgumentTypes[i])) {
+            for (unsigned candidateIdx :
+                 indices(candidateArgumentTypes[argIdx])) {
               // If one of the candidates matched exactly there is no reason
               // to continue checking.
               if (bestCandidateScore == 1)
@@ -262,7 +295,7 @@ static void determineBestChoicesInContext(
               bool isLiteralDefault;
 
               std::tie(candidateType, isLiteralDefault) =
-                  candidateArgumentTypes[i][candidateIdx];
+                  candidateArgumentTypes[argIdx][candidateIdx];
 
               // `inout` parameter accepts only l-value argument.
               if (paramFlags.isInOut() && !candidateType->is<LValueType>()) {