[CSOptimizer] Decrease an operator argument score only if requires optional injection

This makes sure that optional and non-optional types are ranked
uniformly when matched against a generic parameter type that
could accept either of them.

This is a more general fix for #83365

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -970,8 +970,6 @@ static void determineBestChoicesInContext(
 
     bool hasArgumentCandidates = false;
     bool isOperator = isOperatorDisjunction(disjunction);
-    bool isNilCoalescingOperator =
-        isOperator && isOperatorNamed(disjunction, "??");
 
     for (unsigned i = 0, n = argFuncType->getNumParams(); i != n; ++i) {
       const auto &param = argFuncType->getParams()[i];
@@ -1341,17 +1339,26 @@ static void determineBestChoicesInContext(
         paramType = paramType->lookThroughAllOptionalTypes(paramOptionals);
 
         if (!candidateOptionals.empty() || !paramOptionals.empty()) {
+          auto requiresOptionalInjection = [&]() {
+            return paramOptionals.size() > candidateOptionals.size();
+          };
+
           // Can match i.e. Int? to Int or T to Int?
           if ((paramOptionals.empty() &&
                paramType->is<GenericTypeParamType>()) ||
               paramOptionals.size() >= candidateOptionals.size()) {
             auto score = scoreCandidateMatch(genericSig, choice, candidateType,
                                              paramType, options);
-            // Injection lowers the score slightly to comply with
-            // old behavior where exact matches on operator parameter
-            // types were always preferred.
-            return score > 0 && choice->isOperator() ? score.value() - 0.1
-                                                     : score;
+
+            if (score > 0) {
+              // Injection lowers the score slightly to comply with
+              // old behavior where exact matches on operator parameter
+              // types were always preferred.
+              if (choice->isOperator() && requiresOptionalInjection())
+                return score.value() - 0.1;
+            }
+
+            return score;
           }
 
           // Optionality mismatch.
@@ -1605,20 +1612,6 @@ static void determineBestChoicesInContext(
             double bestCandidateScore = 0;
             llvm::BitVector mismatches(argumentCandidates[argIdx].size());
 
-            // `??` is overloaded on optionality of the second parameter,
-            // prevent ranking the argument candidates for this parameter
-            // if there are candidates that come from failable initializer
-            // overloads because non-optional candidates are always going
-            // to be better and that can skew the selection.
-            if (isNilCoalescingOperator && argIdx == 1) {
-              if (llvm::any_of(argumentCandidates[argIdx],
-                               [](const auto &candidate) {
-                                 return candidate.fromInitializerCall &&
-                                        candidate.type->getOptionalObjectType();
-                               }))
-                continue;
-            }
-
             for (unsigned candidateIdx :
                  indices(argumentCandidates[argIdx])) {
               // If one of the candidates matched exactly there is no reason