[CSOptimizer] Keep track of mismatches while evaluating candidates

lib/Sema/CSOptimizer.cpp

@@ -16,6 +16,7 @@
 
 #include "swift/Sema/ConstraintGraph.h"
 #include "swift/Sema/ConstraintSystem.h"
+#include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/TinyPtrVector.h"
@@ -231,20 +232,45 @@ static void determineBestChoicesInContext(
             if (paramType->is<FunctionType>())
               continue;
 
-            double argScore = 0.0;
-            for (auto const &candidate : candidateArgumentTypes[i]) {
-              auto candidateType = candidate.first;
+            if (candidateArgumentTypes[i].empty())
+              continue;
+
+            // The idea here is to match the parameter type against
+            // all of the argument candidate types and pick the best
+            // match (i.e. exact equality one).
+            //
+            // If none of the candidates match exactly and they are
+            // 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());
+
+            for (unsigned candidateIdx : indices(candidateArgumentTypes[i])) {
+              // If one of the candidates matched exactly there is no reason
+              // to continue checking.
+              if (bestCandidateScore == 1)
+                break;
+
+              Type candidateType;
+              bool isLiteralDefault;
+
+              std::tie(candidateType, isLiteralDefault) =
+                  candidateArgumentTypes[i][candidateIdx];
 
               // `inout` parameter accepts only l-value argument.
-              if (paramFlags.isInOut() && !candidateType->is<LValueType>())
+              if (paramFlags.isInOut() && !candidateType->is<LValueType>()) {
+                mismatches.set(candidateIdx);
                 continue;
+              }
 
               // The specifier only matters for `inout` check.
               candidateType = candidateType->getWithoutSpecifierType();
               // Exact match on one of the candidate bindings.
               if (candidateType->isEqual(paramType)) {
-                argScore = std::max(
-                    argScore, /*fromLiteral=*/candidate.second ? 0.3 : 1.0);
+                bestCandidateScore =
+                    std::max(bestCandidateScore, isLiteralDefault ? 0.3 : 1.0);
                 continue;
               }
 
@@ -253,12 +279,23 @@ static void determineBestChoicesInContext(
               // and expect a particular `Scalar` if it's known.
               if (isSIMDType(candidateType) &&
                   isArithmeticOperatorOnSIMDProtocol(decl)) {
-                argScore = std::max(argScore, 1.0);
+                bestCandidateScore = 1.0;
                 continue;
               }
+
+              // Only established arguments could be considered mismatches,
+              // literal default types should be regarded as holes if they
+              // didn't match.
+              if (!isLiteralDefault && !candidateType->hasTypeVariable())
+                mismatches.set(candidateIdx);
             }
 
-            score += argScore;
+            // If none of the candidates for this parameter matched, let's
+            // drop this overload from any further consideration.
+            if (mismatches.all())
+              return;
+
+            score += bestCandidateScore;
           }
 
           // Average the score to avoid disfavoring disjunctions with fewer