Refactor BindingSet a little bit

include/swift/Sema/CSBindings.h

@@ -469,10 +469,6 @@ class BindingSet {
   /// checking.
   bool isViable(PotentialBinding &binding, bool isTransitive);
 
-  explicit operator bool() const {
-    return hasViableBindings() || isDirectHole();
-  }
-
   /// Determine whether this set has any "viable" (or non-hole) bindings.
   ///
   /// A viable binding could be - a direct or transitive binding
@@ -486,6 +482,12 @@ class BindingSet {
            !Defaults.empty();
   }
 
+  /// Determine whether this set can be chosen as the next binding set
+  /// to attempt.
+  bool isViable() const {
+    return hasViableBindings() || isDirectHole();
+  }
+
   ArrayRef<Constraint *> getConformanceRequirements() const {
     return Protocols;
   }
@@ -544,6 +546,8 @@ class BindingSet {
   /// Check if this binding is favored over a conjunction.
   bool favoredOverConjunction(Constraint *conjunction) const;
 
+  void inferTransitiveKeyPathBindings();
+
   /// Detect `subtype` relationship between two type variables and
   /// attempt to infer supertype bindings transitively e.g.
   ///
@@ -553,19 +557,27 @@ class BindingSet {
   ///
   /// \param inferredBindings The set of all bindings inferred for type
   /// variables in the workset.
-  void inferTransitiveBindings();
+  void inferTransitiveSupertypeBindings();
+
+  void inferTransitiveUnresolvedMemberRefBindings();
 
   /// Detect subtype, conversion or equivalence relationship
   /// between two type variables and attempt to propagate protocol
   /// requirements down the subtype or equivalence chain.
   void inferTransitiveProtocolRequirements();
 
-  /// Finalize binding computation for this type variable by
-  /// inferring bindings from context e.g. transitive bindings.
+  /// Check whether the given binding set covers any of the
+  /// literal protocols associated with this type variable.
+  void determineLiteralCoverage();
+
+  /// Finalize binding computation for key path type variables.
   ///
   /// \returns true if finalization successful (which makes binding set viable),
   /// and false otherwise.
-  bool finalize(bool transitive);
+  bool finalizeKeyPathBindings();
+
+  /// Handle diagnostics of unresolved member chains.
+  void finalizeUnresolvedMemberChainResult();
 
   static BindingScore formBindingScore(const BindingSet &b);
 
@@ -590,10 +602,6 @@ class BindingSet {
 
   void addDefault(Constraint *constraint);
 
-  /// Check whether the given binding set covers any of the
-  /// literal protocols associated with this type variable.
-  void determineLiteralCoverage();
-
   StringRef getLiteralBindingKind(LiteralBindingKind K) const {
 #define ENTRY(Kind, String)                                                    \
   case LiteralBindingKind::Kind:                                               \

lib/Sema/CSBindings.cpp

@@ -103,8 +103,7 @@ bool BindingSet::isDirectHole() const {
   if (!CS.shouldAttemptFixes())
     return false;
 
-  return Bindings.empty() && getNumViableLiteralBindings() == 0 &&
-         Defaults.empty() && TypeVar->getImpl().canBindToHole();
+  return !hasViableBindings() && TypeVar->getImpl().canBindToHole();
 }
 
 static bool isGenericParameter(TypeVariableType *TypeVar) {
@@ -494,9 +493,7 @@ void BindingSet::inferTransitiveProtocolRequirements() {
   } while (!workList.empty());
 }
 
-void BindingSet::inferTransitiveBindings() {
-  using BindingKind = AllowedBindingKind;
-
+void BindingSet::inferTransitiveKeyPathBindings() {
   // If the current type variable represents a key path root type
   // let's try to transitively infer its type through bindings of
   // a key path type.
@@ -551,15 +548,17 @@ void BindingSet::inferTransitiveBindings() {
               }
             } else {
               addBinding(
-                  binding.withSameSource(inferredRootTy, BindingKind::Exact),
+                  binding.withSameSource(inferredRootTy, AllowedBindingKind::Exact),
                   /*isTransitive=*/true);
             }
           }
         }
       }
     }
   }
+}
 
+void BindingSet::inferTransitiveSupertypeBindings() {
   for (const auto &entry : Info.SupertypeOf) {
     auto &node = CS.getConstraintGraph()[entry.first];
     if (!node.hasBindingSet())
@@ -609,8 +608,8 @@ void BindingSet::inferTransitiveBindings() {
       // either be Exact or Supertypes in order for it to make sense
       // to add Supertype bindings based on the relationship between
       // our type variables.
-      if (binding.Kind != BindingKind::Exact &&
-          binding.Kind != BindingKind::Supertypes)
+      if (binding.Kind != AllowedBindingKind::Exact &&
+          binding.Kind != AllowedBindingKind::Supertypes)
         continue;
 
       auto type = binding.BindingType;
@@ -621,12 +620,49 @@ void BindingSet::inferTransitiveBindings() {
       if (ConstraintSystem::typeVarOccursInType(TypeVar, type))
         continue;
 
-      addBinding(binding.withSameSource(type, BindingKind::Supertypes),
+      addBinding(binding.withSameSource(type, AllowedBindingKind::Supertypes),
                  /*isTransitive=*/true);
     }
   }
 }
 
+void BindingSet::inferTransitiveUnresolvedMemberRefBindings() {
+  if (!hasViableBindings()) {
+    if (auto *locator = TypeVar->getImpl().getLocator()) {
+      if (locator->isLastElement<LocatorPathElt::MemberRefBase>()) {
+        // If this is a base of an unresolved member chain, as a last
+        // resort effort let's infer base to be a protocol type based
+        // on contextual conformance requirements.
+        //
+        // This allows us to find solutions in cases like this:
+        //
+        // \code
+        // func foo<T: P>(_: T) {}
+        // foo(.bar) <- `.bar` should be a static member of `P`.
+        // \endcode
+        inferTransitiveProtocolRequirements();
+
+        if (TransitiveProtocols.has_value()) {
+          for (auto *constraint : *TransitiveProtocols) {
+            Type protocolTy = constraint->getSecondType();
+
+            // Compiler-known marker protocols cannot be extended with members,
+            // so do not consider them.
+            if (auto p = protocolTy->getAs<ProtocolType>()) {
+              if (ProtocolDecl *decl = p->getDecl())
+                if (decl->getKnownProtocolKind() && decl->isMarkerProtocol())
+                  continue;
+            }
+
+            addBinding({protocolTy, AllowedBindingKind::Exact, constraint},
+                       /*isTransitive=*/false);
+          }
+        }
+      }
+    }
+  }
+}
+
 static Type getKeyPathType(ASTContext &ctx, KeyPathCapability capability,
                            Type rootType, Type valueType) {
   KeyPathMutability mutability;
@@ -664,51 +700,11 @@ static Type getKeyPathType(ASTContext &ctx, KeyPathCapability capability,
   return keyPathTy;
 }
 
-bool BindingSet::finalize(bool transitive) {
-  if (transitive)
-    inferTransitiveBindings();
-
-  determineLiteralCoverage();
-
+bool BindingSet::finalizeKeyPathBindings() {
   if (auto *locator = TypeVar->getImpl().getLocator()) {
-    if (locator->isLastElement<LocatorPathElt::MemberRefBase>()) {
-      // If this is a base of an unresolved member chain, as a last
-      // resort effort let's infer base to be a protocol type based
-      // on contextual conformance requirements.
-      //
-      // This allows us to find solutions in cases like this:
-      //
-      // \code
-      // func foo<T: P>(_: T) {}
-      // foo(.bar) <- `.bar` should be a static member of `P`.
-      // \endcode
-      if (transitive && !hasViableBindings()) {
-        inferTransitiveProtocolRequirements();
-
-        if (TransitiveProtocols.has_value()) {
-          for (auto *constraint : *TransitiveProtocols) {
-            Type protocolTy = constraint->getSecondType();
-
-            // Compiler-known marker protocols cannot be extended with members,
-            // so do not consider them.
-            if (auto p = protocolTy->getAs<ProtocolType>()) {
-              if (ProtocolDecl *decl = p->getDecl())
-                if (decl->getKnownProtocolKind() && decl->isMarkerProtocol())
-                  continue;
-            }
-
-            addBinding({protocolTy, AllowedBindingKind::Exact, constraint},
-                       /*isTransitive=*/false);
-          }
-        }
-      }
-    }
-
     if (TypeVar->getImpl().isKeyPathType()) {
       auto &ctx = CS.getASTContext();
-
-      auto *keyPathLoc = TypeVar->getImpl().getLocator();
-      auto *keyPath = castToExpr<KeyPathExpr>(keyPathLoc->getAnchor());
+      auto *keyPath = castToExpr<KeyPathExpr>(locator->getAnchor());
 
       bool isValid;
       std::optional<KeyPathCapability> capability;
@@ -775,7 +771,7 @@ bool BindingSet::finalize(bool transitive) {
           auto keyPathTy = getKeyPathType(ctx, *capability, rootTy,
                                           CS.getKeyPathValueType(keyPath));
           updatedBindings.insert(
-              {keyPathTy, AllowedBindingKind::Exact, keyPathLoc});
+              {keyPathTy, AllowedBindingKind::Exact, locator});
         } else if (CS.shouldAttemptFixes()) {
           auto fixedRootTy = CS.getFixedType(rootTy);
           // If key path is structurally correct and has a resolved root
@@ -802,10 +798,14 @@ bool BindingSet::finalize(bool transitive) {
 
       Bindings = std::move(updatedBindings);
       Defaults.clear();
-
-      return true;
     }
+  }
 
+  return true;
+}
+
+void BindingSet::finalizeUnresolvedMemberChainResult() {
+  if (auto *locator = TypeVar->getImpl().getLocator()) {
     if (CS.shouldAttemptFixes() &&
         locator->isLastElement<LocatorPathElt::UnresolvedMemberChainResult>()) {
       // Let's see whether this chain is valid, if it isn't then to avoid
@@ -828,8 +828,6 @@ bool BindingSet::finalize(bool transitive) {
       }
     }
   }
-
-  return true;
 }
 
 void BindingSet::addBinding(PotentialBinding binding, bool isTransitive) {
@@ -1143,37 +1141,6 @@ std::optional<BindingSet> ConstraintSystem::determineBestBindings(
       node.initBindingSet();
   }
 
-  // Determine whether given type variable with its set of bindings is
-  // viable to be attempted on the next step of the solver. If type variable
-  // has no "direct" bindings of any kind e.g. direct bindings to concrete
-  // types, default types from "defaultable" constraints or literal
-  // conformances, such type variable is not viable to be evaluated to be
-  // attempted next.
-  auto isViableForRanking = [this](const BindingSet &bindings) -> bool {
-    auto *typeVar = bindings.getTypeVariable();
-
-    // Key path root type variable is always viable because it can be
-    // transitively inferred from key path type during binding set
-    // finalization.
-    if (typeVar->getImpl().isKeyPathRoot())
-      return true;
-
-    // Type variable representing a base of unresolved member chain should
-    // always be considered viable for ranking since it's allow to infer
-    // types from transitive protocol requirements.
-    if (auto *locator = typeVar->getImpl().getLocator()) {
-      if (locator->isLastElement<LocatorPathElt::MemberRefBase>())
-        return true;
-    }
-
-    // If type variable is marked as a potential hole there is always going
-    // to be at least one binding available for it.
-    if (shouldAttemptFixes() && typeVar->getImpl().canBindToHole())
-      return true;
-
-    return bool(bindings);
-  };
-
   // Now let's see if we could infer something for related type
   // variables based on other bindings.
   for (auto *typeVar : getTypeVariables()) {
@@ -1183,6 +1150,16 @@ std::optional<BindingSet> ConstraintSystem::determineBestBindings(
 
     auto &bindings = node.getBindingSet();
 
+    // Special handling for key paths.
+    bindings.inferTransitiveKeyPathBindings();
+    if (!bindings.finalizeKeyPathBindings())
+      continue;
+
+    // Special handling for "leading-dot" unresolved member references,
+    // like .foo.
+    bindings.inferTransitiveUnresolvedMemberRefBindings();
+    bindings.finalizeUnresolvedMemberChainResult();
+
     // Before attempting to infer transitive bindings let's check
     // whether there are any viable "direct" bindings associated with
     // current type variable, if there are none - it means that this type
@@ -1193,12 +1170,12 @@ std::optional<BindingSet> ConstraintSystem::determineBestBindings(
     // associated with given type variable, any default constraints,
     // or any conformance requirements to literal protocols with can
     // produce a default type.
-    bool isViable = isViableForRanking(bindings);
+    bool isViable = bindings.isViable();
 
-    if (!bindings.finalize(true))
-      continue;
+    bindings.inferTransitiveSupertypeBindings();
+    bindings.determineLiteralCoverage();
 
-    if (!bindings || !isViable)
+    if (!isViable)
       continue;
 
     onCandidate(bindings);
@@ -1591,7 +1568,10 @@ BindingSet ConstraintSystem::getBindingsFor(TypeVariableType *typeVar) {
   assert(!typeVar->getImpl().getFixedType(nullptr) && "has a fixed type");
 
   BindingSet bindings(*this, typeVar, CG[typeVar].getPotentialBindings());
-  bindings.finalize(false);
+
+  (void) bindings.finalizeKeyPathBindings();
+  bindings.finalizeUnresolvedMemberChainResult();
+  bindings.determineLiteralCoverage();
 
   return bindings;
 }

lib/Sema/CSOptimizer.cpp

@@ -1105,7 +1105,9 @@ static void determineBestChoicesInContext(
       // Simply adding it as a binding won't work because if the second argument
       // is non-optional the overload that returns `T?` would still have a lower
       // score.
-      if (!bindingSet && isNilCoalescingOperator(disjunction)) {
+      if (!bindingSet.hasViableBindings() &&
+          !bindingSet.isDirectHole() &&
+          isNilCoalescingOperator(disjunction)) {
         auto &cg = cs.getConstraintGraph();
         if (llvm::any_of(cg[typeVar].getConstraints(),
                          [&typeVar](Constraint *constraint) {

lib/Sema/ConstraintGraph.cpp

@@ -921,7 +921,8 @@ bool ConstraintGraph::contractEdges() {
     // us enough information to decided on l-valueness.
     if (tyvar1->getImpl().canBindToInOut()) {
       bool isNotContractable = true;
-      if (auto bindings = CS.getBindingsFor(tyvar1)) {
+      auto bindings = CS.getBindingsFor(tyvar1);
+      if (bindings.isViable()) {
         // Holes can't be contracted.
         if (bindings.isHole())
           continue;

unittests/Sema/BindingInferenceTests.cpp

@@ -125,7 +125,15 @@ TEST_F(SemaTest, TestIntLiteralBindingInference) {
 
     cs.getConstraintGraph()[floatLiteralTy].initBindingSet();
 
-    bindings.finalize(/*transitive=*/true);
+    bindings.inferTransitiveKeyPathBindings();
+    (void) bindings.finalizeKeyPathBindings();
+
+    bindings.inferTransitiveUnresolvedMemberRefBindings();
+    bindings.finalizeUnresolvedMemberChainResult();
+
+    bindings.inferTransitiveSupertypeBindings();
+
+    bindings.determineLiteralCoverage();
 
     // Inferred a single transitive binding through `$T_float`.
     ASSERT_EQ(bindings.Bindings.size(), (unsigned)1);