[ConstraintSystem] Clarify DefaultClosureType constraint and add its own simplify method

lib/Sema/CSSimplify.cpp

@@ -6448,13 +6448,14 @@ ConstraintSystem::simplifyValueWitnessConstraint(
 }
 
 ConstraintSystem::SolutionKind ConstraintSystem::simplifyDefaultableConstraint(
-    ConstraintKind kind, Type first, Type second, TypeMatchOptions flags,
+    Type first, Type second, TypeMatchOptions flags,
     ConstraintLocatorBuilder locator) {
   first = getFixedTypeRecursive(first, flags, true);
 
   if (first->isTypeVariableOrMember()) {
     if (flags.contains(TMF_GenerateConstraints)) {
-      addUnsolvedConstraint(Constraint::create(*this, kind, first, second,
+      addUnsolvedConstraint(
+          Constraint::create(*this, ConstraintKind::Defaultable, first, second,
                              getConstraintLocator(locator)));
       return SolutionKind::Solved;
     }
@@ -6466,6 +6467,28 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyDefaultableConstraint(
   return SolutionKind::Solved;
 }
 
+ConstraintSystem::SolutionKind
+ConstraintSystem::simplifyDefaultClosureTypeConstraint(
+    Type closureType, Type inferredType,
+    ArrayRef<TypeVariableType *> referencedOuterParameters,
+    TypeMatchOptions flags, ConstraintLocatorBuilder locator) {
+  closureType = getFixedTypeRecursive(closureType, flags, /*wantRValue=*/true);
+
+  if (closureType->isTypeVariableOrMember()) {
+    if (flags.contains(TMF_GenerateConstraints)) {
+      addUnsolvedConstraint(Constraint::create(
+          *this, ConstraintKind::DefaultClosureType, closureType, inferredType,
+          getConstraintLocator(locator), referencedOuterParameters));
+      return SolutionKind::Solved;
+    }
+
+    return SolutionKind::Unsolved;
+  }
+
+  // Otherwise, any type is fine.
+  return SolutionKind::Solved;
+}
+
 ConstraintSystem::SolutionKind
 ConstraintSystem::simplifyOneWayConstraint(
     ConstraintKind kind,
@@ -8857,9 +8880,7 @@ ConstraintSystem::addConstraintImpl(ConstraintKind kind, Type first,
     return simplifyOptionalObjectConstraint(first, second, subflags, locator);
 
   case ConstraintKind::Defaultable:
-  case ConstraintKind::DefaultClosureType:
-    return simplifyDefaultableConstraint(kind, first, second, subflags,
-                                         locator);
+    return simplifyDefaultableConstraint(first, second, subflags, locator);
 
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
@@ -8876,6 +8897,7 @@ ConstraintSystem::addConstraintImpl(ConstraintKind kind, Type first,
   case ConstraintKind::Disjunction:
   case ConstraintKind::KeyPath:
   case ConstraintKind::KeyPathApplication:
+  case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Use the correct addConstraint()");
   }
 
@@ -9270,13 +9292,18 @@ ConstraintSystem::simplifyConstraint(const Constraint &constraint) {
                                           constraint.getLocator());
 
   case ConstraintKind::Defaultable:
-  case ConstraintKind::DefaultClosureType:
-    return simplifyDefaultableConstraint(constraint.getKind(),
-                                         constraint.getFirstType(),
+    return simplifyDefaultableConstraint(constraint.getFirstType(),
                                          constraint.getSecondType(),
                                          TMF_GenerateConstraints,
                                          constraint.getLocator());
 
+  case ConstraintKind::DefaultClosureType:
+    return simplifyDefaultClosureTypeConstraint(constraint.getFirstType(),
+                                                constraint.getSecondType(),
+                                                constraint.getTypeVariables(),
+                                                TMF_GenerateConstraints,
+                                                constraint.getLocator());
+
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
     return simplifyFunctionComponentConstraint(constraint.getKind(),

lib/Sema/Constraint.cpp

@@ -608,13 +608,16 @@ bool Constraint::isExplicitConversion() const {
 
 Constraint *Constraint::create(ConstraintSystem &cs, ConstraintKind kind, 
                                Type first, Type second,
-                               ConstraintLocator *locator) {
+                               ConstraintLocator *locator,
+                               ArrayRef<TypeVariableType *> extraTypeVars) {
   // Collect type variables.
   SmallVector<TypeVariableType *, 4> typeVars;
   if (first->hasTypeVariable())
     first->getTypeVariables(typeVars);
   if (second && second->hasTypeVariable())
     second->getTypeVariables(typeVars);
+
+  typeVars.append(extraTypeVars.begin(), extraTypeVars.end());
   uniqueTypeVariables(typeVars);
 
   // Conformance constraints expect an existential on the right-hand side.

lib/Sema/Constraint.h

@@ -161,8 +161,14 @@ enum class ConstraintKind : char {
   /// type). At that point, this constraint will be treated like an `Equal`
   /// constraint.
   OneWayEqual,
-  /// If there are no contextual types e.g. `_ = { 42 }` default first type
-  /// to a second type (inferred closure type).
+  /// If there is no contextual info e.g. `_ = { 42 }` default first type
+  /// to a second type (inferred closure type). This is effectively a
+  /// `Defaultable` constraint which a couple of differences:
+  ///
+  /// - References inferred closure type and all of the outer parameters
+  ///   referenced by closure body.
+  /// - Handled specially by binding inference, specifically contributes
+  ///   to the bindings only if there are no contextual types available.
   DefaultClosureType,
 };
 
@@ -410,8 +416,8 @@ class Constraint final : public llvm::ilist_node<Constraint>,
 public:
   /// Create a new constraint.
   static Constraint *create(ConstraintSystem &cs, ConstraintKind Kind,
-                            Type First, Type Second,
-                            ConstraintLocator *locator);
+                            Type First, Type Second, ConstraintLocator *locator,
+                            ArrayRef<TypeVariableType *> extraTypeVars = {});
 
   /// Create a new constraint.
   static Constraint *create(ConstraintSystem &cs, ConstraintKind Kind, 

lib/Sema/ConstraintSystem.h

@@ -3481,11 +3481,16 @@ private:
       ConstraintLocatorBuilder locator);
 
   /// Attempt to simplify the given defaultable constraint.
-  SolutionKind simplifyDefaultableConstraint(ConstraintKind kind,
-                                             Type first, Type second,
+  SolutionKind simplifyDefaultableConstraint(Type first, Type second,
                                              TypeMatchOptions flags,
                                              ConstraintLocatorBuilder locator);
 
+  /// Attempt to simplify the given defaultable closure type constraint.
+  SolutionKind simplifyDefaultClosureTypeConstraint(
+      Type closureType, Type inferredType,
+      ArrayRef<TypeVariableType *> referencedOuterParameters,
+      TypeMatchOptions flags, ConstraintLocatorBuilder locator);
+
   /// Attempt to simplify a one-way constraint.
   SolutionKind simplifyOneWayConstraint(ConstraintKind kind,
                                         Type first, Type second,