[ConstraintSystem] Add a new endpoint to infer bindings for a given type variable

Since bindings now require finalization we need a new endpoint
which perform all of the required actions before returning complete
`PotentialBindings` object when they are requested for a particular
type variable without any other context.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -491,6 +491,16 @@ bool ConstraintSystem::PotentialBindings::favoredOverDisjunction(
   return !InvolvesTypeVariables;
 }
 
+ConstraintSystem::PotentialBindings
+ConstraintSystem::inferBindingsFor(TypeVariableType *typeVar) {
+  auto bindings = getPotentialBindings(typeVar);
+
+  llvm::SmallDenseMap<TypeVariableType *, ConstraintSystem::PotentialBindings>
+      inferred;
+  bindings.finalize(*this, inferred);
+  return bindings;
+}
+
 Optional<ConstraintSystem::PotentialBinding>
 ConstraintSystem::getPotentialBindingForRelationalConstraint(
     PotentialBindings &result, Constraint *constraint,

lib/Sema/CSSolver.cpp

@@ -2380,7 +2380,7 @@ void DisjunctionChoice::propagateConversionInfo(ConstraintSystem &cs) const {
   if (typeVar->getImpl().getFixedType(nullptr))
     return;
 
-  auto bindings = cs.getPotentialBindings(typeVar);
+  auto bindings = cs.inferBindingsFor(typeVar);
   if (bindings.InvolvesTypeVariables || bindings.Bindings.size() != 1)
     return;
 

lib/Sema/ConstraintGraph.cpp

@@ -1116,7 +1116,7 @@ bool ConstraintGraph::contractEdges() {
     // us enough information to decided on l-valueness.
     if (isParamBindingConstraint && tyvar1->getImpl().canBindToInOut()) {
       bool isNotContractable = true;
-      if (auto bindings = CS.getPotentialBindings(tyvar1)) {
+      if (auto bindings = CS.inferBindingsFor(tyvar1)) {
         // Holes can't be contracted.
         if (bindings.IsHole)
           continue;

lib/Sema/ConstraintSystem.h

@@ -4649,14 +4649,19 @@ class ConstraintSystem {
 
   Optional<Type> checkTypeOfBinding(TypeVariableType *typeVar, Type type) const;
   Optional<PotentialBindings> determineBestBindings();
+
+  /// Infer bindings for the given type variable based on current
+  /// state of the constraint system.
+  PotentialBindings inferBindingsFor(TypeVariableType *typeVar);
+
+private:
   Optional<ConstraintSystem::PotentialBinding>
   getPotentialBindingForRelationalConstraint(
       PotentialBindings &result, Constraint *constraint,
       bool &hasDependentMemberRelationalConstraints,
       bool &hasNonDependentMemberRelationalConstraints) const;
   PotentialBindings getPotentialBindings(TypeVariableType *typeVar) const;
 
-private:
   /// Add a constraint to the constraint system.
   SolutionKind addConstraintImpl(ConstraintKind kind, Type first, Type second,
                                  ConstraintLocatorBuilder locator,