[ConstraintSystem] Record all disjunctions selected along the current path

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -2251,6 +2251,7 @@ class ConstraintSystem {
   friend class SplitterStep;
   friend class ComponentStep;
   friend class TypeVariableStep;
+  friend class DisjunctionStep;
   friend class ConjunctionStep;
   friend class ConjunctionElement;
   friend class RequirementFailure;
@@ -2402,6 +2403,10 @@ class ConstraintSystem {
   /// the current constraint system.
   llvm::MapVector<ConstraintLocator *, unsigned> DisjunctionChoices;
 
+  /// The stack of all disjunctions selected during current path in order.
+  /// This stack is managed by the \c DisjunctionStep.
+  llvm::SmallVector<Constraint *, 4> SelectedDisjunctions;
+
   /// A map from applied disjunction constraints to the corresponding
   /// argument function type.
   llvm::SmallMapVector<ConstraintLocator *, const FunctionType *, 4>

lib/Sema/CSStep.h

@@ -653,6 +653,7 @@ class DisjunctionStep final : public BindingStep<DisjunctionChoiceProducer> {
     assert(Disjunction->getKind() == ConstraintKind::Disjunction);
     pruneOverloadSet(Disjunction);
     ++cs.solverState->NumDisjunctions;
+    cs.SelectedDisjunctions.push_back(Disjunction);
   }
 
   ~DisjunctionStep() override {
@@ -663,6 +664,8 @@ class DisjunctionStep final : public BindingStep<DisjunctionChoiceProducer> {
     // Re-enable previously disabled overload choices.
     for (auto *choice : DisabledChoices)
       choice->setEnabled();
+
+    CS.SelectedDisjunctions.pop_back();
   }
 
   StepResult resume(bool prevFailed) override;