[ConstraintSystem] NFC: Add an ability to check binding producers for exhaustion

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5434,6 +5434,8 @@ class BindingProducer {
   /// for type variables found at one level, before proceeding to
   /// supertypes or literal defaults etc.
   virtual bool needsToComputeNext() const = 0;
+
+  virtual bool isExhausted() const = 0;
 };
 
 class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
@@ -5460,6 +5462,8 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
   /// to that protocol or be wrapped in an optional.
   bool CanBeNil;
 
+  bool IsExhausted = false;
+
 public:
   using Element = TypeVariableBinding;
 
@@ -5469,11 +5473,16 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
   ArrayRef<Binding> getCurrentBindings() const { return Bindings; }
 
   Optional<Element> operator()() override {
+    if (isExhausted())
+      return None;
+
     // Once we reach the end of the current bindings
     // let's try to compute new ones, e.g. supertypes,
     // literal defaults, if that fails, we are done.
-    if (needsToComputeNext() && !computeNext())
+    if (needsToComputeNext() && !computeNext()) {
+      IsExhausted = true;
       return None;
+    }
 
     auto &binding = Bindings[Index++];
 
@@ -5492,7 +5501,11 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
     return TypeVariableBinding(TypeVar, binding);
   }
 
-  bool needsToComputeNext() const override { return Index >= Bindings.size(); }
+  bool needsToComputeNext() const override {
+    return isExhausted() ? false : Index >= Bindings.size();
+  }
+
+  bool isExhausted() const override { return IsExhausted; }
 
 private:
   /// Compute next batch of bindings if possible, this could
@@ -5565,10 +5578,10 @@ class DisjunctionChoiceProducer : public BindingProducer<DisjunctionChoice> {
   }
 
   Optional<Element> operator()() override {
-    unsigned currIndex = Index;
-    if (currIndex >= Choices.size())
+    if (isExhausted())
       return None;
 
+    unsigned currIndex = Index;
     bool isBeginningOfPartition = PartitionIndex < PartitionBeginning.size() &&
                                   PartitionBeginning[PartitionIndex] == Index;
     if (isBeginningOfPartition)
@@ -5594,6 +5607,8 @@ class DisjunctionChoiceProducer : public BindingProducer<DisjunctionChoice> {
 
   bool needsToComputeNext() const override { return false; }
 
+  bool isExhausted() const override { return Index >= Choices.size(); }
+
 private:
   // Partition the choices in the disjunction into groups that we will
   // iterate over in an order appropriate to attempt to stop before we