[CSBindings] Record produced types incrementally

Allow type variable binding producer to record types as they
are being produced. Otherwise it could be possible to re-discover
already explored types during `computeNext()` which leads to
duplicate bindings e.g. inferring fallback `Void` for a closure
result type when `Void` was already inferred as a direct/transitive
binding.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5453,7 +5453,21 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
     if (needsToComputeNext() && !computeNext())
       return None;
 
-    return TypeVariableBinding(TypeVar, Bindings[Index++]);
+    auto &binding = Bindings[Index++];
+
+    // Record produced type as bound/explored early, otherwise
+    // it could be possible to re-discover it during `computeNext()`,
+    // which leads to duplicate bindings e.g. inferring fallback
+    // `Void` for a closure result type when `Void` was already
+    // inferred as a direct/transitive binding.
+    {
+      auto type = binding.BindingType;
+
+      BoundTypes.insert(type.getPointer());
+      ExploredTypes.insert(type->getCanonicalType());
+    }
+
+    return TypeVariableBinding(TypeVar, binding);
   }
 
   bool needsToComputeNext() const override { return Index >= Bindings.size(); }

lib/Sema/CSBindings.cpp

@@ -1705,10 +1705,6 @@ bool TypeVarBindingProducer::computeNext() {
     const auto type = binding.BindingType;
     assert(!type->hasError());
 
-    // After our first pass, note that we've explored these types.
-    if (NumTries == 0)
-      ExploredTypes.insert(type->getCanonicalType());
-
     // If we have a protocol with a default type, look for alternative
     // types to the default.
     if (NumTries == 0 && binding.hasDefaultedLiteralProtocol()) {