[ConstraintSystem] Mark type variable representing closure parameter (in the body) as incomplete

Restore recently removed logic to mark type variable representing
closure parameter used in the body of a closure as potentially
incomplete to delay attempting it until `BindParam` is simplified.

Resolves: rdar://problem/71858936

lib/Sema/CSBindings.cpp

@@ -77,6 +77,25 @@ bool ConstraintSystem::PotentialBindings::isPotentiallyIncomplete() const {
       return true;
   }
 
+  // If there is a `bind param` constraint associated with
+  // current type variable, result should be aware of that
+  // fact. Binding set might be incomplete until
+  // this constraint is resolved, because we currently don't
+  // look-through constraints expect to `subtype` to try and
+  // find related bindings.
+  // This only affects type variable that appears one the
+  // right-hand side of the `bind param` constraint and
+  // represents result type of the closure body, because
+  // left-hand side gets types from overload choices.
+  if (llvm::any_of(
+          EquivalentTo,
+          [&](const std::pair<TypeVariableType *, Constraint *> &equivalence) {
+            auto *constraint = equivalence.second;
+            return constraint->getKind() == ConstraintKind::BindParam &&
+                   constraint->getSecondType()->isEqual(TypeVar);
+          }))
+    return true;
+
   return false;
 }
 

test/Constraints/rdar71858936.swift

@@ -0,0 +1,28 @@
+// RUN: %target-typecheck-verify-swift
+
+@propertyWrapper
+@dynamicMemberLookup
+struct Binding<Value> {
+  var wrappedValue: Value
+
+  init(get: @escaping () -> Value, set: @escaping (Value) -> Void) {
+    self.wrappedValue = get()
+  }
+
+  subscript<Subject>(dynamicMember keyPath: WritableKeyPath<Value, Subject>) -> Binding<Subject> {
+    get { fatalError() }
+  }
+}
+
+class S {
+  var value: String = ""
+  var buffer: String? = nil
+
+  var body: String {
+    let binding = Binding(
+      get: { self.buffer ?? self.value },
+      set: { self.buffer = $0 }
+    )
+    return binding.wrappedValue
+  }
+}