[CSBindings] Transitive key path root inference should make note of adjacent vars

Binding inference through a contextual root variable should include
marking contextual root and all of its adjacent variables as adjacent
to a key path root variable as well otherwise transitively inferred
bindings are ranked incorrectly.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -471,8 +471,16 @@ void BindingSet::inferTransitiveBindings(
                 if (bindings.isDelayed())
                   continue;
 
+                // Copy the bindings over to the root.
                 for (const auto &binding : bindings.Bindings)
                   addBinding(binding);
+
+                // Make a note that the key path root is transitively adjacent
+                // to contextual root type variable and all of its variables.
+                // This is important for ranking.
+                AdjacentVars.insert(contextualRootVar);
+                AdjacentVars.insert(bindings.AdjacentVars.begin(),
+                                    bindings.AdjacentVars.end());
               }
             } else {
               addBinding(

test/Sema/keypath_bidirectional_inference.swift

@@ -62,3 +62,23 @@ class FilteringTest {
              .assign(to: \.innerController, on: viewController) // Ok
   }
 }
+
+extension Sequence {
+  func sorted<T: Comparable>(by keyPath: KeyPath<Element, T>) -> [Element] {
+    []
+  }
+}
+
+func testCollectionUpcastWithTupleLabelErasure() {
+  struct Item {}
+
+  enum Info : Int, Hashable {
+    case one = 1
+  }
+
+
+  func test(data: [Info: [Item]]) -> [(Info, [Item])] {
+    data.map { $0 }
+        .sorted(by: \.key.rawValue) // Ok
+  }
+}