[CSBindings] Optional object type variable should be connected to the optional

If both sides of an `OptionalObject` constraint are un-inferred, their
binding sets need to correctly reflect adjacency - a type variable
that represents optional would get "object" as an adjacency through
its potential binding (the binding is - "object" wrapped in a single
level of optional) and "object" type variable needs to get its parent
optional type variable added to its adjacency list explicitly.
Without this it would be possible to prematurely pick "object" before
its parent optional type variable.

Resolves: swiftlang#73207
Resolves: rdar://126960579

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1671,6 +1671,16 @@ PotentialBindings::inferFromRelational(Constraint *constraint) {
       break;
     }
 
+    case ConstraintKind::OptionalObject: {
+      // Type variable that represents an object type of
+      // an un-inferred optional is adjacent to a type
+      // variable that presents such optional (`bindingTypeVar`
+      // in this case).
+      if (kind == AllowedBindingKind::Supertypes)
+        AdjacentVars.insert({bindingTypeVar, constraint});
+      break;
+    }
+
     default:
       break;
     }

test/stmt/foreach.swift

@@ -341,3 +341,13 @@ do {
     }
   }
 }
+
+// https://github.com/apple/swift/issues/73207
+do {
+  func test(_ levels: [Range<Int>]) {
+    for (i, leaves): (Int, Range<Int>) in levels[8 ..< 15].enumerated() { // Ok
+      _ = i
+      _ = leaves
+    }
+  }
+}