Merge pull request #14698 from rudkx/join

Start stubbing out more parts of Type::join().

Author: rudkx

include/swift/AST/Type.h

@@ -368,15 +368,19 @@ class Type {
   /// class D { }
   /// \endcode
   ///
-  /// The join of B and C is A, the join of A and B is A. However, there is no
-  /// join of D and A (or D and B, or D and C) because there is no common
-  /// superclass. One would have to jump to an existential (e.g., \c AnyObject)
-  /// to find a common type.
+  /// The join of B and C is A, the join of A and B is A.
+  ///
+  /// The Any type is considered the common supertype by default when no
+  /// closer common supertype exists.
+  ///
+  /// In unsupported cases where we cannot yet compute an accurate join,
+  /// we return None.
   ///
   /// \returns the join of the two types, if there is a concrete type
   /// that can express the join, or Any if the only join would be a
-  /// more-general existential type
-  static Type join(Type first, Type second);
+  /// more-general existential type, or None if we cannot yet compute a
+  /// correct join but one better than Any may exist.
+  static Optional<Type> join(Type first, Type second);
 
 private:
   // Direct comparison is disabled for types, because they may not be canonical.