Add rules to allow omission of certain default values

Author: eernstg

working/augmentation-libraries/feature-specification.md

@@ -774,7 +774,8 @@ It's a **compile-time** error if:
     matching] the signature of the augmented function.
 
 *   The augmenting function specifies any default values. *Default values are
-    defined solely by the introductory function.*
+    defined solely by the introductory function. Note that constructors are
+    treated differently from other functions in this respect.*
 
 *   A function is not complete after all augmentations are applied, unless it's
     an instance member and the surrounding class is abstract. *Every function
@@ -845,13 +846,21 @@ Augmenting constructors works similar to augmenting a function, with some extra
 rules to handle features unique to constructors like redirections and
 initializer lists.
 
+It is **not** a compile-time error for an incomplete factory constructor to
+omit default values. *That is, they are treated similarly to abstract
+instance methods in this respect. This allows the augmenting declaration to
+implement the constructor by adding a redirection or a body.*
+
 It's a **compile-time error** if:
 
 *   The signature of the augmenting function does not [match][signature
     matching] the signature of the augmented function.
 
-*   The augmenting constructor parameters specify any default values.
-    *Default values are defined solely by the introductory constructor.*
+*   The augmenting constructor parameters specify any default values,
+    and the constructor is a non-redirecting factory. *Default values are
+    defined by the introductory constructor, except when this precludes the
+    augmentation from choosing whether or not the constructor should be
+    redirecting.*
 
 *   The introductory constructor is `const` and the augmenting constructor
     is not or vice versa. *An augmentation can't change whether or not a