make collapse-module-path-index work on a "self" modidx

pkgs/racket-doc/syntax/scribblings/modcollapse.scrbl

@@ -54,20 +54,26 @@ base.
                                                                     (-> module-path?))])
             module-path?]
            [(collapse-module-path-index [module-path-index module-path-index?])
-            module-path?])]{
+            (or/c module-path? #f)])]{
 
 Like @racket[collapse-module-path] when given two arguments, but the
 input is a @techlink[#:doc refman]{module path index}; in this case,
 the @racket[rel-to-module-path-v] base is used where the module path
-index contains the ``self'' index.
+index contains the ``self'' index (see @racket[module-path-index-split]).
 
 When given a single argument, @racket[collapse-module-path-index]
 returns a module path that is relative if the given module path index
-is relative. The resulting module path is not necessarily normalized.
+is relative, except that it returns @racket[#f] if its argument is the
+``self'' module path index. A resulting module path is
+not necessarily normalized.
 
 @history[#:changed "6.1.1.8" @elem{Added the one-argument variant for
                                    collapsing a relative module path
-                                   index.}]}
+                                   index.}
+         #:changed "6.9.0.5" @elem{Added support for the ``self'' module
+                                   path index as the only argument, which
+                                   meant extending the result contract to
+                                   include @scheme[#f]}]}
 
 
 @close-eval[evaluator]

pkgs/racket-test/tests/syntax/modcollapse.rkt

@@ -10,6 +10,11 @@
 
 (define self (module-path-index-join #f #f))
 
+(check (collapse-module-path-index self)
+       #f)
+(check (collapse-module-path-index self here)
+       here)
+
 (define (check-collapse p expected [relative-expected expected]
                         #:here [here here])
   (check (collapse-module-path p here)

racket/collects/syntax/modcollapse.rkt

@@ -32,7 +32,7 @@
                         . -> . simple-rel-to-module-path-v/c)]
  [collapse-module-path-index (case->
                               (module-path-index?
-                               . -> . module-path?)
+                               . -> . (or/c #f module-path?))
                               ((or/c symbol? module-path-index?)
                                rel-to-module-path-v/c
                                . -> . simple-rel-to-module-path-v/c))])