Change collapse-module-path-index's contract so that it accepts a #false return value, as this can occur when collapsing an empty module-path-index such as the one occurring on an identifier created by syntax-local-lift-expression

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

@@ -54,7 +54,7 @@ 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,
@@ -67,7 +67,14 @@ is relative. The 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 "6.9.0.4" @elem{The one-argument variant may return
+                                   @racket{#false}, e.g. for an empty
+                                   module path index such as the
+                                   @racket{source-mod} and
+                                   @racket{nominal-source-mod} of an
+                                   identifier created with
+                                   @racket{syntax-local-lift-expression}.}]}
 
 
 @close-eval[evaluator]

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

@@ -1,5 +1,7 @@
 #lang racket/base
-(require syntax/modcollapse)
+(require syntax/modcollapse
+         (for-syntax racket/base
+                     syntax/modcollapse))
 
 (define (check got expected)
   (unless (equal? got expected)
@@ -153,3 +155,15 @@
                                    here)
        (build-path here-dir "x" "apple.rkt"))
 
+
+(define-syntax (try-collapse-empty-module-path-index stx)
+  (datum->syntax
+   #f
+   #`'#,(collapse-module-path-index
+         (car
+          (identifier-binding
+           (syntax-local-lift-expression
+            (quote-syntax 1)))))))
+
+(check (try-collapse-empty-module-path-index)
+       #f)

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 module-path? #f))
                               ((or/c symbol? module-path-index?)
                                rel-to-module-path-v/c
                                . -> . simple-rel-to-module-path-v/c))])