diff --git a/guix/build/union.scm b/guix/build/union.scm
index 6640b56523..a2ea72e1f5 100644
--- a/guix/build/union.scm
+++ b/guix/build/union.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2017 Huang Ying <huang.ying.caritas@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -73,9 +74,12 @@ identical, #f otherwise."
                                   (loop)))))))))))))
 
 (define* (union-build output inputs
-                      #:key (log-port (current-error-port)))
-  "Build in the OUTPUT directory a symlink tree that is the union of all
-the INPUTS."
+                      #:key (log-port (current-error-port))
+                      (create-all-directories? #f))
+  "Build in the OUTPUT directory a symlink tree that is the union of all the
+INPUTS.  As a special case, if CREATE-ALL-DIRECTORIES?, creates the
+subdirectories in the output directory to make sure the caller can modify them
+later."
 
   (define (symlink* input output)
     (format log-port "`~a' ~~> `~a'~%" input output)
@@ -104,8 +108,11 @@ the INPUTS."
   (define (union output inputs)
     (match inputs
       ((input)
-       ;; There's only one input, so just make a link.
-       (symlink* input output))
+       ;; There's only one input, so just make a link unless
+       ;; create-all-directories?.
+       (if (and create-all-directories? (file-is-directory? input))
+           (union-of-directories output inputs)
+           (symlink* input output)))
       (_
        (call-with-values (lambda () (partition file-is-directory? inputs))
          (match-lambda*
diff --git a/tests/union.scm b/tests/union.scm
index cccf397181..b63edc757b 100644
--- a/tests/union.scm
+++ b/tests/union.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -124,4 +124,24 @@
                 ;; new 'bin' sub-directory in the profile.
                 (eq? 'directory (stat:type (lstat "bin"))))))))
 
+(test-assert "union-build #:create-all-directories? #t"
+  (let* ((build  `(begin
+                    (use-modules (guix build union))
+                    (union-build (assoc-ref %outputs "out")
+                                 (map cdr %build-inputs)
+                                 #:create-all-directories? #t)))
+         (input  (package-derivation %store %bootstrap-guile))
+         (drv    (build-expression->derivation %store "union-test-all-dirs"
+                                               build
+                                               #:modules '((guix build union))
+                                               #:inputs `(("g" ,input)))))
+    (and (build-derivations %store (list drv))
+         (with-directory-excursion (derivation->output-path drv)
+           ;; Even though there's only one input to the union,
+           ;; #:create-all-directories? #t must have created bin/ rather than
+           ;; making it a symlink to Guile's bin/.
+           (and (file-exists? "bin/guile")
+                (file-is-directory? "bin")
+                (eq? 'symlink (stat:type (lstat "bin/guile"))))))))
+
 (test-end)