The documentation for `with-extensions' says:

> In the same vein, sometimes you want to import not just pure-Scheme
> modules, but also “extensions” such as Guile bindings to C libraries
> or other “full-blown” packages.

However it does not actually add those C libraries into
(guile-extensions-path), which means that trying to actually use a
library imported this way will lead to an error.

I guess `load-path-expression' in (guix gexp) needs to also append to
the `guile-extensions-path' based on the search-paths of the imported
packages?

Reproduction is simple.  Make sure you *do not* have guile-yamlpp
installed in your profile.  Then build the `test' into a store item.

--8<---------------cut here---------------start------------->8---
(use-modules (gnu packages guile-xyz))
(define test
  (with-extensions (list guile-yamlpp)
    (program-file "test" #~(use-modules (yamlpp)))))
--8<---------------cut here---------------end--------------->8---

And try to run it:

--8<---------------cut here---------------start------------->8---
$ /gnu/store/nq0sahcnph5sxms4irq4inv2hq2hfbzr-test
Backtrace:
          19 (primitive-load "/gnu/store/nq0sahcnph5sxms4irq4inv2hq2hfbzr-test")
In ice-9/eval.scm:
   721:20 18 (primitive-eval _)
In ice-9/psyntax.scm:
  1229:36 17 (expand-top-sequence _ _ _ #f _ _ _)
  1221:19 16 (parse _ (("placeholder" placeholder)) ((top) #(ribcage () () ())) _ e (eval) #)
   259:10 15 (parse _ (("placeholder" placeholder)) (()) _ c&e (eval) (hygiene guile-user))
In ice-9/boot-9.scm:
  3935:20 14 (process-use-modules _)
   222:17 13 (map1 (((yamlpp))))
  3936:31 12 (_ ((yamlpp)))
  3327:17 11 (resolve-interface (yamlpp) #:select _ #:hide _ #:prefix _ #:renamer _ # _)
In ice-9/threads.scm:
    390:8 10 (_ _)
In ice-9/boot-9.scm:
  3253:13  9 (_)
In ice-9/threads.scm:
    390:8  8 (_ _)
In ice-9/boot-9.scm:
  3544:20  7 (_)
   2836:4  6 (save-module-excursion _)
  3564:26  5 (_)
In unknown file:
           4 (primitive-load-path "yamlpp" #<procedure 7f405e4fd560 at ice-9/boot-9.scm:35…>)
In yamlpp.scm:
     65:1  3 (_)
In unknown file:
           2 (load-extension "libguile-yamlpp" "init")
In system/foreign-library.scm:
   190:25  1 (load-foreign-library _ #:extensions _ #:search-ltdl-library-path? _ # _ # _ # …)
In unknown file:
           0 (dlopen "libguile-yamlpp.so" 1)

ERROR: In procedure dlopen:
In procedure dlopen: file "libguile-yamlpp.so", message "libguile-yamlpp.so: cannot open shared object file: No such file or directory"
--8<---------------cut here---------------end--------------->8---

Have a nice day,
Tomas

-- 
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.