gexp: Allow character literals in GEXP->SEXP.

Fixes <https://bugs.gnu.org/38628>.

* tests/gexp.scm ("lower-gexp, character literal"): New test.
* guix/gexp.scm (gexp->sexp)[self-quoting?]: Add CHAR? to the tested types.
* guix/repl.scm (self-quoting?): Likewise.
* gnu/tests.scm (marionette-shepherd-service)[self-quoting?]: Likewise.

gnu/tests.scm

@@ -98,7 +98,7 @@
                                                 (or (pred x)
                                                     (one-of rest ...))))))
                        (one-of symbol? string? keyword? pair? null? array?
-                               number? boolean?)))
+                               number? boolean? char?)))
 
                    (match (primitive-fork)
                      (0

guix/gexp.scm

@@ -1028,7 +1028,7 @@ and in the current monad setting (system type, etc.)"
                                (or (pred x)
                                    (one-of rest ...))))))
       (one-of symbol? string? keyword? pair? null? array?
-              number? boolean?)))
+              number? boolean? char?)))
 
   (define* (reference->sexp ref #:optional native?)
     (with-monad %store-monad

guix/repl.scm

@@ -37,7 +37,7 @@
                              (or (pred x)
                                  (one-of rest ...))))))
     (one-of symbol? string? keyword? pair? null? array?
-            number? boolean?)))
+            number? boolean? char?)))
 
 (define (send-repl-response exp output)
   "Write the response corresponding to the evaluation of EXP to PORT, an

tests/gexp.scm

@@ -886,6 +886,12 @@
     (run-with-store %store
       (lower-gexp #~(foo #$+)))))
 
+(test-equal "lower-gexp, character literal"
+  '(#\+)
+  (lowered-gexp-sexp
+   (run-with-store %store
+     (lower-gexp #~(#\+)))))
+
 (test-assertm "gexp->derivation #:references-graphs"
   (mlet* %store-monad
       ((one (text-file "one" (random-text)))