Patch backported by Sage.

Fix from upstream that happens to work around
https://trac.sagemath.org/ticket/23011
diff --git a/src/lsp/format.lsp b/src/lsp/format.lsp
index 77ca799..53b887c 100644
--- a/src/lsp/format.lsp
+++ b/src/lsp/format.lsp
@@ -307,11 +307,13 @@
                   :start (format-directive-start struct)
                   :end (format-directive-end struct))))
 
+(defconstant +format-directive-limit+ (1+ (char-code #\~)))
+
 #+formatter
 (defparameter *format-directive-expanders*
-  (make-array char-code-limit :initial-element nil))
+  (make-array +format-directive-limit+ :initial-element nil))
 (defparameter *format-directive-interpreters*
-  (make-array char-code-limit :initial-element nil))
+  (make-array +format-directive-limit+ :initial-element nil))
 
 (defparameter *default-format-error-control-string* nil)
 (defparameter *default-format-error-offset* nil)
@@ -550,24 +552,24 @@
            (write-string directive stream)
            (interpret-directive-list stream (cdr directives) orig-args args))
           (#-ecl format-directive #+ecl vector
+           (multiple-value-bind
+                 (new-directives new-args)
+               (let* ((code (char-code (format-directive-character directive)))
+                      (function
+                        (and (< code +format-directive-limit+)
+                             (svref *format-directive-interpreters* code)))
+                      (*default-format-error-offset*
+                        (1- (format-directive-end directive))))
+                 (unless function
+                   (error 'format-error
+                          :complaint "Unknown format directive."))
                  (multiple-value-bind
                        (new-directives new-args)
-                     (let ((function
-                            (svref *format-directive-interpreters*
-                                   (char-code (format-directive-character
-                                               directive))))
-                           (*default-format-error-offset*
-                            (1- (format-directive-end directive))))
-                       (unless function
-                         (error 'format-error
-                                :complaint "Unknown format directive."))
-                       (multiple-value-bind
-                             (new-directives new-args)
-                           (funcall function stream directive
-                                    (cdr directives) orig-args args)
-                         (values new-directives new-args)))
-                   (interpret-directive-list stream new-directives
-                                             orig-args new-args)))))
+                     (funcall function stream directive
+                              (cdr directives) orig-args args)
+                   (values new-directives new-args)))
+             (interpret-directive-list stream new-directives
+                                       orig-args new-args)))))
       args))
 
 
@@ -639,11 +641,12 @@
        (values `(write-string ,directive stream)
                more-directives))
       (format-directive
-       (let ((expander
-              (aref *format-directive-expanders*
-                    (char-code (format-directive-character directive))))
-             (*default-format-error-offset*
-              (1- (format-directive-end directive))))
+       (let* ((code (char-code (format-directive-character directive)))
+              (expander
+                (and (< code +format-directive-limit+)
+                     (svref *format-directive-expanders* code)))
+              (*default-format-error-offset*
+                (1- (format-directive-end directive))))
          (if expander
              (funcall expander directive more-directives)
              (error 'format-error
d>Ludovic Courtès</td></tr>
<tr><td><span title='2020-07-18 21:12:38 +0200'>2020-07-18</span></td><td><a href='/guix/commit/.guix-authorizations?id=ba1d9680d61d3b06d9b81a81863448f494d654a7'>.guix-authorizations: Add brettgilio (back) to the committers.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: Add fingerprint for brettgilio.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-07-01 12:50:22 +0200'>2020-07-01</span></td><td><a href='/guix/commit/.guix-authorizations?id=9f5f3932debc72a57a830fc6ca5ab980f6db4941'>.guix-authorizations: Remove keys of two former contributors.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: Remove key of David Craven, who left the project
in 2017, and Federico Beffa, whose last commit was in Feb. 2017.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-07-01 12:50:22 +0200'>2020-07-01</span></td><td><a href='/guix/commit/.guix-authorizations?id=5402f3ed05e72f17509e3e732b67488b4643066d'>.guix-authorizations: Remove former keys that are no longer used.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: Remove old keys of dvc, efraim, and janneke.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-07-01 12:50:22 +0200'>2020-07-01</span></td><td><a href='/guix/commit/.guix-authorizations?id=eb8a79218d33a513d4673162fd752efb1152a084'>.guix-authorizations: Remove brettgilio's old key.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: Remove key "DFC0 C7F7 9EE6 0CA7 AE55  5E19 6722
43C4 A03F 0EEE" (Brett Gilio).  See
&lt;https://lists.gnu.org/archive/html/guix-devel/2020-06/msg00323.html&gt;.
</span></span></td><td>Ludovic Courtès</td></tr>