Avoid fonts with incomplete coverage of MATHEMATICAL chars

* lisp/international/fontset.el (setup-default-fontset): Include both
ends of each sub-range in 'script-representative-chars' (bug#51495).

Some fonts include glyphs for MATHEMATICAL SCRIPT CAPITAL codepoints
(#x1D49C-#x1D4B5) but not for MATHEMATICAL SCRIPT SMALL codepoints
(#x1D4B6-#x1D4CF), see e.g. KpMath[1].

'script-representative-chars' must thus include both CAPITAL and SMALL
codepoints to ensure that we filter those fonts out.

Bug#51495; discussed in help-gnu-emacs[2].

[1] https://mirrors.ctan.org/fonts/kpfonts-otf/doc/unimath-kpfonts.pdf

[2] <878s02u5m6.fsf@gmail.com>
    https://lists.gnu.org/archive/html/help-gnu-emacs/2021-09/msg00150.html

diff --git a/lisp/international/fontset.el b/lisp/international/fontset.el
index fcd22e09d29e257e587dfc9b1d438d3ef5ac9a17..7c3a7cd1a9e7ac3f42dc09a5413c5b065e3b67a0 100644 (file)
--- a/lisp/international/fontset.el
+++ b/lisp/international/fontset.el
@@ -816,11 +816,16 @@
                           (#x1D7EC #x1D7F5 mathematical-sans-serif-bold)
                           (#x1D7F6 #x1D7FF mathematical-monospace)))
     (let ((slot (assq (nth 2 math-subgroup) script-representative-chars)))
+      ;; Add both ends of each subgroup to help filter out some
+      ;; incomplete fonts, e.g. those that cover MATHEMATICAL SCRIPT
+      ;; CAPITAL glyphs but not MATHEMATICAL SCRIPT SMALL ones.
       (if slot
-         (if (vectorp (cdr slot))
-             (setcdr slot (vconcat (cdr slot) (vector (car math-subgroup))))
-           (setcdr slot (vector (cadr slot) (car math-subgroup))))
-       (setq slot (list (nth 2 math-subgroup) (car math-subgroup)))
+          (setcdr slot (append (list (nth 0 math-subgroup)
+                                     (nth 1 math-subgroup))
+                               (cdr slot)))
+        (setq slot (list (nth 2 math-subgroup)
+                         (nth 0 math-subgroup)
+                         (nth 1 math-subgroup)))
        (nconc script-representative-chars (list slot))))
     (set-fontset-font
      "fontset-default"