Avoid assertions in find-composition

* src/font.c (font_range): If called with STRING non-nil and FACE
a NULL pointer, compute face by calling face_at_string_position.
(Bug#29506)

* lisp/composite.el (find-composition): Doc fix.

1 files changed, 15 insertions, 8 deletions

diff --git a/src/font.c b/src/font.c
index f7cebdce78c..441652b0951 100644
--- a/src/font.c
+++ b/src/font.c
@@ -3794,19 +3794,26 @@ font_range (ptrdiff_t pos, ptrdiff_t pos_byte, ptrdiff_t *limit,
   int c;
   Lisp_Object font_object = Qnil;
 
-  if (NILP (string))
+  if (!face)
     {
-      if (! face)
+      struct frame *f = XFRAME (w->frame);
+      int face_id;
+
+      if (NILP (string))
+          face_id = face_at_buffer_position (w, pos, &ignore, *limit,
+                                             false, -1);
+      else
         {
-          int face_id;
+          face_id =
+            NILP (Vface_remapping_alist)
+            ? DEFAULT_FACE_ID
+            : lookup_basic_face (f, DEFAULT_FACE_ID);
 
-          face_id = face_at_buffer_position (w, pos, &ignore,
-                                             *limit, false, -1);
-          face = FACE_FROM_ID (XFRAME (w->frame), face_id);
+          face_id = face_at_string_position (w, string, pos, 0, &ignore,
+                                             face_id, false);
         }
+      face = FACE_FROM_ID (f, face_id);
     }
-  else
-    eassert (face);
 
   while (pos < *limit)
     {