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.

2 files changed, 18 insertions, 10 deletions

diff --git a/lisp/composite.el b/lisp/composite.el
index 72b0ffc8f48..f7e0c4a0912 100644
--- a/lisp/composite.el
+++ b/lisp/composite.el
@@ -337,8 +337,9 @@ When Automatic Composition mode is on, this function also finds a
 chunk of text that is automatically composed.  If such a chunk is
 found closer to POS than the position that has `composition'
 property, the value is a list of FROM, TO, and a glyph-string
-that specifies how the chunk is to be composed.  See the function
-`composition-get-gstring' for the format of the glyph-string."
+that specifies how the chunk is to be composed; DETAIL-P is
+inored this case.  See the function `composition-get-gstring'
+for the format of the glyph-string."
   (let ((result (find-composition-internal pos limit string detail-p)))
     (if (and detail-p (> (length result) 3) (nth 2 result) (not (nth 3 result)))
 	;; This is a valid rule-base composition.
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)
     {