bug#57499: Documentation bug in the docstring of set-face-attribute?

[Gregory Heytings <gregory@heytings.org>]

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>> Would a warning in *Messages* be okay then?
>
> We could try that, yes.  But it has to be only in *Messages*, not an 
> actual warning, since faces get merged and realized as part of 
> redisplay, when signaling an error is a bad idea.
>

Like this?  I just checked, it passes make bootstrap and make check.

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From c37b2cfdf9c5cbcba7cc174fcf8d6d17152c88a2 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Thu, 1 Sep 2022 13:02:27 +0000
Subject: [PATCH] Display a warning when using nil as a face attribute value.

* src/xfaces.c (Finternal_set_lisp_face_attribute): Using the
attribute value nil for the face attributes foreground,
distant-foreground and background is a backward compatibility feature,
that works only with existing frames.  Using it with frame = t (and
therefore also with frame = nil) will not produce the expected effect.
Display a warning in these cases.
---
 src/xfaces.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/xfaces.c b/src/xfaces.c
index 70d5cbeb4c..db7bc028c5 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -3051,6 +3051,22 @@ DEFUN ("internal-copy-lisp-face", Finternal_copy_lisp_face,
   return to;
 }
 
+char warning_attribute_nil[] =
+  "Warning: using :%s nil with frame t has no effect.";
+
+#define WARNING_ATTRIBUTE_NIL(S)					\
+  if (NILP (value) && EQ (frame, Qt))					\
+    {									\
+      if (redisplaying_p)						\
+	CALLN (Fmessage,						\
+	       build_string (warning_attribute_nil),			\
+	       build_string (S));					\
+      else								\
+	Fsignal (Quser_error,						\
+		 list1 (CALLN (Fformat,					\
+			       build_string (warning_attribute_nil),	\
+			       build_string (S))));			\
+    }
 
 DEFUN ("internal-set-lisp-face-attribute", Finternal_set_lisp_face_attribute,
        Sinternal_set_lisp_face_attribute, 3, 4, 0,
@@ -3390,6 +3406,7 @@ DEFUN ("internal-set-lisp-face-attribute", Finternal_set_lisp_face_attribute,
     }
   else if (EQ (attr, QCforeground))
     {
+      WARNING_ATTRIBUTE_NIL ("foreground");
       /* Compatibility with 20.x.  */
       if (NILP (value))
 	value = Qunspecified;
@@ -3409,6 +3426,7 @@ DEFUN ("internal-set-lisp-face-attribute", Finternal_set_lisp_face_attribute,
     }
   else if (EQ (attr, QCdistant_foreground))
     {
+      WARNING_ATTRIBUTE_NIL ("distant-foreground");
       /* Compatibility with 20.x.  */
       if (NILP (value))
 	value = Qunspecified;
@@ -3428,6 +3446,7 @@ DEFUN ("internal-set-lisp-face-attribute", Finternal_set_lisp_face_attribute,
     }
   else if (EQ (attr, QCbackground))
     {
+      WARNING_ATTRIBUTE_NIL ("background");
       /* Compatibility with 20.x.  */
       if (NILP (value))
 	value = Qunspecified;
-- 
2.35.1