With the improved escape sequence parser, it becomes easy to also improve

diagnostics.  Distinguish "incomplete escape sequence", "invalid special
character", and "unknown special character" from the generic "invalid
escape sequence", also promoting them from WARNING to ERROR because
incomplete escape sequences are severe syntax violations and because
encountering an invalid or unknown special character makes it likely
that part of the document content intended by the authors gets lost.

1 files changed, 45 insertions, 15 deletions

diff --git a/mandoc.1 b/mandoc.1
index a730966d..c9d59ccc 100644
--- a/mandoc.1
+++ b/mandoc.1
@@ -1799,21 +1799,9 @@ A new sentence starts in the middle of a text line.
 Start it on a new input line to help formatters produce correct spacing.
 .It Sy "invalid escape sequence"
 .Pq roff
-An escape sequence has an invalid opening argument delimiter, lacks the
-closing argument delimiter, the argument is of an invalid form, or it is
-a character escape sequence with an invalid name.
-If the argument is incomplete,
-.Ic \e*
-and
-.Ic \en
-expand to an empty string,
-.Ic \eB
-to the digit
-.Sq 0 ,
-and
-.Ic \ew
-to the length of the incomplete argument.
-All other invalid escape sequences are ignored.
+An escape sequence has an invalid opening argument delimiter
+or the argument is of an invalid form.
+Invalid escape sequences are ignored.
 .It Sy "undefined escape, printing literally"
 .Pq roff
 In an escape sequence, the first character
@@ -2285,6 +2273,48 @@ with invalid arguments
 .El
 The excess arguments are ignored.
 .El
+.Ss "Errors related to escape sequences"
+.Bl -ohang
+.It Sy "incomplete escape sequence"
+.Pq roff
+The end of the input line is encountered
+while parsing the argument of an escape sequence.
+In this case,
+.Ic \e*
+and
+.Ic \en
+expand to an empty string,
+.Ic \eB
+to the digit
+.Sq 0 ,
+and
+.Ic \ew
+to the length of the incomplete argument.
+All other incomplete escape sequences are ignored.
+.It Sy "invalid special character"
+.Pq roff
+A special character escape sequence is invalid,
+for example a Unicode sequence pointing to a surrogate
+or beyond the Unicode range, a \e[char...] escape sequence
+representing a control character or pointing beyond the
+.Vt unsigned char
+range, or an invalid variable-length form
+of a single-byte character escape sequence, for example writing
+.Qq \e[e]
+or
+.Qq \e[~]
+instead of
+.Qq \ee
+or
+.Qq \e~ ,
+respectively.
+The escape sequence is ignored.
+.It Sy "unknown special character"
+.Pq roff
+The name given in a special character escape sequence is not known to
+.Nm .
+The escape sequence is ignored.
+.El
 .Ss Unsupported features
 .Bl -ohang
 .It Sy "input too large"