P2713R1 Escaping improvements in std::format

Fixes NB US 38-098, FR 5-134 (C++23 CD).

Author: burblebee

source/utilities.tex

@@ -15931,12 +15931,20 @@
 \begin{itemize}
 \item
 \placeholder{CE} is a Unicode encoding and
-\placeholder{C} corresponds to either
-a UCS scalar value whose Unicode property \tcode{General_Category}
-has a value in the groups \tcode{Separator} (\tcode{Z}) or \tcode{Other} (\tcode{C}) or to
-a UCS scalar value which has the Unicode property \tcode{Grapheme_Extend=Yes},
+\placeholder{C} corresponds to a UCS scalar value
+whose Unicode property \tcode{General_Category} has a value in the groups
+%FIXME confusing - is the following C supposed to be the same C as above?
+\tcode{Separator} (\tcode{Z}) or \tcode{Other} (\tcode{C}),
 as described by table 12 of \UAX{44}, or
 
+\item
+\placeholder{CE} is a Unicode encoding and
+\placeholder{C} corresponds to a UCS scalar value
+which has the Unicode property \tcode{Grapheme_Extend=Yes} and
+\placeholder{C} is not immediately preceded in \placeholder{S} by
+a character \placeholder{P} appended to \placeholder{E}
+without translation to an escape sequence, or
+
 \item
 \placeholder{CE} is not a Unicode encoding and
 \placeholder{C} is one of an implementation-defined set
@@ -16018,8 +16026,9 @@
 then \placeholder{C} is appended unchanged.
 \end{itemize}
 
-%% FIXME: Example is incomplete; s2 and s6 are missing below;
+%% FIXME: Example is incomplete; s2 and s6 from P2286R8 are missing below;
 %% FIXME: their Unicode characters are not available in our font (Latin Modern).
+%% FIXME: ... same for s8 from P2713R1
 \begin{example}
 \begin{codeblock}
 string s0 = format("[{}]", "h\tllo");               // \tcode{s0} has value: \tcode{[h    llo]}
@@ -16030,6 +16039,8 @@
 string s4 = format("[{:?}]", string("\0 \n \t \x02 \x1b", 9));
                                                     // \tcode{s4} has value: \tcode{["\textbackslash u\{0\} \textbackslash n \textbackslash t \textbackslash u\{2\} \textbackslash u\{1b\}"]}
 string s5 = format("[{:?}]", "\xc3\x28");           // invalid UTF-8, \tcode{s5} has value: \tcode{["\textbackslash x\{c3\}("]}
+string s6 = format("[{:?}]", "\u0301");             // s6 has value: ["\u{301}"]
+string s7 = format("[{:?}]", "\\\u0301");           // s7 has value: ["\\\u{301}"]
 \end{codeblock}
 \end{example}