P1868R2 width: clarifying units of width and precision in std::format

Also fixes NB US 228 (C++20 CD) and LWG3290.

Author: Dawn Perchik

source/intro.tex

@@ -35,10 +35,6 @@
 \begin{itemize}
 \item Ecma International, \doccite{ECMAScript Language Specification},
 Standard Ecma-262, third edition, 1999.
-% FIXME: Is the following comment refering to the entry above or
-%        the entry removed in LWG3319?  I.e. is it still valid?
-%%% Format for this entry is based on that specified at
-%%% http://www.iec.ch/standardsdev/resources/draftingpublications/directives/principles/referencing.htm
 \item ISO/IEC 2382 (all parts), \doccite{Information technology ---
 Vocabulary}
 \item ISO 8601:2004, \doccite{Data elements and interchange formats ---
@@ -56,6 +52,11 @@
 \item ISO 80000-2:2009, \doccite{Quantities and units ---
 Part 2: Mathematical signs and symbols
 to be used in the natural sciences and technology}
+%%% Format for the following entry is based on that specified at
+%%% http://www.iec.ch/standardsdev/resources/draftingpublications/directives/principles/referencing.htm
+\item The Unicode Consortium. Unicode Standard Annex, UAX \#29, \doccite{Unicode Text Segmentation} [online].
+Edited by Mark Davis. Revision 35; issued for Unicode 12.0.0. 2019-02-15 [viewed 2020-02-23].
+Available at \url{http://www.unicode.org/reports/tr29/tr29-35.html}
 \end{itemize}
 
 \pnum

source/utilities.tex

@@ -19371,6 +19371,55 @@
 there is no minimum field width, and
 the field width is determined based on the content of the field.
 
+\pnum
+\indextext{string!width}%
+The \defn{width} of a string is defined as
+the estimated number of column positions appropriate
+for displaying it in a terminal.
+\begin{note}
+This is similar to the semantics of the POSIX \tcode{wcswidth} function.
+\end{note}
+
+\pnum
+For the purposes of width computation,
+a string is assumed to be in
+a locale-independent, implementation-defined encoding.
+Implementations should use a Unicode encoding
+on platforms capable of displaying Unicode text in a terminal.
+\begin{note}
+This is the case for Windows-based and many POSIX-based operating systems.
+\end{note}
+
+\pnum
+For a string in a Unicode encoding,
+implementations should estimate the width of a string
+as the sum of estimated widths of
+the first code points in its extended grapheme clusters
+as defined by
+Unicode Standard Annex \#29 Unicode Text Segmentation.
+The estimated width of the following code points is 2:
+\begin{itemize}
+\item \tcode{U+1100-U+115F}
+\item \tcode{U+2329}
+\item \tcode{U+232A}
+\item \tcode{U+2E80-U+303E}
+\item \tcode{U+3040-U+A4CF}
+\item \tcode{U+AC00-U+D7A3}
+\item \tcode{U+F900-U+FAFF}
+\item \tcode{U+FE10-U+FE19}
+\item \tcode{U+FE30-U+FE6F}
+\item \tcode{U+FF00-U+FF60}
+\item \tcode{U+FFE0-U+FFE6}
+\item \tcode{U+20000-U+2FFFD}
+\item \tcode{U+30000-U+3FFFD}
+\item \tcode{U+1F300-U+1F64F}
+\item \tcode{U+1F900-U+1F9FF}
+\end{itemize}
+The estimated width of other code points is 1.
+
+\pnum
+For a string in a non-Unicode encoding, the width of a string is unspecified.
+
 \pnum
 A zero (\tcode{0}) character
 preceding the \fmtgrammarterm{width} field
@@ -19398,7 +19447,13 @@
 the precision or maximum field size.
 It can only be used with floating-point and string types.
 For floating-point types this field specifies the formatting precision.
-For string types it specifies how many characters will be used from the string.
+For string types, this field provides an upper bound
+for the estimated width of the prefix of
+the input string that is copied into the output.
+For a string in a Unicode encoding,
+the formatter copies to the output
+the longest prefix of whole extended grapheme clusters
+whose estimated width is no greater than the precision.
 
 \pnum
 When the \tcode{L} option is used, the form used for the conversion is called