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Add paper to reword stuff related to ISO 10646
Addresses sg16-unicode#8
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| # Address wording issues related to ISO 10646 | ||
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| Document Number: DnnnnR0 | ||
| Date: 2018-06-28 | ||
| Audience: SG16, CWG | ||
| Author: R. Martinho Fernandes | ||
| Reply-to: cpp@rmf.io | ||
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| ## Motivation | ||
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| Review of some editorial fixes following the recent update of the normative | ||
| reference to ISO 10646 has unearthed a series of wording issues around the | ||
| subject. This paper intends to fix those issues by rewording relevant | ||
| paragraphs. | ||
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| ## Proposal | ||
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| This paper addresses all of the following issues: | ||
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| 1. The current wording in [lex.charset] does not specify what the behaviour is | ||
| for a universal-character-name without a corresponding short identifier in | ||
| ISO 10646. | ||
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| For example, `\U99004141` and `\U00110000`. Neither of these designates | ||
| a code point in ISO 10646, but the standard is silent about this, which | ||
| makes the behaviour undefined by omission. | ||
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| This paper addresses this by making such uses ill-formed, maintaining | ||
| consistency with the current treatment of surrogate values (`\U0000D800` | ||
| is already ill-formed). | ||
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| 2. The current wording in [lex.charset] uses "hexadecimal value", which is | ||
| confusing because a value is just a number, and hexadecimal is just way to | ||
| represent numbers; "value" alone should suffice. | ||
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| This paper addresses this by removing the need for this term. | ||
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| 3. There is some interest in using the U+ notation (as in U+0041 or U+1F34A) to | ||
| refer to Unicode code points across the entire standard. | ||
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| This paper changes all the relevant wording to use U+ notation. | ||
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| 4. The current text includes explanations of terms from ISO 10646 (like | ||
| "surrogate code point" or "control character") in normative text, which is | ||
| undesirable. | ||
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| This paper moves such explanations to non-normative text. | ||
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| ## Technical Specifications | ||
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| In this description, text that should be deleted is marked red and striked out; | ||
| text that should be added is marked green and underlined. Apply these changes | ||
| on top of the editorial fix provided in [PR #2201]. | ||
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| Edit 5.3 [lex.charset], paragraph 2 as follows. | ||
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| > <sup>2</sup> The *universal-character-name* construct provides a way to name | ||
| > other characters. | ||
| > | ||
| > *hex-quad:* | ||
| > *hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit* | ||
| > | ||
| > *universal-character-name:* | ||
| > *\u hex-quad* | ||
| > *\U hex-quad hex-quad* | ||
| > | ||
| > The character designated by the *universal-character-name* `\U00NNNNNN` is | ||
| > that character whose <del>character</del><ins>code point</ins> short | ||
| > identifier in ISO/IEC 10646 is <del>`NNNNNN`</del><ins>U+NNNNNN</ins>; the | ||
| > character designated by the *universal-character-name* `\uNNNN` is that | ||
| > character whose <del>character</del><ins>code point</ins> short identifier in | ||
| > ISO/IEC 10646 is <del>`NNNN`</del><ins>U+NNNN</ins>. If <del>the hexadecimal | ||
| > value for a *universal-character-name* corresponds to a surrogate code point | ||
| > (in the range 0xD800–0xDFFF, inclusive)</del><ins>If a | ||
| > *universal-character-name* does not correspond to any character in ISO/IEC | ||
| > 10646 [*Note*—ISO/IEC 10646 code points are within the range | ||
| > 0x0-0x10FFFF, inclusive.—*end note*] or if a | ||
| > *universal-character-name* corresponds to a surrogate code point | ||
| > [*Note*—A surrogate code point is a value in the range 0xD800-0xDFFF, | ||
| > inclusive.—*end note*]</ins>, the program is ill-formed. | ||
| > Additionally, if <del>the hexadecimal value for</del> a | ||
| > *universal-character-name* outside the *c-char-sequence*, *s-char-sequence*, | ||
| > or *r-char-sequence* of a character or string literal corresponds to a | ||
| > control character <del>(</del><ins>[*Note*—A control character is a | ||
| > character </ins>in either of the ranges 0x00–0x1F or 0x7F–0x9F, both | ||
| > inclusive<del>)</del><ins>—*end note*]</ins> or to a character in the | ||
| > basic source character set, the program is ill-formed. | ||
| Edit 5.13.3 [lex.ccon], paragraph 3 as follows. | ||
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| > <sup>3</sup> A character literal that begins with `u8`, such as `u8'w'`, is a | ||
| > character literal of type `char`, known as a *UTF-8 character literal*. The | ||
| > value of a UTF-8 character literal is equal to its ISO 10646 code point | ||
| > value, provided that the code point value is representable with a single | ||
| > UTF-8 code unit <del>(that is, provided it is in the C0 Controls and Basic | ||
| > Latin Unicode block)</del><ins>[*Note*—this is true if and only if a | ||
| > code point is in the range 0x0-0x7F, inclusive—*end note*]</ins>. If | ||
| > the value is not representable with a single UTF-8 code unit, the program is | ||
| > ill-formed. A UTF-8 character literal containing multiple *c-chars* is | ||
| > ill-formed. | ||
| Edit 5.13.3 [lex.string], paragraph 10 as follows. | ||
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| > <sup>10</sup> A *string-literal* that begins with `u`, such as `u"asdf"`, is | ||
| > a `char16_t` string literal. A `char16_t` string literal has type “array of | ||
| > *n* `const char16_t`”, where *n* is the size of the string as defined below; | ||
| > it is initialized with the given characters. A single *c-char* may produce | ||
| > more than one `char16_t` character in the form of surrogate pairs | ||
| > <ins>[*Note*— a surrogate pair is a representation representation for a | ||
| > single character as a sequence of two 16-bit code units—*end | ||
| > note*]</ins>. | ||
| Edit 19.8 [cpp.predefined], item (2.4) as follows. | ||
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| > <sup>(2.4)</sup> —`__STDC_ISO_10646__` | ||
| > An integer literal of the form `yyyymmL` (for example, | ||
| > `199712L`). If this symbol is defined, then every character in the Unicode | ||
| > required set, when stored in an object of type `wchar_t`, has the same value | ||
| > as the <del>short identifier</del><ins>code point</ins> of that character. | ||
| > The Unicode required set consists of all the characters that are defined by | ||
| > ISO/IEC 10646, along with all amendments and technical corrigenda as of the | ||
| > specified year and month. | ||
| [PR #2201]: https://github.com/cplusplus/draft/pull/2201 |