P2843R3 Preprocessing is never undefined

Fixes #7937
Fixes cplusplus/papers#1548

Author: AlisdairM

source/lex.tex

@@ -480,10 +480,7 @@
 characters \tcode{*/}. These comments do not nest.
 \indextext{comment!\tcode{//}}%
 The characters \tcode{//} start a comment, which terminates immediately before the
-next new-line character. If there is a form-feed or a vertical-tab
-character in such a comment, only whitespace characters shall appear
-between it and the new-line that terminates the comment; no diagnostic
-is required.
+next new-line character.
 \begin{note}
 The comment characters \tcode{//}, \tcode{/*},
 and \tcode{*/} have no special meaning within a \tcode{//} comment and

source/lib-intro.tex

@@ -3269,16 +3269,6 @@
 \tcode{\#define} or \tcode{\#undef} names declared in any standard
 library header.
 
-\pnum
-\indextext{unit!translation}%
-A translation unit shall not \tcode{\#define} or \tcode{\#undef}
-names lexically identical
-to keywords,
-to the identifiers listed in \tref{lex.name.special}, or
-to the \grammarterm{attribute-token}{s} described in~\ref{dcl.attr},
-except that the names \tcode{likely} and \tcode{unlikely} may be
-defined as function-like macros~\iref{cpp.replace}.
-
 \rSec4[extern.names]{External linkage}
 
 \pnum

source/preprocessor.tex

@@ -532,7 +532,7 @@
 \tcode{defined}
 unary operator does not match one of the two specified forms
 prior to macro replacement,
-the behavior is undefined.
+the program is ill-formed, no diagnostic required.
 
 \pnum
 After all replacements due to macro expansion and
@@ -746,8 +746,8 @@
 (i.e., each identifier currently defined as a macro name is replaced by its
 replacement list of preprocessing tokens).
 If the directive resulting after all replacements does not match
-one of the two previous forms, the behavior is
-undefined.
+one of the two previous forms, the program is ill-formed, no diagnostic required.
+
 \begin{note}
 Adjacent \grammarterm{string-literal}s are not concatenated into
 a single \grammarterm{string-literal}
@@ -1500,6 +1500,16 @@
 replacement list of preprocessing tokens are not considered
 part of the replacement list for either form of macro.
 
+\pnum
+\indextext{unit!translation}%
+A translation unit shall not \tcode{\#define} or \tcode{\#undef}
+names lexically identical
+to keywords,
+to the identifiers listed in \tref{lex.name.special}, or
+to the \grammarterm{attribute-token}{s} described in~\ref{dcl.attr},
+except that the names \tcode{likely} and \tcode{unlikely} may be
+defined as function-like macros.
+
 \pnum
 If a
 \indextext{\#\#0 operator@\tcode{\#} operator}
@@ -1589,7 +1599,7 @@
 \begin{footnote}
 A \grammarterm{conditionally-supported-directive} is a preprocessing directive regardless of whether the implementation supports it.
 \end{footnote}
-the behavior is undefined.
+the program is ill-formed.
 
 \pnum
 \begin{example}
@@ -1783,19 +1793,15 @@
 Otherwise, the original spelling of each preprocessing token in the
 stringizing argument is retained in the character string literal,
 except for special handling for producing the spelling of
-\grammarterm{string-literal}s and \grammarterm{character-literal}s:
-a
-\tcode{\textbackslash}
-character is inserted before each
-\tcode{"}
-and
-\tcode{\textbackslash}
-character of a \grammarterm{character-literal} or \grammarterm{string-literal}
-(including the delimiting
-\tcode{"}
-characters).
+\grammarterm{character-literal}s and \grammarterm{string-literal}s
+(including the delimiting \unicode{0022}{quotation mark} (\tcode{"}))
+contained within the preprocessing token:
+a \unicode{005c}{reverse solidus} character (\tcode{\textbackslash})
+is inserted before each \unicode{0022}{quotation mark} and
+\unicode{005c}{reverse solidus} character and each new-line character is
+replaced by the two-character sequence \tcode{\textbackslash n}.
 If the replacement that results is not a valid character string literal,
-the behavior is undefined. The character string literal corresponding to
+the program is ill-formed. The character string literal corresponding to
 an empty stringizing argument is \tcode{""}.
 The order of evaluation of
 \tcode{\#}
@@ -1844,7 +1850,7 @@
 a \grammarterm{universal-character-name}\iref{lex.charset}.
 \end{note}
 If the result is not a valid preprocessing token,
-the behavior is undefined.
+the program is ill-formed.
 The resulting preprocessing token is available for further macro replacement.
 The order of evaluation of
 \tcode{\#\#}
@@ -2055,7 +2061,7 @@
 by the digit sequence (interpreted as a decimal integer).
 If the digit sequence specifies zero
 or a number greater than 2147483647,
-the behavior is undefined.
+the program is ill-formed.
 
 \pnum
 A preprocessing directive of the form
@@ -2079,7 +2085,7 @@
 (each identifier currently defined as a macro name is replaced by its
 replacement list of preprocessing tokens).
 If the directive resulting after all replacements does not match
-one of the two previous forms, the behavior is undefined;
+one of the two previous forms, the program is ill-formed;
 otherwise, the result is processed as appropriate.
 
 \rSec1[cpp.error]{Diagnostic directives}%
@@ -2420,7 +2426,7 @@
 or a
 \tcode{\#undef}
 preprocessing directive,
-the behavior is undefined.
+the program is ill-formed.
 Any other predefined macro names shall begin with a
 leading underscore followed by an uppercase letter or a second
 underscore.