From 83c5afd6ff86b01c3bd7e4c70072a725dbf71d8e Mon Sep 17 00:00:00 2001
From: Alisdair Meredith <alisdairm@me.com>
Date: Tue, 21 Oct 2025 12:55:05 -0400
Subject: [PATCH] [cpp.line] Simplify specification of line directives

This change set addresses a variety of concerns to produce
a simpler and clearer specification for line directives.

The first issue to address is that the specification refers
to a _string-literal_ that is not present in the grammar.
Howeer, the constraints on that string literal are sufficient
to replace that quoted _s-char-sequence_ in the grammar
production with _string-literal_.

The line directive itself is given its own grammar production
that formally states what used to be described in text
restrictions and interprettions of the generic sequence of
preprocessing tokens.

Refactoring the grammar as its own production provides a
simpler way for incoming proposals to refer to line
directives, such as https://wg21.link/P3868.
---
 source/preprocessor.tex | 56 ++++++++++++++++-------------------------
 1 file changed, 22 insertions(+), 34 deletions(-)

diff --git a/source/preprocessor.tex b/source/preprocessor.tex
index 38c595333c..7e73aa6679 100644
--- a/source/preprocessor.tex
+++ b/source/preprocessor.tex
@@ -47,15 +47,15 @@
 
 \begin{bnf}
 \nontermdef{control-line}\br
-    \terminal{\# include} pp-tokens new-line\br
     pp-import\br
+    line-directive\br
+    \terminal{\# include} pp-tokens new-line\br
     \terminal{\# embed \ } pp-tokens new-line\br
     \terminal{\# define } identifier replacement-list new-line\br
     \terminal{\# define } identifier lparen \opt{identifier-list} \terminal{)} replacement-list new-line\br
     \terminal{\# define } identifier lparen \terminal{... )} replacement-list new-line\br
     \terminal{\# define } identifier lparen identifier-list \terminal{, ... )} replacement-list new-line\br
     \terminal{\# undef \ } identifier new-line\br
-    \terminal{\# line \ \ } pp-tokens new-line\br
     \terminal{\# error \ } \opt{pp-tokens} new-line\br
     \terminal{\# warning} \opt{pp-tokens} new-line\br
     \terminal{\# pragma } \opt{pp-tokens} new-line\br
@@ -2070,12 +2070,20 @@
 \indextext{preprocessing directive!line control}%
 \indextext{\idxcode{\#line}|see{preprocessing directive, line control}}
 
+\begin{bnf}
+\nontermdef{line-directive}\br
+    \terminal{\# line} digit-sequence \opt{string-literal} new-line\br
+    \terminal{\# line} pp-tokens new-line
+\end{bnf}
+
 \pnum
-The \grammarterm{string-literal} of a
-\tcode{\#line}
-directive, if present,
+The \grammarterm{string-literal}, if present,
 shall be a character string literal.
 
+\pnum
+If the digit sequence interpreted as a decimal integer is zero
+or a number greater than 2147483647, the program is ill-formed.
+
 \pnum
 The
 \defn{line number}
@@ -2087,41 +2095,21 @@
 while processing the source file to the current preprocessing token.
 
 \pnum
-A preprocessing directive of the form
-\begin{ncsimplebnf}
-\terminal{\# line} digit-sequence new-line
-\end{ncsimplebnf}
+A \grammarterm{line-directive} that matches the first form
 causes the implementation to behave as if
 the following sequence of source lines begins with a
 source line that has a line number as specified
-by the digit sequence (interpreted as a decimal integer).
-If the digit sequence specifies zero
-or a number greater than 2147483647,
-the program is ill-formed.
+by the digit sequence (interpreted as a decimal integer)
+and the presumed name of the source file becomes
+the contents of the character string literal, if present.
 
 \pnum
-A preprocessing directive of the form
-\begin{ncsimplebnf}
-\terminal{\# line} digit-sequence \terminal{"} \opt{s-char-sequence} \terminal{"} new-line
-\end{ncsimplebnf}
-sets the presumed line number similarly and changes the
-presumed name of the source file to be the contents
-of the character string literal.
-
-\pnum
-A preprocessing directive of the form
-\begin{ncsimplebnf}
-\terminal{\# line} pp-tokens new-line
-\end{ncsimplebnf}
-(that does not match one of the two previous forms)
-is permitted.
-The preprocessing tokens after
-\tcode{line}
-on the directive are processed just as in normal text
-(each identifier currently defined as a macro name is replaced by its
-replacement list of preprocessing tokens).
+The \grammarterm{pp-tokens} of a \grammarterm{line-directive} that does not
+match the first form are processed just as in normal text
+(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 program is ill-formed;
+the first form, the program is ill-formed;
 otherwise, the result is processed as appropriate.
 
 \rSec1[cpp.error]{Diagnostic directives}%