P0306R4 Comma omission and comma deletion

[cpp.subst] Replace "is" with "would be" when talking about a
hypothetical alternative replacement list.

Author: zygoloid

source/preprocessor.tex

@@ -622,15 +622,18 @@
 an ellipsis, the number of arguments (including those arguments consisting
 of no preprocessing tokens)
 in an invocation of a function-like macro shall
-equal the number of parameters in the macro definition. Otherwise, there shall be more arguments in the invocation than there are
+equal the number of parameters in the macro definition.
+Otherwise, there shall be at least as many arguments in the invocation as there are
 parameters in the macro definition (excluding the \tcode{...}). There
 shall exist a
 \tcode{)}
 preprocessing token that terminates the invocation.
 
 \pnum
 \indextext{__VA_ARGS__@\mname{VA_ARGS}}%
-The identifier \mname{VA_ARGS} shall occur only in the \grammarterm{replacement-list}
+\indextext{__VA_OPT__@\mname{VA_OPT}}%
+The identifiers \mname{VA_ARGS} and \mname{VA_OPT}
+shall occur only in the \grammarterm{replacement-list}
 of a function-like macro that uses the ellipsis notation in the parameters.
 
 \pnum
@@ -731,9 +734,10 @@
 \indextext{macro!function-like!arguments}%
 If there is a \tcode{...} immediately preceding the \tcode{)} in the
 function-like macro
-definition, then the trailing arguments, including any separating comma preprocessing
+definition, then the trailing arguments (if any), including any separating comma preprocessing
 tokens, are merged to form a single item: the \term{variable arguments}. The number of
 arguments so combined is such that, following merger, the number of arguments is
+either equal to or
 one more than the number of parameters in the macro definition (excluding the
 \tcode{...}).
 
@@ -760,10 +764,60 @@
 no other preprocessing tokens are available.
 
 \pnum
+\indextext{__VA_ARGS__@\mname{VA_ARGS}}%
 An identifier \mname{VA_ARGS} that occurs in the replacement list
 shall be treated as if it were a parameter, and the variable arguments shall form
 the preprocessing tokens used to replace it.
 
+\pnum
+\indextext{__VA_OPT__@\mname{VA_OPT}}%
+The identifier \mname{VA_OPT}
+shall always occur as part of the token sequence
+\mname{VA_OPT}\tcode{(}\placeholder{content}\tcode{)},
+where \placeholder{content} is
+an arbitrary sequence of \grammarterm{preprocessing-tokens}
+other than \mname{VA_OPT},
+which is terminated by the closing \tcode{)}
+and skips intervening pairs of matching left and right parentheses.
+If \placeholder{content} would be ill-formed
+as the replacement list of the current function-like macro,
+the program is ill-formed.
+The token sequence
+\mname{VA_OPT}\tcode{(}\placeholder{content}\tcode{)}
+shall be treated as if it were a parameter,
+and the preprocessing tokens used to replace it
+are defined as follows.
+If the variable arguments consist of no tokens,
+the replacement consists of
+a single placemarker preprocessing token~(\ref{cpp.concat}, \ref{cpp.rescan}).
+Otherwise, the replacement consists of
+the results of the expansion of \placeholder{content}
+as the replacement list of the current function-like macro
+before rescanning and further replacement.
+\begin{example}
+\begin{codeblock}
+#define F(...)           f(0 __VA_OPT__(,) __VA_ARGS__)
+#define G(X, ...)        f(0, X __VA_OPT__(,) __VA_ARGS__)
+#define SDEF(sname, ...) S sname __VA_OPT__(= { __VA_ARGS__ })
+
+F(a, b, c)          // replaced by \tcode{f(0, a, b, c)}
+F()                 // replaced by \tcode{f(0)}
+
+G(a, b, c)          // replaced by \tcode{f(0, a, b, c)}
+G(a, )              // replaced by \tcode{f(0, a)}
+G(a)                // replaced by \tcode{f(0, a)}
+
+SDEF(foo);          // replaced by \tcode{S foo;}
+SDEF(bar, 1, 2);    // replaced by \tcode{S bar = \{ 1, 2 \};}
+
+#define H1(X, ...) X __VA_OPT__(##) __VA_ARGS__ // ill-formed: \tcode{\#\#} may not appear at
+                                                // the beginning of a replacement list (\ref{cpp.concat})
+#define H2(X, Y, ...) __VA_OPT__(X ## Y,) __VA_ARGS__
+
+H2(a, b, c, d)      // replaced by \tcode{ab, c, d}
+\end{codeblock}
+\end{example}
+
 \rSec2[cpp.stringize]{The \tcode{\#} operator}%
 \indextext{\#\#0 operator@\tcode{\#} operator}%
 \indextext{stringize|see{\tcode{\#} operator}}