[cpp.cond] Keywords are not identifiers while preprocessing

Author: AlisdairM

source/preprocessor.tex

@@ -553,15 +553,19 @@
 \grammarterm{has-embed-expression}s, and
 \grammarterm{has-attribute-expression}s
 have been performed,
-all remaining identifiers and keywords,
+all remaining identifiers
 except for
 \tcode{true}
 and
-\tcode{false},
+\tcode{false}
 are replaced with the \grammarterm{pp-number}
 \tcode{0},
 and then each preprocessing token is converted into a token.
 \begin{note}
+Keywords are identifiers during preprocessing and are not valid macro names,
+so are always replaced by \tcode{0} before being converted into a token.
+\end{note}
+\begin{note}
 An alternative
 token\iref{lex.digraph} is not an identifier,
 even when its spelling consists entirely of letters and underscores.