From 47722e915d2f6d0fbebc84b82d502faa53bdc0c8 Mon Sep 17 00:00:00 2001
From: Jens Maurer <Jens.Maurer@gmx.net>
Date: Mon, 7 Jun 2021 22:43:33 +0200
Subject: [PATCH] P1938R3 if consteval

---
 source/expressions.tex  | 10 ++++++--
 source/preprocessor.tex |  1 +
 source/statements.tex   | 53 +++++++++++++++++++++++++++++++++++++++++
 source/utilities.tex    | 13 ++++++----
 4 files changed, 71 insertions(+), 6 deletions(-)

diff --git a/source/expressions.tex b/source/expressions.tex
index 0383ec9d7e..15c009135e 100644
--- a/source/expressions.tex
+++ b/source/expressions.tex
@@ -7465,9 +7465,15 @@
 
 \pnum
 An expression or conversion is in an \defn{immediate function context}
-if it is potentially evaluated and
+if it is potentially evaluated and either:
+\begin{itemize}
+\item
 its innermost enclosing non-block scope is
-a function parameter scope of an immediate function.
+a function parameter scope of an immediate function, or
+\item
+its enclosing statement is enclosed\iref{stmt.pre} by
+the \grammarterm{compound-statement} of a consteval if statement\iref{stmt.if}.
+\end{itemize}
 An expression or conversion is an \defn{immediate invocation}
 if it is a potentially-evaluated explicit or implicit invocation of
 an immediate function and
diff --git a/source/preprocessor.tex b/source/preprocessor.tex
index 436aa27844..37715aa88f 100644
--- a/source/preprocessor.tex
+++ b/source/preprocessor.tex
@@ -1780,6 +1780,7 @@
 \defnxname{cpp_generic_lambdas}                   & \tcode{201707L} \\ \rowsep
 \defnxname{cpp_guaranteed_copy_elision}           & \tcode{201606L} \\ \rowsep
 \defnxname{cpp_hex_float}                         & \tcode{201603L} \\ \rowsep
+\defnxname{cpp_if_consteval}                      & \tcode{202106L} \\ \rowsep
 \defnxname{cpp_if_constexpr}                      & \tcode{201606L} \\ \rowsep
 \defnxname{cpp_impl_coroutine}                    & \tcode{201902L} \\ \rowsep
 \defnxname{cpp_impl_destroying_delete}            & \tcode{201806L} \\ \rowsep
diff --git a/source/statements.tex b/source/statements.tex
index 95d406a899..cf608ae87c 100644
--- a/source/statements.tex
+++ b/source/statements.tex
@@ -212,6 +212,8 @@
 \nontermdef{selection-statement}\br
     \keyword{if} \opt{\keyword{constexpr}} \terminal{(} \opt{init-statement} condition \terminal{)} statement\br
     \keyword{if} \opt{\keyword{constexpr}} \terminal{(} \opt{init-statement} condition \terminal{)} statement \keyword{else} statement\br
+    \keyword{if} \opt{\terminal{!}} \keyword{consteval} compound-statement\br
+    \keyword{if} \opt{\terminal{!}} \keyword{consteval} compound-statement \keyword{else} statement\br
     \keyword{switch} \terminal{(} \opt{init-statement} condition \terminal{)} statement
 \end{bnf}
 
@@ -315,6 +317,57 @@
 except that the \grammarterm{init-statement} is
 in the same scope as the \grammarterm{condition}.
 
+\pnum
+An \keyword{if} statement of the form \tcode{\keyword{if} \keyword{consteval}}
+is called a \defnadj{consteval if}{statement}.
+The \grammarterm{statement}, if any, in a consteval if statement
+shall be a \grammarterm{compound-statement}.
+\begin{example}
+\begin{codeblock}
+constexpr void f(bool b) {
+  if (true)
+    if consteval { }
+    else ;              // error: not a \grammarterm{compound-statement}; \keyword{else} not associated with outer \keyword{if}
+}
+\end{codeblock}
+\end{example}
+
+\pnum
+If a consteval if statement is evaluated in a context
+that is manifestly constant-evaluated\iref{expr.const},
+the first substatement is executed.
+\begin{note}
+The first substatement is an immediate function context.
+\end{note}
+Otherwise, if the \keyword{else} part of the selection statement is present,
+then the second substatement is executed.
+A \keyword{case} or \keyword{default} label
+appearing within such an \keyword{if} statement
+shall be associated with a \keyword{switch} statement
+within the same \keyword{if} statement.
+A label declared in a substatement of a consteval if statement
+shall only be referred to by a statement in the same substatement.
+
+\pnum
+An \keyword{if} statement of the form
+\begin{ncsimplebnf}
+\keyword{if} \terminal{!} \keyword{consteval} compound-statement
+\end{ncsimplebnf}
+is not itself a consteval if statement,
+but is equivalent to the consteval if statement
+\begin{ncsimplebnf}
+\keyword{if} \keyword{consteval} \terminal{\{} \terminal{\}} \keyword{else} compound-statement
+\end{ncsimplebnf}
+An \keyword{if} statement of the form
+\begin{ncsimplebnf}
+\keyword{if} \terminal{!} \keyword{consteval} compound-statement$_1$ \keyword{else} statement$_2$
+\end{ncsimplebnf}
+is not itself a consteval if statement,
+but is equivalent to the consteval if statement
+\begin{ncsimplebnf}
+\keyword{if} \keyword{consteval} statement$_2$ \keyword{else} compound-statement$_1$
+\end{ncsimplebnf}
+
 \rSec2[stmt.switch]{The \tcode{switch} statement}%
 \indextext{statement!\idxcode{switch}}
 
diff --git a/source/utilities.tex b/source/utilities.tex
index 9f206dc7f9..4d404bedec 100644
--- a/source/utilities.tex
+++ b/source/utilities.tex
@@ -18143,10 +18143,15 @@
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{true} if and only if evaluation of the call occurs
-within the evaluation of an expression or conversion
-that is manifestly constant-evaluated\iref{expr.const}.
+\effects
+Equivalent to:
+\begin{codeblock}
+if consteval {
+  return true;
+} else {
+  return false;
+}
+\end{codeblock}
 
 \pnum
 \begin{example}