P0292R2 constexpr if: a slightly different syntax

Author: tkoeppe

source/basic.tex

@@ -370,12 +370,13 @@
 
 \pnum
 Every program shall contain exactly one definition of every non-inline
-function or variable that is odr-used in that program; no diagnostic required.
+function or variable that is odr-used in that program
+outside of a discarded statement~(\ref{stmt.if}); no diagnostic required.
 The definition can appear explicitly in the program, it can be found in
 the standard or a user-defined library, or (when appropriate) it is
 implicitly defined (see~\ref{class.ctor}, \ref{class.dtor} and
 \ref{class.copy}). An inline function shall be defined in every
-translation unit in which it is odr-used.
+translation unit in which it is odr-used outside of a discarded statement.
 
 \pnum
 \indextext{type!incomplete}%

source/declarations.tex

@@ -1550,7 +1550,8 @@
 the declared return type of the function. Otherwise, the function declarator
 shall declare a function. If the declared return type of the
 function contains a placeholder type, the return type of the function is
-deduced from \tcode{return} statements in the body of the function, if any.
+deduced from non-discarded \tcode{return} statements, if any, in the body
+of the function~(\ref{stmt.if}).
 
 \pnum
 If the \tcode{auto} \grammarterm{type-specifier} appears as one of the
@@ -1633,13 +1634,13 @@
 
 \pnum
 If a function with a declared return type that contains a placeholder type has
-multiple \tcode{return} statements, the return type is deduced for each
+multiple non-discarded \tcode{return} statements, the return type is deduced for each
 \tcode{return} statement. If the type deduced is not the same in each
 deduction, the program is ill-formed.
 
 \pnum
 If a function with a declared return type that uses a placeholder type has no
-\tcode{return} statements, the return type is deduced as though from a
+non-discarded \tcode{return} statements, the return type is deduced as though from a
 \tcode{return} statement with no operand at the closing brace of the function
 body.
 \begin{example}
@@ -1652,7 +1653,7 @@
 \pnum
 If the type of an entity with an undeduced placeholder type is needed to
 determine the type of an expression, the program is ill-formed. Once a
-\tcode{return} statement has been seen in a function, however, the return type deduced
+non-discarded \tcode{return} statement has been seen in a function, however, the return type deduced
 from that statement can be used in the rest of the function, including in other
 \tcode{return} statements.
 \begin{example}
@@ -1786,9 +1787,10 @@
 A type \tcode{T} containing a placeholder type,
 and a corresponding initializer \tcode{e},
 are determined as follows:
+
 \begin{itemize}
 \item
-for a \tcode{return} statement that occurs
+for a non-discarded \tcode{return} statement that occurs
 in a function declared with a return type
 that contains a placeholder type,
 \tcode{T} is the declared return type
@@ -1811,6 +1813,7 @@
 \tcode{T} is the declared type of the non-type template parameter
 and \tcode{e} is the corresponding template argument.
 \end{itemize}
+
 In the case of a \tcode{return} statement with no operand
 or with an operand of type \tcode{void},
 \tcode{T} shall be either
@@ -1885,6 +1888,7 @@
 decltype(auto)*x7d = &i;       // error, declared type is not plain \tcode{decltype(auto)}
 \end{codeblock}
 \end{example}
+
 \rSec1[dcl.enum]{Enumeration declarations}%
 \indextext{enumeration}%
 \indextext{\idxcode{\{\}}!enum declaration@\tcode{enum} declaration}%

source/grammar.tex

@@ -867,8 +867,8 @@
 
 \begin{bnf}
 \nontermdef{selection-statement}\br
-    \terminal{if (} condition \terminal{)} statement\br
-    \terminal{if (} condition \terminal{)} statement \terminal{else} statement\br
+    \terminal{if constexpr\opt (} condition \terminal{)} statement\br
+    \terminal{if constexpr\opt (} condition \terminal{)} statement \terminal{else} statement\br
     \terminal{switch (} condition \terminal{)} statement
 \end{bnf}
 

source/statements.tex

@@ -128,8 +128,8 @@
 %
 \begin{bnf}
 \nontermdef{selection-statement}\br
-    \terminal{if (} condition \terminal{)} statement\br
-    \terminal{if (} condition \terminal{)} statement \terminal{else} statement\br
+    \terminal{if constexpr\opt (} condition \terminal{)} statement\br
+    \terminal{if constexpr\opt (} condition \terminal{)} statement \terminal{else} statement\br
     \terminal{switch (} condition \terminal{)} statement
 \end{bnf}
 
@@ -245,6 +245,50 @@
 an \tcode{else} part.\footnote{In other words, the \tcode{else} is associated with the nearest un-elsed
 \tcode{if}.}
 
+\pnum
+If the parenthesized \grammarterm{condition} is prefixed with
+\tcode{constexpr}, the value of the condition shall be a contextually
+converted constant expression of type \tcode{bool}~(\ref{expr.const}); this
+form is called a \defn{constexpr if} statement. If the value of the
+converted condition is \tcode{false}, the first substatement is a
+\defn{discarded statement}, otherwise the second substatement, if
+present, is a discarded statement. During the instantation of an
+enclosing templated entity, if the condition is not value-dependent
+after its instantiation, the discarded substatement (if any) is not
+instantiated.
+\begin{note}
+Odr-uses~(\ref{basic.def.odr}) in a discarded statement do not require
+an entity to be defined.
+\end{note}
+A \tcode{case} or \tcode{default} label appearing within such an
+\tcode{if} statement shall be associated with a \tcode{switch}
+statement~(\ref{stmt.switch}) within the same \tcode{if} statement.
+A label~(\ref{stmt.label}) declared in a substatement of a \tcode{constexpr if}
+statement shall only be referred to by a statement~(\ref{stmt.goto}) in
+the same substatement.
+\begin{example}
+\begin{codeblock}
+template<typename T, typename ... Rest> void g(T&& p, Rest&& ...rs) {
+  // ... handle \tcode{p}
+
+  if constexpr (sizeof...(rs) > 0)
+    g(rs...);  // never instantiated with an empty argument list.
+  }
+
+  extern int x;   // no definition of \tcode{x} required
+
+  int f() {
+    if constexpr (true)
+      return 0;
+    else if (x)
+      return x;
+    else
+      return -x;
+  }
+}
+\end{codeblock}
+\end{example}
+
 \rSec2[stmt.switch]{The \tcode{switch} statement}%
 \indextext{statement!\idxcode{switch}}
 

source/templates.tex

@@ -1299,6 +1299,8 @@
 definition which is unrelated to
 the function template definition or to any other default arguments or
 \grammarterm{exception-specification}{s}.
+For the purpose of instantiation, the substatements of a \tcode{constexpr if}
+statement~(\ref{stmt.if}) are considered definitions.
 
 \pnum
 Because an \grammarterm{alias-declaration} cannot declare a
@@ -3143,10 +3145,12 @@
 Knowing which names are type names allows the syntax of every template
 to be checked.
 The program is ill-formed, no diagnostic required, if:
+
 \begin{itemize}
 \item
 no valid specialization can be generated for a template
-and that template is not instantiated, or
+or a substatement of a \tcode{constexpr if} statement~(\ref{stmt.if}) within a template
+and the template is not instantiated, or
 \item
 every valid specialization of a variadic template requires an empty template
 parameter pack, or
@@ -3189,6 +3193,7 @@
 \end{itemize}
 \end{note}
 \end{itemize}
+
 Otherwise, no diagnostic shall be issued for a template
 for which a valid specialization can be generated.
 \begin{note}
@@ -4697,8 +4702,9 @@
 \pnum
 An implementation shall not implicitly instantiate a function template,
 a variable template,
-a member template, a non-virtual member function, a member class, or a
-static data member of a class template that does not require instantiation.
+a member template, a non-virtual member function, a member class, a
+static data member of a class template, or a substatement of a \tcode{constexpr if}
+statement (\ref{stmt.if}), unless such instantiation is required.
 It is unspecified whether or not an implementation implicitly instantiates a
 virtual member function of a class template if the virtual member function would
 not otherwise be instantiated.