P1401R5 Narrowing contextual conversions to bool

Also fixes CWG2320.

Author: jensmaurer

source/declarations.tex

@@ -224,10 +224,10 @@
 
 \pnum
 \indextext{\idxgram{static_assert}}%
-In a \grammarterm{static_assert-declaration}, the
-\grammarterm{constant-expression} shall be
-a contextually converted constant expression
-of type \tcode{bool}\iref{expr.const}.
+In a \grammarterm{static_assert-declaration},
+the \grammarterm{constant-expression}
+is contextually converted to \keyword{bool} and
+the converted expression shall be a constant expression\iref{expr.const}.
 If the value of the expression when
 so converted is \tcode{true}, the declaration has no
 effect. Otherwise, the program is ill-formed, and the resulting
@@ -239,6 +239,7 @@
 \begin{example}
 \begin{codeblock}
 static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");
+static_assert(sizeof(int[2]));          // OK, narrowing allowed
 \end{codeblock}
 \end{example}
 
@@ -551,6 +552,14 @@
 the function is explicit. Otherwise, the function is not explicit.
 A \tcode{(} token that follows \keyword{explicit} is parsed as
 part of the \grammarterm{explicit-specifier}.
+\begin{example}
+\begin{codeblock}
+struct S {
+  explicit(sizeof(char[2])) S(char);    // error: narrowing conversion of value 2 to type \keyword{bool}
+  explicit(sizeof(char)) S(bool);       // OK, conversion of value 1 to type \keyword{bool} is non-narrowing
+};
+\end{codeblock}
+\end{example}
 
 \rSec2[dcl.typedef]{The \tcode{typedef} specifier}%
 \indextext{specifier!\idxcode{typedef}}

source/exceptions.tex

@@ -716,6 +716,12 @@
 is
 equivalent to the \grammarterm{noexcept-specifier}
 \tcode{\keyword{noexcept}(\keyword{true})}.
+\begin{example}
+\begin{codeblock}
+void f() noexcept(sizeof(char[2])); // error: narrowing conversion of value 2 to type \keyword{bool}
+void g() noexcept(sizeof(char));    // OK, conversion of value 1 to type \keyword{bool} is non-narrowing
+\end{codeblock}
+\end{example}
 
 \pnum
 If a declaration of a function

source/statements.tex

@@ -249,16 +249,23 @@
 \end{footnote}
 
 \pnum
-If the \keyword{if} statement is of the form \tcode{if constexpr}, the value
-of the condition shall be a contextually
-converted constant expression of type \tcode{bool}\iref{expr.const}; this
+If the \keyword{if} statement is of the form \tcode{if constexpr},
+the value of the condition
+is contextually converted to \keyword{bool} and
+the converted expression shall be a constant expression\iref{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 instantiation of an
 enclosing templated entity\iref{temp.pre}, if the condition is
 not value-dependent after its instantiation, the discarded substatement
 (if any) is not instantiated.
+\begin{example}
+\begin{codeblock}
+if constexpr (sizeof(int[2])) {}        // OK, narrowing allowed
+\end{codeblock}
+\end{example}
 \begin{note}
 Odr-uses\iref{basic.def.odr} in a discarded statement do not require
 an entity to be defined.