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[conv.lval] Determine the value of a constant without accessing an object #4495

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[conv.lval] Determine the value of a constant without accessing an object #4495

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63 changes: 36 additions & 27 deletions source/expressions.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -599,7 +599,7 @@
\pnum
\indextext{conversion!lvalue-to-rvalue}%
\indextext{type!incomplete}%
A glvalue\iref{basic.lval} of a non-function, non-array type \tcode{T}
A glvalue\iref{basic.lval} $E$ of a non-function, non-array type \tcode{T}
can be converted to
a prvalue.
\begin{footnote}
Expand All @@ -618,38 +618,21 @@
cv-qualified types.
\end{footnote}

\pnum
When an lvalue-to-rvalue conversion
is applied to an expression $E$, and either
\begin{itemize}
\item $E$ is not potentially evaluated, or
\item the evaluation of $E$ results in the evaluation of a member
$E_\tcode{x}$ of the set of potential results of $E$, and $E_\tcode{x}$
names a variable \tcode{x} that is not odr-used by
$E_\tcode{x}$\iref{basic.def.odr},
\end{itemize}
the value contained in the referenced object is not accessed.
\begin{example}
\begin{codeblock}
struct S { int n; };
auto f() {
S x { 1 };
constexpr S y { 2 };
return [&](bool b) { return (b ? y : x).n; };
}
auto g = f();
int m = g(false); // undefined behavior: access of \tcode{x.n} outside its lifetime
int n = g(true); // OK, does not access \tcode{y.n}
\end{codeblock}
\end{example}

\pnum
The result of the conversion is determined according to the
following rules:

\begin{itemize}

\item If \tcode{T} is \cv{}~\tcode{std::nullptr_t}, the result is a
\item If $E$ is not potentially evaluated,
the result is an unspecified value of type \tcode{T}.
\begin{note}
An expression that is not potentially evaluated is only
analyzed for its type or other expression properties,
thus its value is not relevant.
\end{note}

\item Otherwise, if \tcode{T} is \cv{}~\tcode{std::nullptr_t}, the result is a
null pointer constant\iref{conv.ptr}.
\begin{note}
Since the conversion does not access the object to which the glvalue refers,
Expand All @@ -661,6 +644,17 @@
type, the conversion copy-initializes the result object from
the glvalue.

\item Otherwise,
if the evaluation of $E$ results in
the evaluation of a member $E_\tcode{x}$ of the set of potential results of $E$,
and $E_\tcode{x}$ names a variable \tcode{x}
that is not odr-used by $E_\tcode{x}$\iref{basic.def.odr},
the result is the value of \tcode{x}.
\begin{note}
The variable \tcode{x} is thus usable in constant expressions\iref{expr.const};
the conversion does not access the object to which the glvalue refers.
\end{note}

\item Otherwise, if the object to which the glvalue refers contains an invalid
pointer value~(\ref{basic.stc.dynamic.deallocation},
\ref{basic.stc.dynamic.safety}), the behavior is
Expand All @@ -675,6 +669,21 @@
See also~\ref{basic.lval}.
\end{note}

\pnum
\begin{example}
\begin{codeblock}
struct S { int n; };
auto f() {
S x { 1 };
constexpr S y { 2 };
return [&](bool b) { return (b ? y : x).n; };
}
auto g = f();
int m = g(false); // undefined behavior: access of \tcode{x.n} outside its lifetime
int n = g(true); // OK, does not access \tcode{y.n}
\end{codeblock}
\end{example}

\rSec2[conv.array]{Array-to-pointer conversion}

\pnum
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