P2641R4 Checking if a union alternative is active

The feature test macro has been renamed from __cpp_lib_within_lifetime
to __cpp_lib_is_within_lifetime, which seems more appropriate and
follows existing practice.

Author: burblebee

source/meta.tex

@@ -583,6 +583,8 @@
 
   // \ref{meta.const.eval}, constant evaluation context
   constexpr bool is_constant_evaluated() noexcept;
+  template<class T>
+    consteval bool is_within_lifetime(const T*) noexcept;
 }
 \end{codeblock}
 
@@ -2432,6 +2434,61 @@
 \end{example}
 \end{itemdescr}
 
+\indexlibraryglobal{is_within_lifetime}%
+\begin{itemdecl}
+template<class T>
+  consteval bool is_within_lifetime(const T* p) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{true} if \tcode{p} is a pointer to an object that is
+within its lifetime\iref{basic.life}; otherwise, \tcode{false}.
+
+\pnum
+\remarks
+During the evaluation of an expression \tcode{E} as a core constant expression,
+a call to this function is ill-formed
+unless \tcode{p} points to an object that is usable
+in constant expressions or
+whose complete object's lifetime began within \tcode{E}.
+
+\pnum
+\begin{example}
+\begin{codeblock}
+struct OptBool {
+  union { bool b; char c; };
+
+  // note: this assumes common implementation properties for \tcode{bool} and \tcode{char}:
+  // * \tcode{sizeof(bool) == sizeof(char)}, and
+  // * the value representations for \tcode{true} and \tcode{false} are distinct
+  //   from the value representation for \tcode{2}
+  constexpr OptBool() : c(2) { }
+  constexpr OptBool(bool b) : b(b) { }
+
+  constexpr auto has_value() const -> bool {
+    if consteval {
+      return std::is_within_lifetime(&b);       // during constant evaluation, cannot read from \tcode{c}
+    } else {
+      return c != 2;                            // during runtime, must read from \tcode{c}
+    }
+  }
+
+  constexpr auto operator*() -> bool& {
+    return b;
+  }
+};
+
+constexpr OptBool disengaged;
+constexpr OptBool engaged(true);
+static_assert(!disengaged.has_value());
+static_assert(engaged.has_value());
+static_assert(*engaged);
+\end{codeblock}
+\end{example}
+\end{itemdescr}
+
 \rSec1[ratio]{Compile-time rational arithmetic}
 
 \rSec2[ratio.general]{In general}

source/support.tex

@@ -679,6 +679,7 @@
 #define @\defnlibxname{cpp_lib_is_pointer_interconvertible}@       201907L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_scoped_enum}@                    202011L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_swappable}@                      201603L // freestanding, also in \libheader{type_traits}
+#define @\defnlibxname{cpp_lib_is_within_lifetime}@                202306L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_jthread}@                           201911L // also in \libheader{stop_token}, \libheader{thread}
 #define @\defnlibxname{cpp_lib_latch}@                             201907L // also in \libheader{latch}
 #define @\defnlibxname{cpp_lib_launder}@                           201606L // freestanding, also in \libheader{new}