From 94f44dd20c65115effcd682276273421919bfd17 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Mon, 9 Mar 2026 12:09:29 +0800
Subject: [PATCH] [meta.const.eval] Use `bit_cast` in the example to avoid UB

It is unclear whether the reading an inactive `char` variant member can
be well-defined. General rules for out-of-lifetime objects disallows
this, but [basic.lval]/11 arguably allows reading the whole union via
such a `char` glvalue. In any case, it seems better to use some more
uncontroversial way to read the value representation of `OptBool` in the
example during runtime.

[basic.fundamental]/10 requires `bool` to have the same object
representation, value representation, and alignment requirements as an
implementation-defined unsigned integer type. Presumably, the example
essentially expected such an underlying-like type to be `unsigned char`.
Since `unsigned char` can't have padding bits, it would be sufficient to
infer that `bool` can't have padding bits either on common
implementations. As a result, `bit_cast` to `char` or `unsigned char`
can work.

This PR clarifies the assumptions for `bool` in comments and switch to
use `bit_cast` to avoid arguable UB.
---
 source/meta.tex | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/source/meta.tex b/source/meta.tex
index 48510a4e39..5da7f5f9c6 100644
--- a/source/meta.tex
+++ b/source/meta.tex
@@ -2867,10 +2867,11 @@
 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
+  // note: this assumes common implementation properties for \tcode{bool}:
+  // * \tcode{bool} has the same object representation, value representation, and
+  //   alignment requirements as \tcode{unsigned char}, and
   // * the value representations for \tcode{true} and \tcode{false} are distinct
-  //   from the value representation for \tcode{2}
+  //   from the value representation for \tcode{char\{2\}}
   constexpr OptBool() : c(2) { }
   constexpr OptBool(bool b) : b(b) { }
 
@@ -2878,7 +2879,7 @@
     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}
+      return std::bit_cast<char>(*this) != 2;   // during runtime, must read the value representation
     }
   }