[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.

Author: frederick-vs-ja

source/meta.tex

@@ -2867,18 +2867,19 @@
 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) { }
 
   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}
+      return std::bit_cast<char>(*this) != 2;   // during runtime, must read the value representation
     }
   }