Mark as [[nodiscard]]

[martinmoene]

See microsoft/STL@7643c27 via PR martinmoene/expected-lite#74.

Plan:

• bad_any_cast
• bad_expected_access
• bad_optional_access
• bad_variant_access

---

Readme, section Synopsis, Configuration:

#### Disable \[\[nodiscard\]\]

-D<b>xxx\_CONFIG\_NO\_NODISCARD</b>=0
Define this to 1 if you want to compile without \[\[nodiscard\]\]. Note that the default of marking `class xxx` with \[\[nodiscard\]\] is not part of the C++23 standard. The rationale to use \[\[nodiscard\]\] is that unnoticed discarded expected error values may break the error handling flow.

Code:

// Control marking class XXX with [[nodiscard]]]:

#if !defined(xxx_CONFIG_NO_NODISCARD)
# define xxx_CONFIG_NO_NODISCARD  0
#else
# define xxx_CONFIG_NO_NODISCARD  1
#endif

// Presence of C++17 language features:

...
#define xxx_HAVE_NODISCARD   xxx_CPP17_000

// C++ feature usage:

...

#if xxx_HAVE_NODISCARD && !xxx_CONFIG_NO_NODISCARD
# define xxx_NODISCARD  [[nodiscard]]
#else
# define xxx_NODISCARD  /*[[nodiscard]]*/
#endif

template<...>
class xxx_NODISCARD XXX