[libc++][expected] Applied [[nodiscard]]

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/expected.bad.void
- https://wg21.link/expected.bad
- https://wg21.link/expected.expected
- https://wg21.link/expected.void
- https://wg21.link/expected.unexpected

Author: H-G-Hristov

libcxx/include/__expected/bad_expected_access.h

@@ -43,9 +43,11 @@ class _LIBCPP_EXPORTED_FROM_ABI bad_expected_access<void> : public exception {
 
 public:
 #  if _LIBCPP_AVAILABILITY_HAS_BAD_EXPECTED_ACCESS_KEY_FUNCTION
-  const char* what() const noexcept override;
+  [[nodiscard]] const char* what() const noexcept override;
 #  else
-  _LIBCPP_HIDE_FROM_ABI_VIRTUAL const char* what() const noexcept override { return "bad access to std::expected"; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI_VIRTUAL const char* what() const noexcept override {
+    return "bad access to std::expected";
+  }
 #  endif
 };
 _LIBCPP_DIAGNOSTIC_POP
@@ -55,10 +57,10 @@ class bad_expected_access : public bad_expected_access<void> {
 public:
   _LIBCPP_HIDE_FROM_ABI explicit bad_expected_access(_Err __e) : __unex_(std::move(__e)) {}
 
-  _LIBCPP_HIDE_FROM_ABI _Err& error() & noexcept { return __unex_; }
-  _LIBCPP_HIDE_FROM_ABI const _Err& error() const& noexcept { return __unex_; }
-  _LIBCPP_HIDE_FROM_ABI _Err&& error() && noexcept { return std::move(__unex_); }
-  _LIBCPP_HIDE_FROM_ABI const _Err&& error() const&& noexcept { return std::move(__unex_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Err& error() & noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const _Err& error() const& noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Err&& error() && noexcept { return std::move(__unex_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const _Err&& error() const&& noexcept { return std::move(__unex_); }
 
 private:
   _Err __unex_;