Mark expected, unexpected, and ALL exception types as [[nodiscard]]

stl/inc/__msvc_ranges_tuple_formatter.hpp

@@ -78,7 +78,7 @@ _NODISCARD constexpr const _CharT* _Choose_literal(const char* const _Str, const
 // It's defined here, so that both headers can use this definition.
 #define _STATICALLY_WIDEN(_CharT, _Literal) (_Choose_literal<_CharT>(_Literal, L##_Literal))
 
-_EXPORT_STD class format_error : public runtime_error {
+_EXPORT_STD class _NODISCARD format_error : public runtime_error {
     using runtime_error::runtime_error;
 };
 

stl/inc/any

@@ -28,7 +28,7 @@ _STL_DISABLE_CLANG_WARNINGS
 
 _STD_BEGIN
 
-_EXPORT_STD class bad_any_cast : public bad_cast { // thrown by failed any_cast
+_EXPORT_STD class _NODISCARD bad_any_cast : public bad_cast { // thrown by failed any_cast
 public:
     _NODISCARD const char* __CLR_OR_THIS_CALL what() const noexcept override {
         return "Bad any_cast";

stl/inc/chrono

@@ -1720,7 +1720,7 @@ namespace chrono {
         sys_info second;
     };
 
-    _EXPORT_STD class nonexistent_local_time : public runtime_error {
+    _EXPORT_STD class _NODISCARD nonexistent_local_time : public runtime_error {
     public:
         template <class _Duration>
         nonexistent_local_time(const local_time<_Duration>& _Tp, const local_info& _Info)
@@ -1736,7 +1736,7 @@ namespace chrono {
         _THROW(nonexistent_local_time{_Tp, _Info});
     }
 
-    _EXPORT_STD class ambiguous_local_time : public runtime_error {
+    _EXPORT_STD class _NODISCARD ambiguous_local_time : public runtime_error {
     public:
         template <class _Duration>
         ambiguous_local_time(const local_time<_Duration>& _Tp, const local_info& _Info)

stl/inc/exception

@@ -69,7 +69,7 @@ _STD_END
 #undef stdext
 
 _STDEXT_BEGIN
-class exception;
+class _NODISCARD exception;
 _STDEXT_END
 
 _STD_BEGIN
@@ -83,7 +83,7 @@ extern _CRTIMP2_PURE_IMPORT _Prhand _Raise_handler; // pointer to raise handler
 _STD_END
 
 _STDEXT_BEGIN
-class exception { // base of all library exceptions
+class _NODISCARD exception { // base of all library exceptions
 public:
     static _STD _Prhand _Set_raise_handler(_STD _Prhand _Pnew) { // register a handler for _Raise calls
         const _STD _Prhand _Pold = _STD _Raise_handler;
@@ -123,7 +123,7 @@ protected:
     const char* _Ptr; // the message pointer
 };
 
-class bad_exception : public exception { // base of all bad exceptions
+class _NODISCARD bad_exception : public exception { // base of all bad exceptions
 public:
     __CLR_OR_THIS_CALL bad_exception(const char* _Message = "bad exception") noexcept : exception(_Message) {}
 
@@ -135,9 +135,9 @@ protected:
     }
 };
 
-class bad_array_new_length;
+class _NODISCARD bad_array_new_length;
 
-class bad_alloc : public exception { // base of all bad allocation exceptions
+class _NODISCARD bad_alloc : public exception { // base of all bad allocation exceptions
 public:
     __CLR_OR_THIS_CALL bad_alloc() noexcept
         : exception("bad allocation", 1) {} // construct from message string with no memory allocation
@@ -156,7 +156,7 @@ protected:
     }
 };
 
-class bad_array_new_length : public bad_alloc {
+class _NODISCARD bad_array_new_length : public bad_alloc {
 public:
     bad_array_new_length() noexcept : bad_alloc("bad array new length") {}
 };
@@ -390,8 +390,8 @@ _EXPORT_STD template <class _Ty>
 void rethrow_if_nested(const _Ty&) = delete; // requires /GR option
 #endif // ^^^ !defined(_CPPRTTI) ^^^
 
-_EXPORT_STD class bad_variant_access
-    : public exception { // exception for visit of a valueless variant or get<I> on a variant with index() != I
+_EXPORT_STD class _NODISCARD bad_variant_access : public exception {
+    // exception for visit of a valueless variant or get<I> on a variant with index() != I
 public:
     bad_variant_access() noexcept = default;
 

stl/inc/execution

@@ -161,7 +161,7 @@ void _Implicitly_construct_in_place_by_binary_op_transform_deref_rhs(
         _Ty([&]() -> _Ty { return _Reduce_op(_STD forward<_LeftTy>(_Left), _Transform_op(*_Iter)); }());
 }
 
-struct _Parallelism_resources_exhausted : exception {
+struct _NODISCARD _Parallelism_resources_exhausted : exception {
     _NODISCARD const char* __CLR_OR_THIS_CALL what() const noexcept override {
         // return pointer to message string
         return "Insufficient resources were available to use additional parallelism.";

stl/inc/expected

@@ -25,7 +25,7 @@ _STL_DISABLE_CLANG_WARNINGS
 _STD_BEGIN
 
 _EXPORT_STD template <class _Err>
-class unexpected;
+class _NODISCARD unexpected;
 
 template <class _Err>
 struct _Check_unexpected_argument : true_type {
@@ -39,7 +39,7 @@ struct _Check_unexpected_argument : true_type {
 
 // [expected.un.general]
 _EXPORT_STD template <class _Err>
-class unexpected {
+class _NODISCARD unexpected {
     static_assert(_Check_unexpected_argument<_Err>::value);
 
     template <class _Ty, class _Err2>
@@ -107,10 +107,10 @@ template <class _Err>
 unexpected(_Err) -> unexpected<_Err>;
 
 _EXPORT_STD template <class _Err>
-class bad_expected_access;
+class _NODISCARD bad_expected_access;
 
 template <>
-class bad_expected_access<void> : public exception {
+class _NODISCARD bad_expected_access<void> : public exception {
 public:
     _NODISCARD const char* __CLR_OR_THIS_CALL what() const noexcept override {
         return "Bad expected access";
@@ -131,7 +131,7 @@ protected:
 };
 
 _EXPORT_STD template <class _Err>
-class bad_expected_access : public bad_expected_access<void> {
+class _NODISCARD bad_expected_access : public bad_expected_access<void> {
 public:
     explicit bad_expected_access(_Err _Unex) noexcept(is_nothrow_move_constructible_v<_Err>) // strengthened
         : _Unexpected(_STD move(_Unex)) {}
@@ -205,7 +205,7 @@ concept _Trivially_move_constructible_assignable_destructible =
     && is_trivially_destructible_v<_Type>;
 
 _EXPORT_STD template <class _Ty, class _Err>
-class expected {
+class _NODISCARD expected {
 private:
     static_assert(_Check_expected_argument<_Ty>::value);
     static_assert(_Check_unexpected_argument<_Err>::value);
@@ -1210,7 +1210,7 @@ concept _Expected_unary_move_assignable = is_move_assignable_v<_Err> && is_move_
 
 template <class _Ty, class _Err>
     requires is_void_v<_Ty>
-class expected<_Ty, _Err> {
+class _NODISCARD expected<_Ty, _Err> {
 private:
     static_assert(_Check_unexpected_argument<_Err>::value);
 

stl/inc/experimental/filesystem

@@ -1284,7 +1284,7 @@ _NODISCARD path u8path(const basic_string<char, _Traits, _Alloc>& _Str) { // mak
     return path{_Path_cvt<_Char8_t, _Pchar>::_Cvt(_Str_out, _Str.c_str(), _Str.size())};
 }
 
-class filesystem_error : public system_error { // base of all filesystem-error exceptions
+class _NODISCARD filesystem_error : public system_error { // base of all filesystem-error exceptions
 public:
     explicit filesystem_error(
         const string& _Message, error_code _Errcode = make_error_code(errc::operation_not_permitted))

stl/inc/filesystem

@@ -1790,7 +1790,7 @@ namespace filesystem {
         return iterator(_Text.cend(), this);
     }
 
-    _EXPORT_STD class filesystem_error : public system_error { // base of all filesystem-error exceptions
+    _EXPORT_STD class _NODISCARD filesystem_error : public system_error { // base of all filesystem-error exceptions
     public:
         filesystem_error(const string& _Message, const error_code _Errcode)
             : system_error(_Errcode, _Message), _What(runtime_error::what()) {}

stl/inc/functional

@@ -690,7 +690,8 @@ _NODISCARD _CONSTEXPR20 _Not_fn<decay_t<_Callable>> not_fn(_Callable&& _Obj)
 }
 #endif // _HAS_CXX17
 
-_EXPORT_STD class bad_function_call : public exception { // exception thrown when an empty std::function is called
+_EXPORT_STD class _NODISCARD bad_function_call : public exception {
+    // exception thrown when an empty std::function is called
 public:
     bad_function_call() noexcept {}
 

stl/inc/future

@@ -113,7 +113,7 @@ _NODISCARD inline const char* _Future_error_map(int _Errcode) noexcept { // conv
     }
 }
 
-_EXPORT_STD class future_error : public logic_error { // future exception
+_EXPORT_STD class _NODISCARD future_error : public logic_error { // future exception
 public:
     explicit future_error(error_code _Errcode) // internal, TRANSITION, will be removed
         : logic_error(""), _Mycode(_Errcode) {}

stl/inc/memory

@@ -1086,7 +1086,8 @@ public:
 };
 #endif // _HAS_AUTO_PTR_ETC
 
-_EXPORT_STD class bad_weak_ptr : public exception { // exception type for invalid use of expired weak_ptr object
+_EXPORT_STD class _NODISCARD bad_weak_ptr : public exception {
+    // exception type for invalid use of expired weak_ptr object
 public:
     bad_weak_ptr() noexcept {}
 

stl/inc/optional

@@ -35,7 +35,7 @@ _EXPORT_STD struct nullopt_t { // no-value state indicator
 };
 _EXPORT_STD inline constexpr nullopt_t nullopt{nullopt_t::_Tag{}};
 
-_EXPORT_STD class bad_optional_access : public exception {
+_EXPORT_STD class _NODISCARD bad_optional_access : public exception {
 public:
     _NODISCARD const char* __CLR_OR_THIS_CALL what() const noexcept override {
         return "Bad optional access";

stl/inc/regex

@@ -460,7 +460,7 @@ public:
     }
 };
 
-_EXPORT_STD class regex_error : public runtime_error { // type of all regular expression exceptions
+_EXPORT_STD class _NODISCARD regex_error : public runtime_error { // type of all regular expression exceptions
 public:
     explicit regex_error(regex_constants::error_type _Ex) : runtime_error(_Stringify(_Ex)), _Err(_Ex) {}
 

stl/inc/stdexcept

@@ -17,7 +17,7 @@ _STL_DISABLE_CLANG_WARNINGS
 #pragma push_macro("new")
 #undef new
 _STD_BEGIN
-_EXPORT_STD class logic_error : public exception { // base of all logic-error exceptions
+_EXPORT_STD class _NODISCARD logic_error : public exception { // base of all logic-error exceptions
 public:
     using _Mybase = exception;
 
@@ -33,7 +33,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class domain_error : public logic_error { // base of all domain-error exceptions
+_EXPORT_STD class _NODISCARD domain_error : public logic_error { // base of all domain-error exceptions
 public:
     using _Mybase = logic_error;
 
@@ -49,7 +49,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class invalid_argument : public logic_error { // base of all invalid-argument exceptions
+_EXPORT_STD class _NODISCARD invalid_argument : public logic_error { // base of all invalid-argument exceptions
 public:
     using _Mybase = logic_error;
 
@@ -65,7 +65,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class length_error : public logic_error { // base of all length-error exceptions
+_EXPORT_STD class _NODISCARD length_error : public logic_error { // base of all length-error exceptions
 public:
     using _Mybase = logic_error;
 
@@ -81,7 +81,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class out_of_range : public logic_error { // base of all out-of-range exceptions
+_EXPORT_STD class _NODISCARD out_of_range : public logic_error { // base of all out-of-range exceptions
 public:
     using _Mybase = logic_error;
 
@@ -97,7 +97,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class runtime_error : public exception { // base of all runtime-error exceptions
+_EXPORT_STD class _NODISCARD runtime_error : public exception { // base of all runtime-error exceptions
 public:
     using _Mybase = exception;
 
@@ -113,7 +113,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class overflow_error : public runtime_error { // base of all overflow-error exceptions
+_EXPORT_STD class _NODISCARD overflow_error : public runtime_error { // base of all overflow-error exceptions
 public:
     using _Mybase = runtime_error;
 
@@ -129,7 +129,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class underflow_error : public runtime_error { // base of all underflow-error exceptions
+_EXPORT_STD class _NODISCARD underflow_error : public runtime_error { // base of all underflow-error exceptions
 public:
     using _Mybase = runtime_error;
 
@@ -145,7 +145,7 @@ protected:
 #endif // !_HAS_EXCEPTIONS
 };
 
-_EXPORT_STD class range_error : public runtime_error { // base of all range-error exceptions
+_EXPORT_STD class _NODISCARD range_error : public runtime_error { // base of all range-error exceptions
 public:
     using _Mybase = runtime_error;
 

stl/inc/system_error

@@ -458,7 +458,7 @@ struct hash<error_condition> {
     }
 };
 
-class _System_error : public runtime_error { // base of all system-error exceptions
+class _NODISCARD _System_error : public runtime_error { // base of all system-error exceptions
 private:
     static string _Makestr(error_code _Errcode, string _Message) { // compose error message
         if (!_Message.empty()) {
@@ -478,7 +478,7 @@ protected:
     error_code _Mycode; // the stored error code
 };
 
-_EXPORT_STD class system_error : public _System_error { // base of all system-error exceptions
+_EXPORT_STD class _NODISCARD system_error : public _System_error { // base of all system-error exceptions
 private:
     using _Mybase = _System_error;
 

stl/inc/typeinfo

@@ -29,7 +29,7 @@ _STD_BEGIN
 _INLINE_VAR constexpr int _Small_object_num_ptrs = 6 + 16 / sizeof(void*);
 
 #if !_HAS_EXCEPTIONS
-_EXPORT_STD class bad_cast : public exception { // base of all bad cast exceptions
+_EXPORT_STD class _NODISCARD bad_cast : public exception { // base of all bad cast exceptions
 public:
     bad_cast(const char* _Message = "bad cast") noexcept : exception(_Message) {}
 
@@ -41,7 +41,7 @@ protected:
     }
 };
 
-_EXPORT_STD class bad_typeid : public exception { // base of all bad typeid exceptions
+_EXPORT_STD class _NODISCARD bad_typeid : public exception { // base of all bad typeid exceptions
 public:
     bad_typeid(const char* _Message = "bad typeid") noexcept : exception(_Message) {}
 
@@ -53,7 +53,7 @@ protected:
     }
 };
 
-class __non_rtti_object : public bad_typeid { // report a non-RTTI object
+class _NODISCARD __non_rtti_object : public bad_typeid { // report a non-RTTI object
 public:
     __non_rtti_object(const char* _Message) : bad_typeid(_Message) {}
 };

stl/inc/xiosbase

@@ -108,7 +108,7 @@ public:
     using seek_dir  = unsigned int;
 #endif // _HAS_OLD_IOSTREAMS_MEMBERS
 
-    class failure : public system_error { // base of all iostreams exceptions
+    class _NODISCARD failure : public system_error { // base of all iostreams exceptions
     public:
         explicit failure(const string& _Message, const error_code& _Errcode = _STD make_error_code(io_errc::stream))
             : system_error(_Errcode, _Message) {} // construct with message

tests/libcxx/expected_results.txt

@@ -36,6 +36,12 @@ std/numerics/rand/rand.dist/rand.dist.uni/rand.dist.uni.real/param_ctor.pass.cpp
 # LLVM-113609: [libc++][test] Non-rebindable test_alloc in string.capacity/deallocate_size.pass.cpp
 std/strings/basic.string/string.capacity/deallocate_size.pass.cpp FAIL
 
+# LLVM-119174: [libcxx][test] Silence nodiscard warnings for std::expected
+std/utilities/expected/expected.expected/monadic/and_then.pass.cpp FAIL
+std/utilities/expected/expected.expected/monadic/or_else.pass.cpp FAIL
+std/utilities/expected/expected.expected/monadic/transform.pass.cpp FAIL
+std/utilities/expected/expected.expected/monadic/transform_error.pass.cpp FAIL
+
 # Non-Standard regex behavior.
 # "It seems likely that the test is still non-conforming due to how libc++ handles the 'w' character class."
 std/re/re.traits/lookup_classname.pass.cpp FAIL

tests/std/tests/P2505R5_monadic_functions_for_std_expected/test.cpp

@@ -217,8 +217,6 @@ constexpr void test_impl(Expected&& engaged, Expected&& unengaged) {
             assert(result.value().x == 77);
         }
     }
-
-    engaged.transform([](auto...) { return ""; });
 }
 
 template <IsNothrowConstructible nothrowConstructible, IsNothrowConvertible nothrowConvertible>