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3629. make_error_code and make_error_condition are customization points

Section: 19.5 [syserr] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-10-31 Last modified: 2023-11-22

Priority: 2

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Discussion:

The rule in 16.4.2.2 [contents] means that the calls to make_error_code in 19.5.4.2 [syserr.errcode.constructors] and 19.5.4.3 [syserr.errcode.modifiers] are required to call std::make_error_code, which means program-defined error codes do not work. The same applies to the make_error_condition calls in 19.5.5.2 [syserr.errcondition.constructors] and 19.5.5.3 [syserr.errcondition.modifiers].

They need to use ADL. This is what all known implementations (including Boost.System) do.

[2022-01-29; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

This wording is relative to N4901.

1. Modify 19.5.2 [system.error.syn] as indicated:

   -1- The value of each enum errc constant shall be the same as the value of the <cerrno> macro shown in the above synopsis. Whether or not the <system_error> implementation exposes the <cerrno> macros is unspecified.

   -?- Invocations of make_error_code and make_error_condition shown in subclause 19.5 [syserr] select a function to call via overload resolution (12.2 [over.match]) on a candidate set that includes the lookup set found by argument dependent lookup (6.5.4 [basic.lookup.argdep]).

   -2- The is_error_code_enum and is_error_condition_enum templates may be specialized for program-defined types to indicate that such types are eligible for class error_code and class error_condition implicit conversions, respectively.

   [Note 1: Conversions from such types are done by program-defined overloads of make_error_code and make_error_condition, found by ADL. —end note]

[2022-08-25; Jonathan Wakely provides improved wording]

Discussed in LWG telecon and decided on new direction:

• Add make_error_code and make_error_condition to 16.4.2.2 [contents] as done for swap. Describe form of lookup used for them.
• Respecify error_code and error_condition constructors in terms of "Effects: Equivalent to" so that the requirements on program-defined overloads found by ADL are implied by those effects.

[2022-09-07; Jonathan Wakely revises wording]

Discussed in LWG telecon. Decided to change "established as-if by performing unqualified name lookup and argument-dependent lookup" to simply "established as-if by performing argument-dependent lookup".

This resolves the question of whether std::make_error_code(errc), std::make_error_code(io_errc), etc. should be visible to the unqualified name lookup. This affects whether a program-defined type that specializes is_error_code_enum but doesn't provide an overload of make_error_code should find the overloads in namespace std and consider them for overload resolution, via implicit conversion to std::errc, std::io_errc, etc.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

• Modify 16.4.2.2 [contents] as indicated:

  -3- Whenever an unqualified name other than swap, make_error_code, or make_error_condition is used in the specification of a declaration D in 17 [support] through 32 [thread] or D [depr], its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of D.

  [Note 1: Argument-dependent lookup is not performed. — end note]

  Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of D.

  [Example 1: The reference to is_array_v in the specification of std::to_array (23.3.3.6 [array.creation]) refers to ::std::is_array_v. — end example]

  [Note 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

  The meaning of the unqualified name swap is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]). The meanings of the unqualified names make_error_code and make_error_condition are established as-if by performing argument-dependent lookup (6.5.4 [basic.lookup.argdep]).

• Modify 19.5.4.2 [syserr.errcode.constructors] as indicated:

  error_code() noexcept;

  -1- Postconditions: val_ == 0 and cat_ == &system_category().
  -1- Effects: Initializes val_ with 0 and cat_ with &system_category().

  error_code(int val, const error_category& cat) noexcept;

  -2- Postconditions: val_ == val and cat_ == &cat.
  -2- Effects: Initializes val_ with val and cat_ with &cat.

  template<class ErrorCodeEnum>
    error_code(ErrorCodeEnum e) noexcept;

  -3- Constraints: is_error_code_enum_v<ErrorCodeEnum> is true.

  -4- Postconditions: *this == make_error_code(e).
  -4- Effects: Equivalent to:

  error_code ec = make_error_code(e);
  assign(ec.value(), ec.category());
  

• Modify 19.5.4.3 [syserr.errcode.modifiers] as indicated:

  template<class ErrorCodeEnum>
    error_code& operator=(ErrorCodeEnum e) noexcept;

  -2- Constraints: is_error_code_enum_v<ErrorCodeEnum> is true.

  -3- Postconditions: *this == make_error_code(e).
  -3- Effects: Equivalent to:

  error_code ec = make_error_code(e);
  assign(ec.value(), ec.category());
  

  -4- Returns: *this.

• Modify 19.5.5.2 [syserr.errcondition.constructors] as indicated:

  error_condition() noexcept;

  -1- Postconditions: val_ == 0 and cat_ == &generic_category().
  -1- Effects: Initializes val_ with 0 and cat_ with &generic_category().

  error_condition(int val, const error_category& cat) noexcept;

  -2- Postconditions: val_ == val and cat_ == &cat.
  -2- Effects: Initializes val_ with val and cat_ with &cat.

  template<class ErrorConditionEnum>
    error_condition(ErrorConditionEnum e) noexcept;

  -3- Constraints: is_error_condition_enum_v<ErrorConditionEnum> is true.

  -4- Postconditions: *this == make_error_condition(e).
  -4- Effects: Equivalent to:

  error_condition ec = make_error_condition(e);
  assign(ec.value(), ec.category());
  

• Modify 19.5.5.3 [syserr.errcondition.modifiers] as indicated:

  template<class ErrorConditionEnum>
    error_condition& operator=(ErrorConditionEnum e) noexcept;

  -2- Constraints: is_error_condition_enum_v<ErrorConditionEnum> is true.

  -3- Postconditions: *this == make_error_condition(e).
  -3- Effects: Equivalent to:

  error_condition ec = make_error_condition(e);
  assign(ec.value(), ec.category());
  

  -4- Returns: *this.