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3973. Monadic operations should be ADL-proof

Section: 22.8.6.7 [expected.object.monadic], 22.5.3.8 [optional.monadic] Status: C++26 Submitter: Jiang An Opened: 2023-08-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [expected.object.monadic].

View all issues with C++26 status.

Discussion:

LWG 3938(i) switched to use **this to access the value stored in std::expected. However, as shown in LWG 3969(i), **this can trigger ADL and find an unwanted overload, and thus may caused unintended behavior.

Current implementations behave correctly (Godbolt link): they don't direct use **this, but use the name of the union member instead.

Moreover, P2407R5 will change the monadic operations of std::optional to use **this, which is also problematic.

[2023-09-19; Wording update]

Several people preferred to replace operator*() by the corresponding union members, so this part of the proposed wording has been adjusted, which is a rather mechanical replacement.

[2023-10-30; Reflector poll]

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

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

  1. Modify 22.8.6.7 [expected.object.monadic] as indicated:

    [Drafting note: Effectively replace all occurrences of **this by val, except for decltype.] 

    template constexpr auto and_then(F&& f) &;
template constexpr auto and_then(F&& f) const &;

    -1- Let U be remove_cvref_t**this(val))>>.

    -2- […]

    -3- […]

    -4- Effects: Equivalent to: 

    if (has_value())
  return invoke(std::forward(f), **thisval);
else
  return U(unexpect, error());

    template constexpr auto and_then(F&& f) &&;
template constexpr auto and_then(F&& f) const &&;

    -5- Let U be remove_cvref_t**thisval))>>.

    -6- […]

    -7- […]

    -8- Effects: Equivalent to: 

    if (has_value())
  return invoke(std::forward(f), std::move(**thisval));
else
  return U(unexpect, std::move(error()));

    template constexpr auto or_else(F&& f) &;
template constexpr auto or_else(F&& f) const &;

    -9- Let G be remove_cvref_t>.

    -10- Constraints: is_constructible_v**this(val))> is true.

    -11- […]

    -12- Effects: Equivalent to: 

    if (has_value())
  return G(in_place, **thisval);
else
  return invoke(std::forward(f), error());

    template constexpr auto or_else(F&& f) &&;
template constexpr auto or_else(F&& f) const &&;

    -13- Let G be remove_cvref_t>.

    -14- Constraints: is_constructible_v**thisval))> is true.

    -15- […]

    -16- Effects: Equivalent to: 

    if (has_value())
  return G(in_place, std::move(**thisval));
else
  return invoke(std::forward(f), std::move(error()));

    template constexpr auto transform(F&& f) &;
template constexpr auto transform(F&& f) const &;

    -17- Let U be remove_cvref_t**this(val))>>.

    -18- […]

    -19- Mandates: U is a valid value type for expected. If is_void_v is false, the declaration 

    U u(invoke(std::forward(f), **thisval));

    is well-formed.

    -20- Effects:

    1. (20.1) — […]

    2. (20.2) — Otherwise, if is_void_v is false, returns an expected object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward(f), **thisval).

    3. (20.3) — Otherwise, evaluates invoke(std::forward(f), **thisval) and then returns expected().

    template constexpr auto transform(F&& f) &&;
template constexpr auto transform(F&& f) const &&;

    -21- Let U be remove_cvref_t**thisval))>>.

    -22- […]

    -23- Mandates: U is a valid value type for expected. If is_void_v is false, the declaration 

    U u(invoke(std::forward(f), std::move(**thisval)));

    is well-formed.

    -24- Effects:

    1. (24.1) — […]

    2. (24.2) — Otherwise, if is_void_v is false, returns an expected object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward(f), std::move(**thisval)).

    3. (24.3) — Otherwise, evaluates invoke(std::forward(f), std::move(**thisval)) and then returns expected().

    template constexpr auto transform_error(F&& f) &;
template constexpr auto transform_error(F&& f) const &;

    -25- Let G be remove_cvref_t>.

    -26- Constraints: is_constructible_v**this(val))> is true.

    -27- Mandates: […]

    -28- Returns: If has_value() is true, expected(in_place, **thisval); otherwise, an expected object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward(f), error()). 

    template constexpr auto transform_error(F&& f) &&;
template constexpr auto transform_error(F&& f) const &&;

    -29- Let G be remove_cvref_t>.

    -30- Constraints: is_constructible_v**thisval))> is true.

    -31- Mandates: […]

    -32- Returns: If has_value() is true, expected(in_place, std::move(**thisval)); otherwise, an expected object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward(f), std::move(error())).

  2. Modify 22.5.3.8 [optional.monadic] as indicated:

    [Drafting note: Effectively replace all occurrences of value() by *val.] 

    template constexpr auto and_then(F&& f) &;
template constexpr auto and_then(F&& f) const &;

    -1- Let U be invoke_result_tvalue()*val)>.

    -2- […]

    -3- Effects: Equivalent to: 

    if (*this) {
  return invoke(std::forward(f), value()*val);
} else {
  return remove_cvref_t();
}

    template constexpr auto and_then(F&& f) &&;
template constexpr auto and_then(F&& f) const &&;

    -4- Let U be invoke_result_tvalue()*val))>.

    -5- […]

    -6- Effects: Equivalent to: 

    if (*this) {
  return invoke(std::forward(f), std::move(value()*val));
} else {
  return remove_cvref_t();
}

    template constexpr auto transform(F&& f) &;
template constexpr auto transform(F&& f) const &;

    -7- Let U be remove_cv_tvalue()*val)>>.

    -8- Mandates: U is a non-array object type other than in_place_t or nullopt_t. The declaration 

    U u(invoke(std::forward(f), value()*val));

    is well-formed for some invented variable u.

    […]

    -9- Returns: If *this contains a value, an optional object whose contained value is direct-non-list-initialized with invoke(std::forward(f), value()*val); otherwise, optional(). 

    template constexpr auto transform(F&& f) &&;
template constexpr auto transform(F&& f) const &&;

    -10- Let U be remove_cv_tvalue()*val))>>.

    -11- Mandates: U is a non-array object type other than in_place_t or nullopt_t. The declaration 

    U u(invoke(std::forward(f), std::move(value()*val)));

    is well-formed for some invented variable u.

    […]

    -12- Returns: If *this contains a value, an optional object whose contained value is direct-non-list-initialized with invoke(std::forward(f), std::move(value()*val)); otherwise, optional().