Issue 4015: LWG 3973 broke const overloads of std::optional monadic operations

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4015. LWG 3973 broke `const` overloads of `std::optional` monadic operations

Section: 22.5.3.8 [optional.monadic] Status: C++26 Submitter: Jonathan Wakely Opened: 2023-11-24 Last modified: 2026-06-09

Priority: 1

View all other issues in [optional.monadic].

View all issues with C++26 status.

Discussion:

The resolution of LWG 3973⁽ⁱ⁾ (adopted in Kona) changed all occurrences of value() to *val. The intention was not to change the meaning, just avoid the non-freestanding value() function, and avoid ADL that would be caused by using **this. However, in the const overloads such as and_then(F&&) const the type of value() was const T&, but the type of *val is always T&. This implies that the const overloads invoke the callable with a non-const argument, which is incorrect (and would be undefined behaviour for a const std::optional<T>).

On the LWG reflector it was suggested that we should rewrite the specification of std::optional to stop using an exposition-only data member of type T*. No such member ever exists in real implemetations, so it is misleading and leads to specification bugs of this sort.

Change the class definition in 22.5.3.1 [optional.optional.general] to use a union, and update every use of val accordingly throughout 22.5.3 [optional.optional]. For consistency with 22.8.6.1 [expected.object.general] we might also want to introduce a bool has_val member and refer to that in the specification.


  private:
    T *val;         // exposition only
    
    
      
    
  };

For example, in 22.5.3.9 [optional.mod]:

-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.

[2023-11-26; Daniel provides wording]

The proposed wording is considerably influenced by that of the specification of expected, but attempts to reduce the amount of changes to not perfectly mimic it. Although "the contained value" is a magic word of power it seemed feasible and simpler to use the new exposition-only member val directly in some (but not all) places, usually involved with initializations.

Furthermore, I have only added "and sets has_val to true/false" where either the Effects wording says "otherwise no effect" or in other cases if the postconditions did not already say that indirectly. I also added extra mentioning of has_val changes in tables where different cells had very different effects on that member (unless these cells specify postconditions), to prevent misunderstanding.

[2024-03-11; Reflector poll]

Set priority to 1 after reflector poll in November 2023. Six votes for 'Tentatively Ready' but enough uncertainty to deserve discussion at a meeting.

Previous resolution [SUPERSEDED]

This wording is relative to N4964 after application of the wording of LWG 3973⁽ⁱ⁾.

Modify 22.5.3.1 [optional.optional.general], class template optional synopsis, as indicated:

namespace std {
  template<class T>
  class optional {
  public:
    using value_type = T;
    […]
  private:
    
      T *val; // exposition only
    
  };

  […]
}

Modify 22.5.3.1 [optional.optional.general] as indicated:

-2- ~~When an optional<T> object contains a value, member val points to the contained value~~.
Modify 22.5.3.2 [optional.ctor] as indicated:

[Drafting note: Normatively, this subclause doesn't require any changes, but I'm suggesting to replace phrases of the form "[…]initializes the contained value with"] by "[…]initializes val with" as we do in 22.8.6.2 [expected.object.cons]. I intentionally did not add extra "and sets has_val to true/false" since those effects are already guaranteed by the postconditions]
```
constexpr optional(const optional& rhs);
```
-4- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *rhs.
-5- Postconditions: rhs.has_value() == this->has_value().
[…]
```
constexpr optional(optional&& rhs) noexcept(see below);
```
-8- Constraints: […]
-9- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with std::move(*rhs). rhs.has_value() is unchanged.
-10- Postconditions: rhs.has_value() == this->has_value().
[…]
```
template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
```
-13- Constraints: […]
-14- Effects: Direct-non-list-initializes ~~the contained value~~ with std::forward<Args>(args)....
-15- Postconditions: *this contains a value.
[…]
```
template<class U, class... Args>
 constexpr explicit optional(in_place_t, initializer_list il, Args&&... args);
```
-18- Constraints: […]
-19- Effects: Direct-non-list-initializes ~~the contained value~~ with il, std::forward<Args>(args)....
-20- Postconditions: *this contains a value.
[…]
```
template<class U = T> constexpr explicit(see below) optional(U&& v);
```
-23- Constraints: […]
-24- Effects: Direct-non-list-initializes ~~the contained value~~ with std::forward(v).
-25- Postconditions: *this contains a value.
[…]
```
template<class U> constexpr explicit(see below) optional(const optional& rhs);
```
-28- Constraints: […]
-29- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *rhs.
-30- Postconditions: rhs.has_value() == this->has_value().
[…]
```
template<class U> constexpr explicit(see below) optional(optional&& rhs);
```
-33- Constraints: […]
-34- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with std::move(*rhs). rhs.has_value() is unchanged.
-35- Postconditions: rhs.has_value() == this->has_value().
[…]
Modify 22.5.3.3 [optional.dtor] as indicated:
```
constexpr ~optional();
```
-1- Effects: If is_trivially_destructible_v<T> != true and *this contains a value, calls val->T::~T()

Modify 22.5.3.4 [optional.assign] as indicated:

constexpr optional<T>& operator=(nullopt_t) noexcept;
-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value ; otherwise no effect.
-2- Postconditions: *this does not contain a value.
constexpr optional<T>& operator=(const optional& rhs);
-4- Effects: See Table 58.

Table 58 — optional::operator=(const optional&) effects [tab:optional.assign.copy]
*this contains a value *this does not contain a value

rhs contains a value assigns *rhs to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with *rhs

rhs does not contain a value destroys the contained value by calling val->T::~T()
no effect

-5- Postconditions: rhs.has_value() == this->has_value().
[…]
constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);
-8- Constraints: […]
-9- Effects: See Table 59. The result of the expression rhs.has_value() remains unchanged.
-10- Postconditions: rhs.has_value() == this->has_value().
-11- Returns: *this.

Table 59 — optional::operator=(optional&&) effects [tab:optional.assign.move]
*this contains a value *this does not contain a value

rhs contains a value assigns std::move(*rhs) to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with std::move(*rhs)

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-12- Remarks: […]
-13- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor. If an exception is thrown during the call to T's move assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.
template<class U = T> constexpr optional<T>& operator=(U&& v);
-14- Constraints: […]
-15- Effects: If *this contains a value, assigns std::forward(v) to ~~the contained value~~; otherwise direct-non-list-initializes ~~the contained value~~ with std::forward(v).
-16- Postconditions: *this contains a value.
-17- Returns: *this.
-18- Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and v is determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(const optional& rhs);
-19- Constraints: […]
-20- Effects: See Table 60.

Table 60 — optional::operator=(const optional&) effects [tab:optional.assign.copy.templ]
*this contains a value *this does not contain a value

rhs contains a value assigns *rhs to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with *rhs

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-21- Postconditions: rhs.has_value() == this->has_value().
-22- Returns: *this.
-23- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(optional&& rhs);
-24- Constraints: […]
-25- Effects: See Table 61. The result of the expression rhs.has_value() remains unchanged.

Table 61 — optional::operator=(optional&&) effects [tab:optional.assign.move.templ]
*this contains a value *this does not contain a value

rhs contains a value assigns std::move(*rhs) to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with
std::move(*rhs)

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-26- Postconditions: rhs.has_value() == this->has_value().
-27- Returns: *this.
-28- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
template<class... Args> constexpr T& emplace(Args&&... args);
-29- Mandates: […]
-30- Effects: Calls *this = nullopt. Then direct-non-list-initializes ~~the contained value~~ with std::forward<Args>(args)....
-31- Postconditions: *this contains a value.
-32- Returns: ~~A reference to the new contained value~~.
[…]
-34- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.
template<class U, class... Args> constexpr T& emplace(initializer_list il, Args&&... args);
-35- Constraints: […]
-36- Effects: Calls *this = nullopt. Then direct-non-list-initializes ~~the contained value~~ with il, std::forward<Args>(args)....
-37- Postconditions: *this contains a value.
-38- Returns: ~~A reference to the new contained value~~.
[…]
-40- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

Table 58 — `optional::operator=(const optional&)` effects [tab:optional.assign.copy]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*rhs` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*rhs`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 59 — `optional::operator=(optional&&)` effects [tab:optional.assign.move]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `std::move(*rhs)` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `std::move(*rhs)`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 60 — `optional::operator=(const optional<U>&)` effects [tab:optional.assign.copy.templ]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*rhs` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*rhs`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 61 — `optional::operator=(optional<U>&&)` effects [tab:optional.assign.move.templ]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `std::move(*rhs)` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `std::move(*rhs)`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Modify 22.5.3.5 [optional.swap] as indicated:

constexpr void swap(optional& rhs) noexcept(see below);
-1- Mandates: […]
-2- Preconditions: […]
-3- Effects: See Table 62.

Table 62 — optional::swap(optional&) effects [tab:optional.swap]
*this contains a value *this does not contain a value

rhs contains a value calls swap(*(*this), *rhs) direct-non-list-initializes ~~the contained value of *this~~
with std::move(*rhs), followed by rhs.val->T::~T();
postcondition is that *this contains a value and rhs does
not contain a value

rhs does not contain a value direct-non-list-initializes ~~the contained value of~~ rhs
with std::move(*(*this)), followed by val->T::~T();
postcondition is that *this does not contain a value and rhs
contains a value no effect

-4- Throws: […]
-5- Remarks: […]
-6- If any exception is thrown, the results of the expressions this->has_value() and rhs.has_value() remain unchanged. If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T. If an exception is thrown during the call to T's move constructor, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor.

Table 62 — `optional::swap(optional&)` effects [tab:optional.swap]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	calls `swap((this), *rhs)`	direct-non-list-initializes ~~the contained value of `this`~~ with `std::move(rhs)`, followed by `rhs.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value
`rhs` does not contain a value	direct-non-list-initializes ~~the contained value of~~ `rhs` with `std::move((this))`, followed by `val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value	no effect

Modify 22.5.3.7 [optional.observe] as indicated:

constexpr const T* operator->() const noexcept;
constexpr T* operator->() noexcept;
-1- Preconditions: *this contains a value.
-2- Returns: val.
-3- […]
constexpr const T& operator*() const & noexcept;
constexpr T& operator*() & noexcept;
-4- Preconditions: *this contains a value.
-5- Returns: *val.
-6- […]
constexpr T&& operator*() && noexcept;
constexpr const T&& operator*() const && noexcept;
-7- Preconditions: *this contains a value.
-8- Effects: Equivalent to: return std::move(*val);
constexpr explicit operator bool() const noexcept;
~~-9- Returns: true if and only if *this contains a value.~~
~~-10- Remarks: This function is a constexpr function.~~
constexpr bool has_value() const noexcept;
-11- Returns: ~~true if and only if *this contains a value~~.
-12- Remarks: ~~This function is a~~ constexpr function.
constexpr const T& value() const &;
constexpr T& value() &;
-13- Effects: Equivalent to:
return has_value() ? *val : throw bad_optional_access();
constexpr T&& value() &&;
constexpr const T&& value() const &&;
-14- Effects: Equivalent to:
return has_value() ? std::move(*val) : throw bad_optional_access();
template<class U> constexpr T value_or(U&& v) const &;
-15- Mandates: […]
-16- Effects: Equivalent to:
return has_value() ? **this : static_cast<T>(std::forward(v));
template<class U> constexpr T value_or(U&& v) &&;
-17- Mandates: […]
-18- Effects: Equivalent to:
return has_value() ? std::move(**this) : static_cast<T>(std::forward(v));

Modify 22.5.3.8 [optional.monadic] as indicated:
```
template<class F> constexpr auto and_then(F&& f) &;
template<class F> constexpr auto and_then(F&& f) const &;
```
-1- Let U be invoke_result_t<F, decltype(*val)>.
-2- Mandates: […]
-3- Effects: Equivalent to:
```
if (*this) {
 return invoke(std::forward<F>(f), *val);
} else {
 return remove_cvref_t();
}
```
```
template<class F> constexpr auto and_then(F&& f) &&;
template<class F> constexpr auto and_then(F&& f) const &&;
```
-4- Let U be invoke_result_t<F, decltype(std::move(*val))>.
-5- Mandates: […]
-6- Effects: Equivalent to:
```
if (*this) {
 return invoke(std::forward<F>(f), std::move(*val));
} else {
 return remove_cvref_t();
}
```
```
template<class F> constexpr auto transform(F&& f) &;
template<class F> constexpr auto transform(F&& f) const &;
```
-7- Let U be remove_cv_t<invoke_result_t<F, decltype(*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>(f), *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>(f), *val); otherwise, optional().
```
template<class F> constexpr auto transform(F&& f) &&;
template<class F> constexpr auto transform(F&& f) const &&;
```
-10- Let U be remove_cv_t<invoke_result_t<F, decltype(std::move(*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>(f), std::move(*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>(f), std::move(*val)); otherwise, optional().
Modify 22.5.3.9 [optional.mod] as indicated:
```
constexpr void reset() noexcept;
```
-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value ; otherwise no effect.
-2- Postconditions: *this does not contain a value.

[St. Louis 2024-06-24; Jonathan provides improved wording]

[2024-08-21; LWG telecon]

During telecon review it was suggested to replace 22.5.3.1 [optional.optional.general] p1 and p2. On the reflector Daniel requested to keep the "additional storage" prohibition, so that will be addressed by issue 4141⁽ⁱ⁾ instead.

[2024-10-02; Jonathan tweaks proposed resolution]

On the reflector we decided that the union member should use remove_cv_t, as proposed for expected by issue 3891⁽ⁱ⁾. The rest of the proposed resolution is unchanged, so that edit was made in-place below, instead of as a new resolution that supersedes the old one.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

Modify 22.5.3.1 [optional.optional.general], class template optional synopsis, as indicated:

namespace std {
  template<class T>
  class optional {
  public:
    using value_type = T;
    […]
  private:
    *val // exposition only;
    
      
    
  };

  […]
}

Modify 22.5.3.1 [optional.optional.general] as indicated:

-1- Any instance of optional<T> at any given time either contains a value or does not contain a value. When an instance of optional<T> contains a value, it means that an object of type T, referred to as the optional object's contained value, is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value. ~~When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false.~~

~~-2- When an optional<T> object contains a value, member val points to the contained value.~~
Modify 22.5.3.2 [optional.ctor] as indicated:
```
constexpr optional(const optional& rhs);
```
-4- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *rhs.
-5- Postconditions: rhs.has_value() == this->has_value().
[…]
```
constexpr optional(optional&& rhs) noexcept(see below);
```
-8- Constraints: […]
-9- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with std::move(*rhs). rhs.has_value() is unchanged.
-10- Postconditions: rhs.has_value() == this->has_value().
[…]
```
template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
```
-13- Constraints: […]
-14- Effects: Direct-non-list-initializes ~~the contained value~~ with std::forward<Args>(args)....
-15- Postconditions: *this contains a value.
[…]
```
template<class U, class... Args>
 constexpr explicit optional(in_place_t, initializer_list il, Args&&... args);
```
-18- Constraints: […]
-19- Effects: Direct-non-list-initializes ~~the contained value~~ with il, std::forward<Args>(args)....
-20- Postconditions: *this contains a value.
[…]
```
template<class U = T> constexpr explicit(see below) optional(U&& v);
```
-23- Constraints: […]
-24- Effects: Direct-non-list-initializes ~~the contained value~~ with std::forward(v).
-25- Postconditions: *this contains a value.
[…]
```
template<class U> constexpr explicit(see below) optional(const optional& rhs);
```
-28- Constraints: […]
-29- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *rhs.
-30- Postconditions: rhs.has_value() == this->has_value().
[…]
```
template<class U> constexpr explicit(see below) optional(optional&& rhs);
```
-33- Constraints: […]
-34- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with std::move(*rhs). rhs.has_value() is unchanged.
-35- Postconditions: rhs.has_value() == this->has_value().
[…]
Modify 22.5.3.3 [optional.dtor] as indicated:
```
constexpr ~optional();
```
-1- Effects: If is_trivially_destructible_v<T> != true and *this contains a value, calls val->T::~T()

Modify 22.5.3.4 [optional.assign] as indicated:

constexpr optional<T>& operator=(nullopt_t) noexcept;
-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.
-2- Postconditions: *this does not contain a value.
constexpr optional<T>& operator=(const optional& rhs);
-4- Effects: See Table 58.

Table 58 — optional::operator=(const optional&) effects [tab:optional.assign.copy]
*this contains a value *this does not contain a value

rhs contains a value assigns *rhs to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with *rhs

rhs does not contain a value destroys the contained value by calling val->T::~T()
no effect

-5- Postconditions: rhs.has_value() == this->has_value().
[…]
constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);
-8- Constraints: […]
-9- Effects: See Table 59. The result of the expression rhs.has_value() remains unchanged.
-10- Postconditions: rhs.has_value() == this->has_value().
-11- Returns: *this.

Table 59 — optional::operator=(optional&&) effects [tab:optional.assign.move]
*this contains a value *this does not contain a value

rhs contains a value assigns std::move(*rhs) to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with std::move(*rhs)

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-12- Remarks: […]
-13- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor. If an exception is thrown during the call to T's move assignment, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.
template<class U = T> constexpr optional<T>& operator=(U&& v);
-14- Constraints: […]
-15- Effects: If *this contains a value, assigns std::forward(v) to ~~the contained value~~; otherwise direct-non-list-initializes ~~the contained value~~ with std::forward(v).
-16- Postconditions: *this contains a value.
-17- Returns: *this.
-18- Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the ~~state~~ of *val and v is determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(const optional& rhs);
-19- Constraints: […]
-20- Effects: See Table 60.

Table 60 — optional::operator=(const optional&) effects [tab:optional.assign.copy.templ]
*this contains a value *this does not contain a value

rhs contains a value assigns *rhs to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with *rhs

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-21- Postconditions: rhs.has_value() == this->has_value().
-22- Returns: *this.
-23- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(optional&& rhs);
-24- Constraints: […]
-25- Effects: See Table 61. The result of the expression rhs.has_value() remains unchanged.

Table 61 — optional::operator=(optional&&) effects [tab:optional.assign.move.templ]
*this contains a value *this does not contain a value

rhs contains a value assigns std::move(*rhs) to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with
std::move(*rhs)

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-26- Postconditions: rhs.has_value() == this->has_value().
-27- Returns: *this.
-28- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
template<class... Args> constexpr T& emplace(Args&&... args);
-29- Mandates: […]
-30- Effects: Calls *this = nullopt. Then direct-non-list-initializes ~~the contained value~~ with std::forward<Args>(args)....
-31- Postconditions: *this contains a value.
-32- Returns: ~~A reference to the new contained value~~.
[…]
-34- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.
template<class U, class... Args> constexpr T& emplace(initializer_list il, Args&&... args);
-35- Constraints: […]
-36- Effects: Calls *this = nullopt. Then direct-non-list-initializes ~~the contained value~~ with il, std::forward<Args>(args)....
-37- Postconditions: *this contains a value.
-38- Returns: ~~A reference to the new contained value~~.
[…]
-40- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

Table 58 — `optional::operator=(const optional&)` effects [tab:optional.assign.copy]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*rhs` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*rhs`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 59 — `optional::operator=(optional&&)` effects [tab:optional.assign.move]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `std::move(*rhs)` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `std::move(*rhs)`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 60 — `optional::operator=(const optional<U>&)` effects [tab:optional.assign.copy.templ]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*rhs` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*rhs`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 61 — `optional::operator=(optional<U>&&)` effects [tab:optional.assign.move.templ]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `std::move(*rhs)` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `std::move(*rhs)`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Modify 22.5.3.5 [optional.swap] as indicated:

constexpr void swap(optional& rhs) noexcept(see below);
-1- Mandates: […]
-2- Preconditions: […]
-3- Effects: See Table 62.

Table 62 — optional::swap(optional&) effects [tab:optional.swap]
*this contains a value *this does not contain a value

rhs contains a value calls swap(*(*this), *rhs) direct-non-list-initializes ~~the contained value of *this~~
with std::move(*rhs), followed by rhs.val->T::~T();
postcondition is that *this contains a value and rhs does
not contain a value

rhs does not contain a value direct-non-list-initializes ~~the contained value of~~ rhs
with std::move(*(*this)), followed by val->T::~T();
postcondition is that *this does not contain a value and rhs
contains a value no effect

-4- Throws: […]
-5- Remarks: […]
-6- If any exception is thrown, the results of the expressions this->has_value() and rhs.has_value() remain unchanged. If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T. If an exception is thrown during the call to T's move constructor, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor.

Table 62 — `optional::swap(optional&)` effects [tab:optional.swap]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	calls `swap((this), *rhs)`	direct-non-list-initializes ~~the contained value of `this`~~ with `std::move(rhs)`, followed by `rhs.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value
`rhs` does not contain a value	direct-non-list-initializes ~~the contained value of~~ `rhs` with `std::move((this))`, followed by `val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value	no effect

Modify 22.5.3.7 [optional.observe] as indicated:

constexpr const T* operator->() const noexcept;
constexpr T* operator->() noexcept;
-1- Preconditions: *this contains a value.
-2- Returns: val.
-3- […]
constexpr const T& operator*() const & noexcept;
constexpr T& operator*() & noexcept;
-4- Preconditions: *this contains a value.
-5- Returns: *val.
-6- […]
constexpr T&& operator*() && noexcept;
constexpr const T&& operator*() const && noexcept;
-7- Preconditions: *this contains a value.
-8- Effects: Equivalent to: return std::move(*val);
constexpr explicit operator bool() const noexcept;
-9- Returns: true if and only if *this contains a value.
-10- Remarks: This function is a constexpr function.
constexpr bool has_value() const noexcept;
-11- Returns: true if and only if *this contains a value.
-12- Remarks: This function is a constexpr function.
constexpr const T& value() const &;
constexpr T& value() &;
-13- Effects: Equivalent to:
return has_value() ? *val : throw bad_optional_access();
constexpr T&& value() &&;
constexpr const T&& value() const &&;
-14- Effects: Equivalent to:
return has_value() ? std::move(*val) : throw bad_optional_access();
template<class U> constexpr T value_or(U&& v) const &;
-15- Mandates: […]
-16- Effects: Equivalent to:
return has_value() ? **this : static_cast<T>(std::forward(v));
template<class U> constexpr T value_or(U&& v) &&;
-17- Mandates: […]
-18- Effects: Equivalent to:
return has_value() ? std::move(**this) : static_cast<T>(std::forward(v));

Modify 22.5.3.8 [optional.monadic] as indicated:
```
template<class F> constexpr auto and_then(F&& f) &;
template<class F> constexpr auto and_then(F&& f) const &;
```
-1- Let U be invoke_result_t<F, decltype(*val)>.
-2- Mandates: […]
-3- Effects: Equivalent to:
```
if (*this) {
 return invoke(std::forward<F>(f), *val);
} else {
 return remove_cvref_t();
}
```
```
template<class F> constexpr auto and_then(F&& f) &&;
template<class F> constexpr auto and_then(F&& f) const &&;
```
-4- Let U be invoke_result_t<F, decltype(std::move(*val))>.
-5- Mandates: […]
-6- Effects: Equivalent to:
```
if (*this) {
 return invoke(std::forward<F>(f), std::move(*val));
} else {
 return remove_cvref_t();
}
```
```
template<class F> constexpr auto transform(F&& f) &;
template<class F> constexpr auto transform(F&& f) const &;
```
-7- Let U be remove_cv_t<invoke_result_t<F, decltype(*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>(f), *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>(f), *val); otherwise, optional().
```
template<class F> constexpr auto transform(F&& f) &&;
template<class F> constexpr auto transform(F&& f) const &&;
```
-10- Let U be remove_cv_t<invoke_result_t<F, decltype(std::move(*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>(f), std::move(*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>(f), std::move(*val)); otherwise, optional().
Modify 22.5.3.9 [optional.mod] as indicated:
```
constexpr void reset() noexcept;
```
-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.
-2- Postconditions: *this does not contain a value.

[2025-11-03; Tomasz tweaks proposed resolution]

Updated converting constructor and assignments to use operator*() directly, required to correctly support optional<T&>. Also update corresponding constructor in specialization.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

Modify 22.5.3.1 [optional.optional.general], class template optional synopsis, as indicated:

namespace std {
  template<class T>
  class optional {
  public:
    using value_type = T;
    […]
  private:
    T* val; // exposition only
    
      
    
  };

  […]
}

Modify 22.5.3.1 [optional.optional.general] as indicated:

-1- An object of type optional<T> at any given time either contains a value or does not contain a value. When an object of type optional<T> contains a value, it means that an object of type T, referred to as the optional object's contained value, is nested within (6.8.2 [intro.object]) the optional object. ~~When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false.~~

~~-2- When an optional<T> object contains a value, member val points to the contained value.~~
Modify 22.5.3.2 [optional.ctor] as indicated:
```
constexpr optional(const optional& rhs);
```
-4- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *rhs.
-5- Postconditions: rhs.has_value() == this->has_value().
[…]
```
constexpr optional(optional&& rhs) noexcept(see below);
```
-8- Constraints: […]
-9- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with std::move(*rhs). rhs.has_value() is unchanged.
-10- Postconditions: rhs.has_value() == this->has_value().
[…]
```
template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
```
-13- Constraints: […]
-14- Effects: Direct-non-list-initializes ~~the contained value~~ with std::forward<Args>(args)....
-15- Postconditions: *this contains a value.
[…]
```
template<class U, class... Args>
 constexpr explicit optional(in_place_t, initializer_list il, Args&&... args);
```
-18- Constraints: […]
-19- Effects: Direct-non-list-initializes ~~the contained value~~ with il, std::forward<Args>(args)....
-20- Postconditions: *this contains a value.
[…]
```
template<class U = T> constexpr explicit(see below) optional(U&& v);
```
-23- Constraints: […]
-24- Effects: Direct-non-list-initializes ~~the contained value~~ with std::forward(v).
-25- Postconditions: *this contains a value.
[…]
```
template<class U> constexpr explicit(see below) optional(const optional& rhs);
```
-28- Constraints: […]
-29- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *rhs.
-30- Postconditions: rhs.has_value() == this->has_value().
[…]
```
template<class U> constexpr explicit(see below) optional(optional&& rhs);
```
-33- Constraints: […]
-34- Effects: If rhs contains a value, direct-non-list-initializes ~~the contained value~~ with *std::move(rhs). rhs.has_value() is unchanged.
-35- Postconditions: rhs.has_value() == this->has_value().
[…]
Modify 22.5.3.3 [optional.dtor] as indicated:
```
constexpr ~optional();
```
-1- Effects: If is_trivially_destructible_v<T> != true and *this contains a value, calls val->T::~T()

Modify 22.5.3.4 [optional.assign] as indicated:

constexpr optional<T>& operator=(nullopt_t) noexcept;
-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.
-2- Postconditions: *this does not contain a value.
constexpr optional<T>& operator=(const optional& rhs);
-4- Effects: See Table 58.

Table 58 — optional::operator=(const optional&) effects [tab:optional.assign.copy]
*this contains a value *this does not contain a value

rhs contains a value assigns *rhs to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with *rhs

rhs does not contain a value destroys the contained value by calling val->T::~T()
no effect

-5- Postconditions: rhs.has_value() == this->has_value().
[…]
constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);
-8- Constraints: […]
-9- Effects: See Table 59. The result of the expression rhs.has_value() remains unchanged.
-10- Postconditions: rhs.has_value() == this->has_value().
-11- Returns: *this.

Table 59 — optional::operator=(optional&&) effects [tab:optional.assign.move]
*this contains a value *this does not contain a value

rhs contains a value assigns std::move(*rhs) to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with std::move(*rhs)

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-12- Remarks: […]
-13- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor. If an exception is thrown during the call to T's move assignment, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.
template<class U = T> constexpr optional<T>& operator=(U&& v);
-14- Constraints: […]
-15- Effects: If *this contains a value, assigns std::forward(v) to ~~the contained value~~; otherwise direct-non-list-initializes ~~the contained value~~ with std::forward(v).
-16- Postconditions: *this contains a value.
-17- Returns: *this.
-18- Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the ~~state~~ of *val and v is determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(const optional& rhs);
-19- Constraints: […]
-20- Effects: See Table 60.

Table 60 — optional::operator=(const optional&) effects [tab:optional.assign.copy.templ]
*this contains a value *this does not contain a value

rhs contains a value assigns *rhs to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with *rhs

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-21- Postconditions: rhs.has_value() == this->has_value().
-22- Returns: *this.
-23- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(optional&& rhs);
-24- Constraints: […]
-25- Effects: See Table 61. The result of the expression rhs.has_value() remains unchanged.

Table 61 — optional::operator=(optional&&) effects [tab:optional.assign.move.templ]
*this contains a value *this does not contain a value

rhs contains a value assigns *std::move(rhs) to ~~the contained value~~ direct-non-list-initializes ~~the contained value~~ with
*std::move(rhs)

rhs does not contain a value destroys the contained value by calling
val->T::~T() no effect

-26- Postconditions: rhs.has_value() == this->has_value().
-27- Returns: *this.
-28- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
template<class... Args> constexpr T& emplace(Args&&... args);
-29- Mandates: […]
-30- Effects: Calls *this = nullopt. Then direct-non-list-initializes ~~the contained value~~ with std::forward<Args>(args)....
-31- Postconditions: *this contains a value.
-32- Returns: ~~A reference to the new contained value~~.
[…]
-34- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.
template<class U, class... Args> constexpr T& emplace(initializer_list il, Args&&... args);
-35- Constraints: […]
-36- Effects: Calls *this = nullopt. Then direct-non-list-initializes ~~the contained value~~ with il, std::forward<Args>(args)....
-37- Postconditions: *this contains a value.
-38- Returns: ~~A reference to the new contained value~~.
[…]
-40- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

Table 58 — `optional::operator=(const optional&)` effects [tab:optional.assign.copy]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*rhs` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*rhs`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 59 — `optional::operator=(optional&&)` effects [tab:optional.assign.move]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `std::move(*rhs)` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `std::move(*rhs)`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 60 — `optional::operator=(const optional<U>&)` effects [tab:optional.assign.copy.templ]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*rhs` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*rhs`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Table 61 — `optional::operator=(optional<U>&&)` effects [tab:optional.assign.move.templ]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	assigns `*std::move(rhs)` to ~~the contained value~~	direct-non-list-initializes ~~the contained value~~ with `*std::move(rhs)`
`rhs` does not contain a value	destroys the contained value by calling `val->T::~T()`	no effect

Modify 22.5.3.5 [optional.swap] as indicated:

constexpr void swap(optional& rhs) noexcept(see below);
-1- Mandates: […]
-2- Preconditions: […]
-3- Effects: See Table 62.

Table 62 — optional::swap(optional&) effects [tab:optional.swap]
*this contains a value *this does not contain a value

rhs contains a value calls swap(*(*this), *rhs) direct-non-list-initializes ~~the contained value of *this~~
with std::move(*rhs), followed by rhs.val->T::~T();
postcondition is that *this contains a value and rhs does
not contain a value

rhs does not contain a value direct-non-list-initializes ~~the contained value of~~ rhs
with std::move(*(*this)), followed by val->T::~T();
postcondition is that *this does not contain a value and rhs
contains a value no effect

-4- Throws: […]
-5- Remarks: […]
-6- If any exception is thrown, the results of the expressions this->has_value() and rhs.has_value() remain unchanged. If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T. If an exception is thrown during the call to T's move constructor, the ~~state~~ of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor.

Table 62 — `optional::swap(optional&)` effects [tab:optional.swap]
	*`this` contains a value**	*`this` does not contain a value**
`rhs` contains a value	calls `swap((this), *rhs)`	direct-non-list-initializes ~~the contained value of `this`~~ with `std::move(rhs)`, followed by `rhs.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value
`rhs` does not contain a value	direct-non-list-initializes ~~the contained value of~~ `rhs` with `std::move((this))`, followed by `val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value	no effect

Modify 22.5.3.7 [optional.observe] as indicated:

constexpr const T* operator->() const noexcept;
constexpr T* operator->() noexcept;
-1- Preconditions: *this contains a value.
-2- Returns: val.
-3- […]
constexpr const T& operator*() const & noexcept;
constexpr T& operator*() & noexcept;
-4- Preconditions: *this contains a value.
-5- Returns: *val.
-6- […]
constexpr T&& operator*() && noexcept;
constexpr const T&& operator*() const && noexcept;
-7- Preconditions: *this contains a value.
-8- Effects: Equivalent to: return std::move(*val);
constexpr explicit operator bool() const noexcept;
-9- Returns: true if and only if *this contains a value.
-10- Remarks: This function is a constexpr function.
constexpr bool has_value() const noexcept;
-11- Returns: true if and only if *this contains a value.
-12- Remarks: This function is a constexpr function.
constexpr const T& value() const &;
constexpr T& value() &;
-13- Effects: Equivalent to:
return has_value() ? *val : throw bad_optional_access();
constexpr T&& value() &&;
constexpr const T&& value() const &&;
-14- Effects: Equivalent to:
return has_value() ? std::move(*val) : throw bad_optional_access();
template<class U> constexpr T value_or(U&& v) const &;
-15- Mandates: […]
-16- Effects: Equivalent to:
return has_value() ? **this : static_cast<T>(std::forward(v));
template<class U> constexpr T value_or(U&& v) &&;
-17- Mandates: […]
-18- Effects: Equivalent to:
return has_value() ? std::move(**this) : static_cast<T>(std::forward(v));

Modify 22.5.3.8 [optional.monadic] as indicated:
```
template<class F> constexpr auto and_then(F&& f) &;
template<class F> constexpr auto and_then(F&& f) const &;
```
-1- Let U be invoke_result_t<F, decltype(*val)>.
-2- Mandates: […]
-3- Effects: Equivalent to:
```
if (*this) {
 return invoke(std::forward<F>(f), *val);
} else {
 return remove_cvref_t();
}
```
```
template<class F> constexpr auto and_then(F&& f) &&;
template<class F> constexpr auto and_then(F&& f) const &&;
```
-4- Let U be invoke_result_t<F, decltype(std::move(*val))>.
-5- Mandates: […]
-6- Effects: Equivalent to:
```
if (*this) {
 return invoke(std::forward<F>(f), std::move(*val));
} else {
 return remove_cvref_t();
}
```
```
template<class F> constexpr auto transform(F&& f) &;
template<class F> constexpr auto transform(F&& f) const &;
```
-7- Let U be remove_cv_t<invoke_result_t<F, decltype(*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>(f), *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>(f), *val); otherwise, optional().
```
template<class F> constexpr auto transform(F&& f) &&;
template<class F> constexpr auto transform(F&& f) const &&;
```
-10- Let U be remove_cv_t<invoke_result_t<F, decltype(std::move(*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>(f), std::move(*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>(f), std::move(*val)); otherwise, optional().
Modify 22.5.3.9 [optional.mod] as indicated:
```
constexpr void reset() noexcept;
```
-1- Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.
-2- Postconditions: *this does not contain a value.

Modify 22.5.4.2 [optional.ref.ctor] as indicated:

template<class U>
  constexpr explicit(!is_convertible_v<U&, T&>)
  optional(optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, U&>);

-8- Constraints: […]
-9- Effects: Equivalent to:
  if (rhs.has_value()) convert-ref-init-val(*rhs);
-10- Remarks: […]

template<class U>
  constexpr explicit(!is_convertible_v<const U&, T&>)
  optional(const optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, const U&>);

-11- Constraints: […]
-12- Effects: Equivalent to:
  if (rhs.has_value()) convert-ref-init-val(*rhs);
-13- Remarks: […]

template<class U>
  constexpr explicit(!is_convertible_v<U, T&>)
  optional(optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, U>);

-14- Constraints: […]
-15- Effects: Equivalent to:
  if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));
-16- Remarks: […]

template<class U>
  constexpr explicit(!is_convertible_v<const U, T&>)
  optional(const optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, const U>);

-17- Constraints: […]
-18- Effects: Equivalent to:
  if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));
-19- Remarks: […]

4015. LWG 3973 broke const overloads of std::optional monadic operations

4015. LWG 3973 broke `const` overloads of `std::optional` monadic operations