3718. P2418R2 broke the overload resolution for std::basic_format_arg

Section: 28.5.8.1 [format.arg] Status: Resolved Submitter: Jiang An Opened: 2022-06-17 Last modified: 2023-03-23

Priority: 2

View all other issues in [format.arg].

View all issues with Resolved status.

Discussion:

While correcting some bugs in MSVC STL, it is found that P2418R2 broke the overload resolution involving non-const lvalue: constructing basic_format_arg from a non-const basic_string lvalue incorrectly selects the T&& overload and uses handle, while the separated basic_string overload should be selected (i.e. the old behavior before P2418R2 did the right thing). Currently MSVC STL is using a workaround that treats basic_string to be as if passed by value during overload resolution.

I think the T&& overload should not interfere the old result of overload resolution, which means that when a type is const-formattable, the newly added non-const mechanism shouldn't be considered.

[2022-07-06; Reflector poll]

Set priority to 2 after reflector poll.

[2022-10-19; Would be resolved by 3631]

Previous resolution [SUPERSEDED]:

This wording is relative to N4910.

[Drafting note: The below presented wording adds back the const T& constructor and its specification that were present before P2418R2. Furthermore it adds an additional constraint to the T&& constructor and simplifies the Effects of this constructors to the handle construction case.]

  1. Modify 28.5.8.1 [format.arg] as indicated:

    namespace std {
  template<class Context>
  class basic_format_arg {
  public:
    class handle;

  private:
    […]
    template<class T> explicit basic_format_arg(T&& v) noexcept; // exposition only
    template<class T> explicit basic_format_arg(const T& v) noexcept; // exposition only
    […]
  };
}
    […] 
    template<class T> explicit basic_format_arg(T&& v) noexcept;

    -4- Constraints: The template specialization 

    typename Context::template formatter_type<remove_cvref_t<T>>

    meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression 

    typename Context::template formatter_type<remove_cvref_t<T>>()
  .format(declval<T&>(), declval<Context&>())

    shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]), and if this overload were not declared, the overload resolution would find no usable candidate or be ambiguous. [Note ?: This overload has no effect if the overload resolution among other overloads succeeds. — end note].

    -5- Effects:

    1. (5.1) — if T is bool or char_type, initializes value with v;

    2. (5.2) — otherwise, if T is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);

    3. (5.3) — otherwise, if T is a signed integer type (6.8.2 [basic.fundamental]) and sizeof(T) <= sizeof(int), initializes value with static_cast<int>(v);

    4. (5.4) — otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);

    5. (5.5) — otherwise, if T is a signed integer type and sizeof(T) <= sizeof(long long int), initializes value with static_cast<long long int>(v);

    6. (5.6) — otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);

    7. (5.7) — otherwise, iInitializes value with handle(v).

    template<class T> explicit basic_format_arg(const T& v) noexcept;

    -?- Constraints: The template specialization 

    typename Context::template formatter_type<T>

    meets the Formatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the Formatter requirements is unspecified, except that as a minimum the expression 

    typename Context::template formatter_type<T>()
  .format(declval<const T&>(), declval<Context&>())

    shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

    -?- Effects:

    1. (?.1) — if T is bool or char_type, initializes value with v;

    2. (?.2) — otherwise, if T is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);

    3. (?.3) — otherwise, if T is a signed integer type (6.8.2 [basic.fundamental]) and sizeof(T) <= sizeof(int), initializes value with static_cast<int>(v);

    4. (?.4) — otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);

    5. (?.5) — otherwise, if T is a signed integer type and sizeof(T) <= sizeof(long long int), initializes value with static_cast<long long int>(v);

    6. (?.6) — otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);

    7. (?.7) — otherwise, initializes value with handle(v).

[2023-03-22 Resolved by the adoption of 3631 in Issaquah. Status changed: New → Resolved.]

Proposed resolution: