Proposed resolution:

This wording is relative to N4928.

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

   template<class F> constexpr auto transform_error(F&& f) &;
   template<class F> constexpr auto transform_error(F&& f) const &;
   

   […]

   -27- Mandates: G is a valid value type for expectedtemplate argument for unexpected ([expected.un.general]) and the declaration

   G g(invoke(std::forward<F>(f), error()));
   

   is well-formed.

   […]

   […]

   template<class F> constexpr auto transform_error(F&& f) &&;
   template<class F> constexpr auto transform_error(F&& f) const &&;
   

   […]

   -31- Mandates: G is a valid value type for expectedtemplate argument for unexpected ([expected.un.general]) and the declaration

   G g(invoke(std::forward<F>(f), std::move(error())));
   

   is well-formed.

   […]

2. Modify [expected.void.monadic] as indicated:

   template<class F> constexpr auto transform_error(F&& f) &;
   template<class F> constexpr auto transform_error(F&& f) const &;
   

   […]

   -24- Mandates: G is a valid value type for expectedtemplate argument for unexpected ([expected.un.general]) and the declaration

   G g(invoke(std::forward<F>(f), error()));
   

   is well-formed.

   […]

   […]

   template<class F> constexpr auto transform_error(F&& f) &&;
   template<class F> constexpr auto transform_error(F&& f) const &&;
   

   […]

   -27- Mandates: G is a valid value type for expectedtemplate argument for unexpected ([expected.un.general]) and the declaration

   G g(invoke(std::forward<F>(f), std::move(error())));
   

   is well-formed.

   […]

[ 2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP. ]

[ 2023-02-06; Reflector poll ]

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

The overloads of expected::transform_error mandate that "[type] G is a valid value type for expected" ([expected.object.monadic]/27 and 31 as well as [expected.void.monadic]/24 and 27)).

All of these overloads then instantiate expected<T, G> (for some type T) which doesn't require G to be a valid value type for expected ([expected.object.general]/2) but instead requires that G is "a valid template argument for unexpected" ([expected.object.general]/2). Comparing [expected.object.general]/2 with [expected.un.general]/2 it's clear that there are types — const int, for example — which are valid value types for expected but not valid template arguments for unexpected. Presumably this unimplementable requirement is a typo, and the subject paragraphs intended to require that G be a valid template argument for unexpected.