Suggested resolution:

Change in 6.6 [basic.link] paragraph 15.2 as follows:

An entity is TU-local if it is

• a type, function, variable, or template that
  • has a name with internal linkage, or
  • does not have a name with linkage and is declared, or introduced by a lambda-expression, within the definition of a TU-local entity,
• a type with no name that is defined outside a class-specifier, function body, concept-definition, or initializer or is introduced by a defining-type-specifier that is used to declare only TU-local entities,
• ...

Consider:

   template<typename _Tp>
    concept __is_derived_from_optional = requires (const _Tp& __t) {
     []<typename _Up>(const optional<_Up>&){ }(__t);
    };

According to 6.6 [basic.link] paragraph 15, the closure type of the lambda-expression is a TU-local entity, thus the concept is an exposure, thus it cannot appear in a module interface unit. There is no fundamental reason why this needs to be the case.