Proposed resolution:

This wording is relative to N5014.

1. Modify [meta.reflection.substitute] as indicated:

   -1- Let TARG-SPLICE(x) be:

   • -1.1- template [: x :] if `is_template(x)` is `true`, otherwise
   • -1.2- typename [: x :] if `is_type(x)` is `true`, otherwise
   • -1.3- ([: x :])

   template<reflection_range R = initializer_list<info>>
     consteval bool can_substitute(info templ, R&& arguments);
   

   -1- Let `Z` be the template represented by `templ` and let `Args...` be a sequence of prvalue constant expressions that compute the reflections held by the elements of `arguments`, in order.

   -2- Returns: `true` if Z<TARG-SPLICE([:Args:])...> is a valid template-id ([temp.names]) that does not name a function whose type contains an undeduced placeholder type. Otherwise, `false`.

   -3- Throws: `meta::exception` unless `templ` represents a template, and every reflection in `arguments` represents a construct usable as a template argument ([temp.arg]).

   -4- [Note: If forming Z<TARG-SPLICE([:Args:])...> leads to a failure outside of the immediate context, the program is ill-formed. — end note]

   template<reflection_range R = initializer_list<info>>
     consteval info substitute(info templ, R&& arguments);
   

   -5- Let `Z` be the template represented by `templ` and let `Args...` be a sequence of prvalue constant expressions that compute the reflections held by the elements of `arguments`, in order.

   -6- Returns: Z<TARG-SPLICE([:Args:])...>.

   -7- Throws: `meta::exception` unless `can_substitute(templ, arguments)` is `true`.

   -8- [Note: If forming Z<TARG-SPLICE([:Args:])...> leads to a failure outside of the immediate context, the program is ill-formed. — end note]

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

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

[ 2025-11-03; Tomasz provides wording. ]

[ 2025-10-27; Reflector comments. ]

We lost definition of `Z`. Use TARG-SPLICE([:Args:])....

[ 2025-10-27; Tomasz provides wording. ]

[ 2025-10-22; Reflector poll. ]

Set priority to 1 after reflector poll.

`can_substitute` and `substitute` are currently specified in terms of splices in a template argument list:

[meta.reflection.substitute] p3:

Returns: `true` if Z<[:Args:]...> is a valid template-id ([temp.names]) that does not name a function whose type contains an undeduced placeholder type. Otherwise, `false`.

[meta.reflection.substitute] p7:

Returns: ^^Z<[:Args:]...>.

This wording was introduced in P2996R11. However, merging in changes from P3687 "Final Adjustments to C++26 Reflection" in P2996R13 changed the rules for splices in this context. This makes `can_substitute` and `substitute` as specified currently ill-formed. We cannot use the given syntax to splice an arbitrary choice of values, types and templates anymore.

While the intent seems clear, this should be rephrased to be more technically correct.