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Showing content from https://cplusplus.github.io/LWG/issue4316 below:

Issue 4316: {can_}substitute specification is ill-formed

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Section: 21.4.13 [meta.reflection.substitute] Status: New Submitter: Matthias Wippich Opened: 2025-08-15 Last modified: 2025-08-17

Priority: Not Prioritized

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Discussion:

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

Returns: true if Z<[:Args:]...> is a valid template-id (13.3 [temp.names]) that does not name a function whose type contains an undeduced placeholder type. Otherwise, false.

21.4.13 [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.

Proposed resolution:

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