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4535. Disallow user specialization of <simd> templates

Section: 29.10.1 [simd.general] Status: New Submitter: Tim Song Opened: 2026-03-05 Last modified: 2026-03-06

Priority: Not Prioritized

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Discussion:

Addresses US 176-280.

It does not appear that any of the class templates in <simd> can be profitably specialized by the user – or that the non-member functions can be used with such user specializations.

Proposed resolution:

This wording is relative to N5032.

  1. Add the following paragraph to the end of 29.10.1 [simd.general]:

    -?- If a program declares an explicit or partial specialization of any of the templates specified in subclause 29.10 [simd], the program is ill-formed, no diagnostic required.

  2. Strike 29.10.4 [simd.traits] p3 as redundant:

    -3- The behavior of a program that adds specializations for alignment is undefined.