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2909. User specializations of is_execution_policy should be ill-formed

Section: 19.18.3 [execpol.type] Status: NAD Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-16

Priority: Not Prioritized

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Addresses US 14

A user specialization of any is_execution_policy should produce an ill-formed program, not merely one whose behavior is unspecified. See also the related comment re [meta.type.synop] (20.15.2). (US 10)

Proposed change: Reword the paragraph as follows:

Unless otherwise specified, a program that adds specializations for is_execution_policy is ill-formed; no diagnostic required.

See also LWG 2871.

[2017-03-03, Kona Friday AM]

Jonathan: This breaks all known implementations of parallel algorithms, since they use custom execution policies.

Alisdair to check with SG1 to see if that's what they meant.

[2017-07 Toronto Thurs Issue Prioritization]

NAD; what the standard says is that this is "undefined", not "unspecified".

Proposed resolution: