Issue 4260: Query objects must be default constructible

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4260. Query objects must be default constructible

Section: 33.2.1 [exec.queryable.general] Status: New Submitter: Eric Niebler Opened: 2025-05-07 Last modified: 2025-05-18

Priority: Not Prioritized

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Discussion:

Imported from cplusplus/sender-receiver #333.

We require the types of query objects such as get_scheduler to be customization point objects. 16.3.3.3.5 [customization.point.object] requires them to be semiregular but that concept does not require default constructability. Much of std::execution assumes query object types to be default constructible.

I propose adding a (nothrow) default-constructibility requirement.

Proposed resolution:

This wording is relative to N5008.

Modify 33.2.1 [exec.queryable.general] as indicated:

-1- A queryable object is a read-only collection of key/value pair where each key is a customization point object known as a query object. The type of a query object satisfies default_initializable, and its default constructor is not potentially throwing. A query is an invocation of a query object with a queryable object as its first argument and a (possibly empty) set of additional arguments. A query imposes syntactic and semantic requirements on its invocations.