Implement parallelism and cardinality

[maikel]

This PR introduces 2 queries for sequence senders, which are useful for optimizations within algorithms.

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[ericniebler]

/ok to test

[maikel]

I think parallelism and cardinality should not be forwarded queries. But join_env works only with forwarding. Is that an unnecessary requirement of join_env ?

[ericniebler]

I think parallelism and cardinality should not be forwarded queries. But join_env works only with forwarding. Is that an unnecessary requirement of join_env ?

Possibly. The use I had in mind for join_env is when a sender adaptor is augmenting its child sender's environment, in which case the exclusion of non-forwarding queries is correct.

But if you are just cobbling together a sender's environment from bits and bobs, none of which are from a child sender, then the current behavior is not what you want. In that case, perhaps we want a variant of join_env that doesn't exclude non-forwarding queries.

[maikel]

Thats a very fair argument. I will provide a helper for what I really need

[ericniebler]

I think parallelism and cardinality should not be forwarded queries. But join_env works only with forwarding. Is that an unnecessary requirement of join_env?

@maikel stdexec now has the stdexec::env<> template for joining environments without any special handling for forwarding queries.