[Cra3z]

Forward non-completion-queries to affine and starts_on sender attributes

• Forward non-completion-queries from the child sender's sender-attributes to the sender-attributes of affine and starts_on.
• Fix an issue where affine's forwarding of non-completion-queries did not actually invoke the query and unnecessarily required the query result to be default-constructible.

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[ericniebler]

/ok to test 0ce499b

[Cra3z]

@ericniebler
The CI failures were caused by starts_on's constrained query member template for forwarding non-completion queries not SFINAE-friendly. Its trailing return type directly uses __query_result_t, causing a hard error during instantiation of the enclosing class template when the child environment does not support the query, rather than allowing the constraint to remove the member template from the candidate set. A minimal reproducer is available on https://godbolt.org/z/xY37b5eGq.

The same pattern occurs in several other adaptors. It can be addressed either by using return type deduction to defer type instantiation or by applying a workaround like the defaulted class _Env2 = env_of_t<_Sender> parameter in

stdexec/include/stdexec/__detail/__queries.hpp

Line 70 in 362cdc7

template <class _Env2 = env_of_t<_Sender>, __forwarding_query _Query, class... _Args>

This change uses return type deduction.

[ericniebler]

/ok to test ae6346c