core(basic sender): fix trailing return type

[romintomasetti]

This PR changes a decltype(auto) to a proper trailing return type, avoiding instantiation of stuff that ends up trying to instantiate stuff for incomplete types.

I've added a new test that does not compile if the trailing return type is turned back to decltype(auto).

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[romintomasetti]

@maartenarnst

[romintomasetti]

@ericniebler Not sure about the quality of the trailing return type but we need it in our code base to avoid incomplete type issues. Would you have a minute to take a look ? Thks!