(My apologies if this has been discussed before, as I didn't have the time to read the full thread)
I worry that this proposal adds additional complexity to an already overcomplicated feature. Please keep in mind that trailing closures are optional, in that a user is never required to use them, yet library designers often design their APIs with trailing closures in mind. However, for many newcomers, trailing closures are a confusing and alien-looking feature. I've seen many students avoid them for months, because they find it easier to learn the language using regular closure arguments, which are more consistent with the rest of the language.
Is it not possible to resolve this "first parameter" discussion by removing complexity from the language, instead of adding it? If the Standard Library is open for changes, can we not simply remove the exception that a trailing closure argument drops its argument label, and return to a state of symmetry where it's up to a library author to decide whether an argument requires a label or not, and that this label is required, both for a regular closure argument, and for a trailing closure?