I might be asking about something obvious here but isn't this what is explained in this and this section of SE-0143, ie:
As noted in the section on multiple conformances, Swift already bans programs that attempt to make the same type conform to the same protocol twice. This proposal extends the ban to cases where the conformances are conditional.
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For these reasons, this proposal bans overlapping conformances entirely.
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Conditional conformances may exacerbate existing problems with overloading behaving differently with concrete types vs. in a generic context. For example, consider:
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This is not a new problem to Swift. We can write a similar example using a constrained extension and non-conditional conformances:
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That said, the introduction of conditional conformances might increase the likelihood of these problems surprising developers.
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If so, I keep making mistakes having to do with this, even though I understand it every time I read it. It seems like I can't truly internalize it.