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Cannot subclass such a class. (Well, we could subclass a sealed
class in the same compilation unit. We ignore this for simplicity.)
This is a bit of a sneaky fix for this bug, but our hand
is pretty much forced by other constraints, in this intersection
of overload resolution involving built-in function types and SAMs,
and type inference for higher-order function literals (#5307).
Luckily, in this particular issue, the overloading clash seems
accidental. The `sealed` `<:<` class is not a SAM type as it
cannot be subclassed outside of `Predef`. For simplicity,
we don't consider where the SAM conversion occurs and exclude
all sealed classes from yielding SAM types.
Thanks to Miles for pointing out that `final` was missing in my
first iteration of this fix.
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