Fix nested excess property checks when target is a union with non-discriminated + non-object constituents #30771
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After checking an object type for excess properties, we un-fresh it if the target is a union with nullable members. The intent of that is that code like this
doesn't error because
a
is excess inPartial<{ b: string>}
(even though it's assignable to it).Similarly, we don't want to consider
a
excess here:This un-freshening doesn't happen if there's a findable discriminating property, as you would expect.
However, we don't consider this example to have a discriminant
... even though the
number
type is sort of "self-discriminating" in the context of a union with other types.The fix is to a) rename the singly-called function to match its use and b) skip non-object targets when considering whether or not we need to un-fresh the source type.
The only non-new-test baselines are improvements in error spans