Fix discriminated unions with primtive types #10296
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| return type.flags & TypeFlags.Union ? | ||
| getUnionType(filter((<UnionType>type).types, f)) : | ||
| f(type) ? type : neverType; | ||
| if (!(type.flags & TypeFlags.Union)) { |
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The rewrite here makes sense (I'm assuming this is a perf thing?) but deserves a comment as to why it isn't using the existing filter code so that a maintainer doesn't just undo this change later.
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Yes, it is an optimization to avoid doing any work if the filter function returns true for all constituents. But now that you mention it, it would make more sense to push the optimization into the filter function itself. Similar to how concatenate doesn't allocate a new array unless it has to, so could filter just return the argument array itself when the predicate is true for all elements.
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Fixes #10257.