Type guards should not affect values of type any

[ahejlsberg]

This PR changes type guards to not affect values of type any.

Fixes #1426.

[DanielRosenwasser]

This needs to get merged into release-1.4, or at least cherry-picked in.

[CyrusNajmabadi]

Add comment why 'any' is considered to be a 'structured' type. (for that matter, comment what we mean by a 'structured type' and thus why a TypeParameter is also considered one).

[ahejlsberg]

This change removes Any from StructuredType, leaving just ObjectType, Union, and TypeParameter.

[JsonFreeman]

Why doesn't getNarrowedTypeOfSymbol call isStructuredType? Instead it checks the flag directly

[ahejlsberg]

The TypeFlags.Structured flag is set for a union type regardless of the state of the flag for each of the constituent types. The isStructuredType performs the deep check. In the case of narrowing we only care about the shallow state (e.g. you can narrow a union of primitive types).

[CyrusNajmabadi]

This is somethign that would compltely break my expectations from reading the code. can you rename the isStructuredType flag to 'isStructuredTypeAtAnyDepth()' or something that can at least indicate that it's doing something extra.

[ahejlsberg]

I've gotten rid of the confusing TypeFlags.Structured flag and just inlined the actual flags in the few cases it was used. I think isStructuredType is a fine name for what it does and the comment on the function explains it further.

[JsonFreeman]

Thanks, that is helpful

[mhegazy]

👍

as @DanielRosenwasser noted, this needs to go into branch release-1.4

[ahejlsberg]

Cherry picked and applied changes to release-1.4 branch as well.

[sophiajt]

Does it make sense to remove narrowing 'any' in all cases? I can see why things like instanceof are a bit iffy, but for examples like this:

function f(x) {
   if (typeof x === "string") {
     return x.lengthh;
   }
}

It feels like we have enough information to know we should give the user an error. Is that not true here as well?

[DanielRosenwasser]

Looks like we should really revisit this to get a sense of what we could still do with any.