JavaScript boy and “lovely thoughtleader” 
he/him 
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Flow at least invalidates refinements when calling local functions that mutate variables, but does not follow any indirection https://flow.org/try/#0PQKgBAAgZgNg9gdzAZwC4CcCWBjVYTABQhMApngG4CGMAXGAHYCuAtgEanpgA+KGmDAOZgAvGADk4gNzEoTBrkxwGYKsmSZBDAJIMAJpnSlFygBQBKMAG9CYVes0MAcqw7oLMgL6z5JlWo0tF3ZOC2tbMGoYUTAAJi8fBVQlFRYqATCbO0woMFNUAE8AB1I4XKjRETFxZhD0cUssuzBUAAt0REZSJABRdA73cSc4RldOZFUYeARSPQaZO28I4GAwbVy2zAmENTAAAwDHYLc9ltbSFQAxabAEOCYYPTBsOH7jVBgCsAEozD0qVCkM5AoxQASkFgXVARQ5aXQGIx+DzEOwrMAAFVaWxQrXujzAUHSMAANN9UOIJlRRnUIqYovQ0FghOYvEA …
TS won't ever invalidate local refinements in this case, even when it can easily statically tell that a function call will https://www.typescriptlang.org/play/#code/DYUwLgBAbghsBcEB2BXAtgIxAJwgHwgGcxsBLJAcwgF4IByOgbgChmAzFJAYzFIHskEGIUKkKSAHLos2ABQBKCAG9mEaHBoQATCwC+rDt14CIaGOQXLVEUmwiywATwAOIPndjAa1WnVSYcOkUVNTUwAAtsPgB3ZBBYgFFsKLk6CT5kaRxCIWBgGJAAEyCWNX1rAHoKiAARATpIJBAioSRHGyRC0mwQHn5BcJwQa2FRcSkAuXkWSuqAFXDSHMJwvhRgQog2c2AAGhswOhyYTMnrLgFiIhJEYjJKTU89IA …
Where are you seeing flow break down with invalidation? Pretty sure it handles that TS example fine.
It does handle the TS case. My Flow tweet above has an example that does not fail when indirection is involved though.
Did you use any hack or anything to work around this?
Nope. I just live with it. There's a bunch of things to be aware of, which is what motivates my tweets so others can be aware too.
To be completely honest, these are not _local_ variables, these are _closed-over_ variables.
Yeah, this is a known limitation unfortunately. There is a discussion here https://github.com/microsoft/TypeScript/issues/9998 …. Some inlining might help catch some more errors but there are limits to how deep it would be practical to do that.
Ouch, I didn't know this. Thanks for including a clear example!
lol cool. Closure is, I think it's fair to say, a bit *overly* cautious in similar situations, and of course people always used to get mad about that too.
Looks like an outright bug. I can't find an open issue for it, though, have you reported it? This one is similar, but probably not the same:https://github.com/microsoft/TypeScript/issues/15835 …
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