JavaScript boy and “lovely thoughtleader” 
he/him 
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The real problem is that the "safe" assumption (if you can't see a function body, assume it changes everything) is also the one most likely to be wrong in practice -- function bodies you can't see *almost never* change locally-visible data
Dunno, I've found that TypeScript is way too loose. I never personally found a problem with refinment invalidation when writing code with Flow but I'm a masochistic and am fine writing weird code.
This is another case I ran into which I'm pretty sure is in the same class as refinement invalidation: https://www.typescriptlang.org/play/#code/MYGwhgzhAEBiD29oG8BQ1rHgOwgFwCcBXYPeAgCgEoV0No8ALASwgDoxTmA3AU2gC80AGZgQEXgG46AX1R1OeHrwBc0AEaIQvMNml0AJjq7cweXtVr0GLdouWCRYidIxy6zbMzyW01pqwcJvxChERS8v62bEb2pubU+tYA9MnQsSZmFjQSeNAABgF2wfkMSPmi4ryl6kR53gDkMPjMICDQALZgBADWvAbQkAVh1enhZTb8kBDMAObYHbzYeWCafHQYzMLQFEVBSnyCAkKVEjR+1pg4EPDabCDwsxQNRB0dDVSu9HJuqDJAA …
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