Hack does not refine variables of unknown type when using the `as` operator
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hack
- Dominant language
- C++
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Description
### HHVM Version
3.30.3 + 4.0 + 2019.03.20
### Operating System and Version
macOS Mojave
### Standalone code, or other way to reproduce the problem
Hack does not refine variables of unknown type when using the `as` operator. It works as expected at runtime, but the typechecker seems to just ignore it.
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