Flow: Switch cases don't find unreachable cases with disjoint unions

Created on 6 Sep 2016 · 5Comments · Source: facebook/flow

When using a disjoint union and switching on the literal type, flow doesn't understand that some cases cannot happen. It could be nice to know that this is actually dead code. See this example . Or perhaps there's something I don't understand with disjoint unions?

soundness feature request reachability

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AugustinLF

👍4

Most helpful comment

Yup, there's some reachability work in progress that would properly detect this. No definite ETA on landing it, unfortunately.

bhosmer on 16 Sep 2016

🎉3 😄2 👍2

All 5 comments

@bhosmer has been working on generalized reachability, so I'm curious what he has to say here.

jeffmo on 9 Sep 2016

Yup, there's some reachability work in progress that would properly detect this. No definite ETA on landing it, unfortunately.

bhosmer on 16 Sep 2016

🎉3 😄2 👍2

I'm looking forward to this because I've seen some bugs due to typos that this would have caught. How's progress, @bhosmer?

dmnd on 30 Jan 2017

@bhosmer Any progress on this? Flow becomes truly valuable when it can report me that parts of my code are unreachable because of improper if statements and switch statements etc. It would save me lots of work debugging, especially when refactoring.

hmeerlo on 21 Jul 2017

Seems like this was partially fixed in v0.58 when not all possible branches are covered.

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgCqAdgJZzFgC8YA3mNvgFxgDkAhq2AL5gA+dBrjwtWAIy7d0AYwoBnDGGkALPNIDWAFWHUwACjww8AWzzEMLEuWIBKagD46qMHISkMK-YZNmMAOkY8O1pnMCV2OQIOViZQsLAAJzwMAFcEylYoUgSFVgBuOOkIqOkYuLCk1PS2DGVsgBN85ykpVBU1LWE9ekDRTh4bIA

However exhaustive switch do not error about additional cases