microsoft / microsoft/TypeScript
Narrowing doesn't recognize string constant as truthy
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Description
TypeScript Version: 3.7-beta
Search Terms: cfa narrowing string truthy
Code
declare var a: { b: number } | null
!a || a.b > 3; // works
!a && true || a.b > 3 && true; // works
!a && "a was not defined" || a.b > 3 && "a.b was too big"; // error "Object is possibly null"
Expected behavior:
The narrowings all work
Actual behavior:
The third narrowing does not work and produces an error
Playground Link:
Playground
Related Issues:
I suspect this is a duplicate, as most CFA narrowing bugs are. But I searched as best I could and didn't find any other bugs that matched what was going on here. It seems like typescript doesn't understand that a non-empty string-literal is true in this situation?
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked TypeScript Playground reproduction using TypeScript 3.7-beta and compare the three narrowing expressions. Investigate control-flow analysis for non-empty string literals in the type checker; done means the third expression narrows a to exclude null without an error and the behavior is covered by a regression test.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100