microsoft / microsoft/TypeScript
Type guards can't deal with unknown type as default
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Description
We can't know the failures of type inference in type guards because of this problem. Maybe a is any[] should also return unknown[].
TypeScript Version: 3.7.x-dev.20200410
Search Terms:
Code
declare function f(a: any): a is any[];
declare function g(a: any): a is unknown[];
declare const a: readonly number[];
f(a) && a; // any[]
g(a) && a; // not unknown[]
Expected behavior:
a's type narrowed by g is unknown[].
Actual behavior:
a's type narrowed by g is readonly number[] & unknown[].
Related Issues: #17002
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
Reproduce the narrowing behavior in the linked TypeScript Playground using the declarations for f, g, and readonly number[] a, then inspect the type-guard narrowing logic. Done means that g(a) narrows a to unknown[] as expected, with the behavior covered by a regression test; related issue #17002 may provide context.
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