microsoft / microsoft/TypeScript
Union properties like `0 | number` or `"" | string` should be discriminants in truthiness checks
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Bug
Domain: check: Control Flow
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Description
TypeScript Version: 3.7.2
Code
type A = { readonly [0]: 0 }
type B = { [0]: number }
declare var x: A | B;
if (x[0]) x[0] = 2;
Expected behavior:
No error will be reported
Actual behavior:
error TS2540: Cannot assign to '[0]' because it is a read-only property.
4 if (x[0]) x[0] = 2;
~
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 TypeScript 3.7.2 reproduction in the issue and trace truthiness narrowing for the union property types 0 | number and "" | string. Done means the shown assignment produces no error and a regression test covers the behavior.
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
- 42/100