microsoft / microsoft/TypeScript
Returned tuples don't match compatible types when using function union types
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Description
The first error is especially odd.
TypeScript Version: 3.7.x-dev.20200104
Search Terms:
Code
type C1 = (a: 1) => [0];
type C2 = (a: 2) => [number];
type C3 = (a: 3) => undefined;
declare function f(a: C1 | C3): void;
f(() => [0]);
declare function g(a: C2 | C3): void;
g(() => [0]);
Expected behavior:
pass
Actual behavior:
Type 'number' is not assignable to type '0'.(2322)
Property '0' is missing in type 'number[]' but required in type '[number]'.(2741)
Playground Link: http://www.typescriptlang.org/play/index.html?ts=3.8.0-dev.20200104&ssl=1&ssc=1&pln=8&pc=14#code/C4TwDgpgBAwgjFAvFAFAQwFxTgSiQPigG0AGAXQG4AoUSWAJiVUynr0UKIDsBXAWwBGEAE6Ua4aDADMTdFintCPLgBMIAMwCWXCCupU1AYwA2aYdHXLDwTQHsuUdXNgIAPrAVYAbrc16qTiiKxOQ41Eam5o5WNvZQAObOMIzu0jjevv6JQQQhZGFAA
Related Issues:
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 by reproducing the reported TypeScript 3.7.x behavior with the code snippet or its linked Playground example. The payload names no repository files or tests, so locate the type-checking path for unioned function arguments and add a regression test that accepts both calls without the reported errors.
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