microsoft / microsoft/TypeScript
Tuple + Record Intersection Assignment Compatibility Inconsistency
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Description
TypeScript Version: 4.1.0-dev.20200824
Search Terms: tuple, object, intersection, type, assignment, coercion
Code
Here is the type that I'd like to use in an assignment:
type X = {example: "Field"} & number[];
The following produces TS error 2322.
const x: X = {
...[1, 2],
example: "Field",
} as const;
If we update this example like so, the type error disappears:
- const x: X = {
+ const x = {
...[1, 2],
example: "Field",
} as const;
+ const y: X = x;
Expected behavior:
I would have expected the first assignment to not produce a type error. I'm wondering if this inconsistency (if that's the correct word for it) is an edge case, or by design? Apologies if this issue is unhelpful! Thank you!
Actual behavior:
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 and reproduce the two assignment forms shown in the issue. Trace the type-checking path for intersection types, array or tuple spreads, and object literal assignments; done means the behavior is explained and, if confirmed as a bug, a regression test captures the expected result.
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
- 25/100