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:

Playground Link: https://www.typescriptlang.org/play?#code/HYQwtgpgzgDiDGEAEAhATgewNYWEg3gLABQSSALgJ4zIAaSAvARAB7gwA2EAXEgEQAxAJYQOAEz4BfJADIkwAK5gARhDQBtALoBuEiTLwMwKOSQte9JkVJkkAOgfqAjABokAJk0v9t1uy68giLifN420iBQSIbG5LrEknrEoJCwCMgA6hhoWFHWZFQ0SJbMbGCcPPzCohLScooqalrxPjEmZowEPmQOds5unmG2SH7lAVXBEkNIEVFtcUkGRu2UFp0s8ZJAA

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. 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

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