microsoft / microsoft/TypeScript

Infer from usages should not widen literal types

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Domain: LS: Refactorings Experience Enhancement Help Wanted Suggestion
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Description

Bug Report

🔎 Search Terms

string literal to string, infer type from usages, code fix, quickfix

⏯ Playground Link

Playground link with relevant code

💻 Code
let a

a = "foo" as "foo" | "bar"

let b

b = getB()

checkB(b)

function checkB(toCheck: "foo" | "bar") {
}

function getB(): "foo" | "bar" {
    return "foo"
}
🙁 Actual behavior

After running infer from usages quickfix, you get string type in a and b positions

🙂 Expected behavior

Should get inffered type "foo"

I think in case of as and satisfies clauses the type should be just copied literally?

As you can see from example this is common case for string enums, like when some function expects some string enum instead of just string, but this information gets lost

Contributor guide

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the behavior in the linked TypeScript Playground using the supplied code, then trace the “infer from usages” quickfix and its handling of assertions or satisfies clauses. Done means the inferred types for a and b preserve the literal "foo" rather than widening to string, with coverage for the shown case.

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

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