Inferred type parameter does not satisfy constraint

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
32/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
typescript
Domain
compilers

Research direction

Reproduce the linked TypeScript Playground example and compare the reported behavior with TypeScript 3.3 and 3.5+. Start by tracing generic type inference and constraint checking for the shown function calls. Done means the compiler handles the inferred U extends T constraint consistently and has regression coverage for the selected expected behavior.

Written by the indexing model from the issue text.

Description

Bug Domain: check: Type Inference

Bug Report

🔎 Search Terms

generic constraint, inference, invalid, specified, manually, explicitly, invalid

🕗 Version & Regression Information
  • This changed between versions 3.3 and 3.5
⏯ Playground Link

Playground link with relevant code

💻 Code
declare function f<T, U extends T>(cb: (u: U) => T): U

const v = f(u => "x");
// const v: unknown <-- wait, what?
// function f<string, unknown>(cb: (u: unknown) => string): unknown

f<string, unknown>(u => "x"); // error, of course
// -----> ~~~~~~~
// Type 'unknown' does not satisfy the constraint 'string'.
🙁 Actual behavior

In TS3.5+, the type parameter T is inferred to be string, and U is inferred to be unknown in the first call to f(). But then U extends T is not met; you couldn't even manually specify T and U like this when calling f(), as shown in the second call to f().

(In TS3.3, T and U are both inferred as {}.)

🙂 Expected behavior

Either I'd expect both T and U to be string, or I'd expect an error where the compiler says it cannot find a suitable candidate for U. Presumably the compiler has not inferred T before it needs to infer U, and so it constrains U to T's constraint which is implicitly unknown, and then falls back to that. But it never circles back to make sure that such a fallback actually works for U extends T.

I came upon this while looking for an answer to this SO question. This behavior is weird, right? I haven't found an existing issue for it.

Dominant language
Go
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Merged PRs (30d)
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