microsoft / microsoft/TypeScript

Unhelpful `--strictFunctionTypes` error when first method is `self(): this;`

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Domain: Error Messages In Discussion Suggestion
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Description

TypeScript Version: 2.7.0-dev.20171214

Code

declare class C<T> {
    self(): this;
    covariant(): T;
    contravariant: (t: T) => void;
    m(other: C<{}>): void;
}

new C<number>().m(new C<number>());

Expected behavior:

Error that C<number> isn't C<{}> because C<{}>'s contravariant can accept (t: {}) but C<number> can only accept (t: number).

Actual behavior:

src/a.ts(8,19): error TS2345: Argument of type 'C<number>' is not assignable to parameter of type 'C<{}>'.
  Types of property 'self' are incompatible.
    Type '() => C<number>' is not assignable to type '() => C<{}>'.
      Type 'C<number>' is not assignable to type 'C<{}>'.

Contributor guide

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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

Reproduce the issue with the TypeScript snippet in the report using --strictFunctionTypes and compare the diagnostic with the stated expected behavior. Trace the type comparison triggered by the first self(): this method, and consider the issue complete when the error identifies the contravariant property incompatibility instead.

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
38/100

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