microsoft / microsoft/TypeScript
Running a type through a mapped type loses polymorphic `this` type
Open
Nobody has claimed this yet.
Bug
Domain: Mapped Types
- Dominant language
- Go
- Stars
- 111k
- Forks
- 14.3k
- Avg merge
- 2d 4h
- Merged PRs (30d)
- 132
Description
TypeScript Version: 4.0.0-dev.20200622
Search Terms: polymorphic this mapped type
Expected behavior: b.foo() returns B
Actual behavior: b.foo() returns A even though the tooltip on foo says it returns this
Related Issues: None
Code
interface A {
foo(): this
}
interface B extends Pick<A, keyof A> {
bar(): this
}
declare let b: B;
b.foo().bar() // error
Output
"use strict";
b.foo().bar(); // error
Compiler Options
{
"compilerOptions": {
"noImplicitAny": true,
"strictNullChecks": true,
"strictFunctionTypes": true,
"strictPropertyInitialization": true,
"strictBindCallApply": true,
"noImplicitThis": true,
"noImplicitReturns": true,
"useDefineForClassFields": false,
"alwaysStrict": true,
"allowUnreachableCode": false,
"allowUnusedLabels": false,
"downlevelIteration": false,
"noEmitHelpers": false,
"noLib": false,
"noStrictGenericChecks": false,
"noUnusedLocals": false,
"noUnusedParameters": false,
"esModuleInterop": true,
"preserveConstEnums": false,
"removeComments": false,
"skipLibCheck": false,
"checkJs": false,
"allowJs": false,
"declaration": true,
"experimentalDecorators": false,
"emitDecoratorMetadata": false,
"target": "ES2017",
"module": "ESNext"
}
}
Playground Link: Provided
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 TypeScript Playground repro and the interfaces using Pick<A, keyof A>. Trace how the mapped type preserves the polymorphic this type, then verify that b.foo().bar() type-checks and that foo is treated as returning B rather than A.
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
- 42/100