generic parameter is not narrowed down correctly in extends clause when using keyof operator about typescript HOT 4 CLOSED

if R extends {[key: string]: ...} then keyof R is string, so why P in keyof R is string | number | symbol and not string?

{ a: string, b: string } extends { a: string }, but its keyof is "a" | "b". The same rationale applies here.

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https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html#support-number-and-symbol-named-properties-with-keyof-and-mapped-types

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Yes, this behavior is by design. number keys are already subsumed by string index signatures, and symbol keys are not prohibited by string index signatures because types in TS are not sealed. There's no bug in this code.

The statement "if R extends {[key: string]: ...} then keyof R is string" is demonstrably false:

type Ex<R extends { [key: string]: number }> = void;

type A = { a: 1, 2: 3, [Symbol.toPrimitive]: "abc" };
type B = Ex<A>; // <-- this is fine
type C = {} & keyof A; 
//   ^? type C = typeof Symbol.toPrimitive | "a" | 2

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thank you all. I finally got it.

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