Adding generic parameter to a lambda function removes typing from the function argument

TypeScript Version: 3.2.2

Search Terms: generic removes parameter type

Code

// OKAY
// declare const labmda function with typings, then implement with minimal type annotations.
// "input" has type of "string".
export const foo: (input: string) => string
    = (input) => (input);

// OKAY
// declare const lambda generic function without typings, then implement with full inline type annotations.
// "input" has type of "string".
export const bar
    = <TResult>(input: string) => ({ } as Partial<TResult>);

// ERROR
// declare const labmda generic function with typings, then implement with minimal type annotations.
// "input" has type of "any".
export const baz: <TResult>(input: string) => Partial<TResult>
    = <TResult>(input) => ({ [input]: input } as Partial<TResult>);

Expected behavior:
input is typed as string.

Actual behavior:
input is typed as any.

Playground Link: https://www.typescriptlang.org/play/index.html#src=%2F%2F%20OKAY%0D%0A%2F%2F%20declare%20const%20labmda%20function%20with%20typings%2C%20then%20implement%20with%20minimal%20type%20annotations.%0D%0A%2F%2F%20%22input%22%20has%20type%20of%20%22string%22.%0D%0Aexport%20const%20foo%3A%20(input%3A%20string)%20%3D%3E%20string%0D%0A%20%20%20%20%3D%20(input)%20%3D%3E%20(input)%3B%0D%0A%0D%0A%2F%2F%20OKAY%0D%0A%2F%2F%20declare%20const%20lambda%20generic%20function%20without%20typings%2C%20then%20implement%20with%20full%20inline%20type%20annotations.%0D%0A%2F%2F%20%22input%22%20has%20type%20of%20%22string%22.%0D%0Aexport%20const%20bar%0D%0A%20%20%20%20%3D%20%3CTResult%3E(input%3A%20string)%20%3D%3E%20(%7B%20%7D%20as%20Partial%3CTResult%3E)%3B%0D%0A%0D%0A%2F%2F%20ERROR%0D%0A%2F%2F%20declare%20const%20labmda%20generic%20function%20with%20typings%2C%20then%20implement%20with%20minimal%20type%20annotations.%0D%0A%2F%2F%20%22input%22%20has%20type%20of%20%22any%22.%0D%0Aexport%20const%20baz%3A%20%3CTResult%3E(input%3A%20string)%20%3D%3E%20Partial%3CTResult%3E%0D%0A%20%20%20%20%3D%20%3CTResult%3E(input)%20%3D%3E%20(%7B%20%5Binput%5D%3A%20input%20%7D%20as%20Partial%3CTResult%3E)%3B

(NOTE: options -> noImplicitAny to have the issue highlighted as an error.)

Related Issues:

https://github.com/Microsoft/TypeScript/issues/18606
I think this issue might be related, but I don't understand it enough to judge.
If this is the same issue then the answer is "this is correct, and a limitation of the type system", which is fair enough.

Inizia riproducendo l’esempio nel TypeScript Playground collegato con noImplicitAny abilitato, quindi confronta le tre dichiarazioni lambda e la issue correlata #18606. Traccia il modo in cui il tipaggio contestuale viene applicato all’assegnazione della lambda generica. Il lavoro è completato quando si determina se l’argomento debba essere inferito come string e si aggiunge un test di regressione se il comportamento viene confermato come bug.