Almost no type checking for mapped type with generic parameter #50558
Description
Bug Report
I am able to make a mapped type with a generic parameter that defines the keys of the new type. However, when the generic parameter is itself a generic, rather than a literal, type-checking fails almost completely.
May be a dupe of #49302
🔎 Search Terms
generic mapped type function
🕗 Version & Regression Information
Occurs in all of current release 4.7.4, current beta 4.8.0-beta, and current nightly.
⏯ Playground Link
Playground link with relevant code
💻 Code
type Model = {
a: string;
b: number;
}
type MappedModel<Suffix extends string> = {
[K in keyof Model as `${K}${Suffix}`]: Model[K];
}
// Works as expected:
const foo: MappedModel<'Foo'> = { aFoo: 'test', bFoo: 42 };
// @ts-expect-error
const fooBad: MappedModel<'Foo'> = { bFoo: 'bar' };
function getModel<T extends string>(name: T): MappedModel<T> {
// With a generic, there is almost no type-checking on assignment
const okay: MappedModel<T> = 42;
const okay2: MappedModel<T> = 'test';
const okay3: MappedModel<T> = [1,2,3];
const okay4: MappedModel<T> = false;
const okay5: MappedModel<T> = {foo: 'bar'};
// Only null and undefined error out
// @ts-expect-error
const bad: MappedModel<T> = undefined;
// @ts-expect-error
const bad2: MappedModel<T> = null;
return okay;
}
// Consumers of the function get the correct output type
const model = getModel('Foo');
const str: string = model.aFoo;
// @ts-expect-error
const str2: string = model.bFoo;🙁 Actual behavior
Within a function using the generic T, the type MappedModel<T> provides almost no type safety.
🙂 Expected behavior
Within a function using the generic T, the type MappedModel<T> should accurately protect against type errors in assignment or in the return value.