Missing error when mistyped value overwrites property of spreaded generic type parameter

Bug Report

🔎 Search Terms

generic argument, rest spread, false positive

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about object spread and generics

I found a few relevant issues, but none of them fully correspond to the current one:

• https://github.com/microsoft/TypeScript/issues/35858
• https://github.com/microsoft/TypeScript/issues/38469
• https://github.com/microsoft/TypeScript/issues/30129

⏯ Playground Link

Playground link with relevant code

💻 Code

interface InputProps {
  value: string;
}

const getProps = (props: InputProps): InputProps => ({
  ...props,
  value: 123, // when there is no generic type, it shows an error, as expected
});

const getPropsGenericNoSpead = <T extends InputProps>(props: T): T => {
  const newProps = {...props};
  newProps.value = 123; // also error, as expected
  return newProps;
};


const getPropsGeneric = <T extends InputProps>(props: T): T => ({
  ...props,
  value: 123, // no error here
});

🙁 Actual behavior

Typescript allows assigning incompatible types

🙂 Expected behavior

Typescript shows an error

[teynar]

I am also experiencing this issue and I feel quite surprised since this looks like a fairly common case when using reduce with the spread operator.

function foo<T extends string>(keys: T[]): Record<T, number> {
  return keys.reduce((acc, key) => {
    return {
      ...acc,
      [key]: 'string' // this is not a number, but there is no error
    }
  }, {} as Record<T, number>);
}

TypeScript Playground

[misuken-now]

In situations where generics are not used, the use of “Object.assign” may be a solution.

function test(param: { [key: string]: (string | number) }) {
  // Type '{ foo: string; } & { [key: string]: string | number; }' is not assignable to type '{ [key: string]: string; }'.
  // 'string' index signatures are incompatible.
  //   Type 'string | number' is not assignable to type 'string'.
  //     Type 'number' is not assignable to type 'string'.(2322)
  const stringValues: { [key: string]: string } = Object.assign({}, {
    foo: 'bar'
  }, param);
  console.log(stringValues);
}

playground