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

Inizia dall’esempio Playground collegato e confronta il caso di spread generico con i casi non generico e non spread mostrati nel report. Traccia il comportamento del controllo dei tipi per l’assegnazione value: 123 nello spread di oggetti generico e considera conclusa l’issue quando TypeScript segnala l’incompatibilità prevista senza introdurre regressioni negli altri esempi.