Object key signature is not taken into account with destructive assignment on method definition level

TypeScript Version: 2.3.2

Code

interface IMyInterface {
  onChange(changes : {[key : string] : {newValue : any, oldValue : any}})
}

class MyClass implements IMyInterface {
  onChange({config}) {
    console.log(config);
  }
}

class MyAnotherClass implements IMyInterface {
  onChange(changes) {
    const {config} = changes;

    console.log(config);
  }
}

Expected behavior: compiled without errors

Actual behavior:

test.ts(5,7): error TS2420: Class 'MyClass' incorrectly implements interface 'IMyInterface'.
  Types of property 'onChange' are incompatible.
    Type '({config}: { config: any; }) => void' is not assignable to type '(changes: { [key: string]: { newValue: any; oldValue: any; }; }) => any'.
      Types of parameters '__0' and 'changes' are incompatible.
        Type '{ [key: string]: { newValue: any; oldValue: any; }; }' is not assignable to type '{ config: any; }'.
          Property 'config' is missing in type '{ [key: string]: { newValue: any; oldValue: any; }; }'.

Note: this affects Angular >1.5 users when writing component controllers and implementing $onChanges method

[mhegazy]

IMyInterface.onChange says it can be called by any object, as long as its properties have a certain type.

MyClass.onChange has a stricter requirement, saying it has to be called with an object with a property called config.

as a result MyClass is not substituatble for IMyInterface.

The second class MyAnotherClass, says it can be called with any arguments, no requirements here…

If MyClass.onChange does not care if change is not provided, consider making it optional:

class MyClass implements IMyInterface {
  onChange({config = undefined}) {
    console.log(config);
  }
}

or give it an explicit type:

class MyClass implements IMyInterface {
  onChange({config} : any) {
    console.log(config);
  }
}

you can always give type, and you would see a similar error.

class MyAnotherClass implements IMyInterface {
    onChange(changes: { config: number }) {
        const { config } = changes;

        console.log(config);
    }
}

[sandersn]

Since this is comparing assignability of parameters, there is bivariance. So it shouldn’t be an error.