Improve error message for stricter types not matching generic T extends SomeInterface

[patrys]

Search Terms

generic type is not assignable to type error

Suggestion

It's impossible for a non-generic type to be assignable to a generic that extends an interface. This is understandable as at the time of type checking it's impossible to determine the generic type's fields. It would be nice if the error message explained that instead of just saying that the types are incompatible.

Use Cases

See the example below:

Examples

interface Foo {
    bar: string;
}

function baz<T extends Foo> {
    const fizz: T = { bar: "hello world " };
}

Current error:

Type '{ bar: string; }' is not assignable to type 'T'.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[jack-williams]

This is fixed by #30394