Bug Report

🔎 Search Terms

TS2322 unhelpful message

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about TS2322

⏯ Playground Link

Playground link with relevant code

💻 Code

type StringMap = {
	[x: string]: string;
}

type Message<TMap extends StringMap, T extends keyof TMap> = {
	bar: "bar";
	type: T;
}

type MessageHelper<TMap extends StringMap, T extends keyof TMap> =
	T extends keyof TMap ?
		Message<TMap, T> :
		never;

function foo<TMap extends StringMap, T extends keyof TMap>(type: T) {
	const message : Message<TMap, T> = {
		bar: "bar",
		type,
	}

	const message2 : MessageHelper<TMap, T> = message;
	//    ^^^^^^^^------- Type 'Message<TMap, T>' is not assignable to type 'MessageHelper<TMap, T>'.

	message.type; // T extends keyof TMap
	message2.type; // (keyof TMap & string) | (keyof TMap & number) | (keyof TMap & symbol)

	message.type = message2.type;
	// ^^^^^^^^^ This is more helpful, but I had to jump through quite some hoops to finally figure
	// out this was the cause.
}

🙁 Actual behavior

The error that occurs when assigning message to message2 shows just

Type 'Message<TMap, T>' is not assignable to type 'MessageHelper<TMap, T>'.

🙂 Expected behavior

A better explanation of why the types are not assignable. I'm assuming it's because the types of message.type are different. But this isn't clearly shown in the error message.