unknown types should not able to concat with strings

[hronro]

TypeScript Version: 3.3.0

Code

declare const a: unknown;
const b = a + 123;
const c = a + '123';
const d = `${a}123`;

Expected behavior:
All the declarations of variables b c d should have errors.

Actual behavior:
Only declaration of variable b have error.

Playground Link:
https://www.typescriptlang.org/play/index.html#src=declare%20const%20a%3A%20unknown%3B%0Aconst%20b%20%3D%20a%20%2B%20123%3B%0Aconst%20c%20%3D%20a%20%2B%20'123'%3B%0Aconst%20d%20%3D%20%60%24%7Ba%7D123%60%3B%0A

Although we won't get runtime errors in this case, it may cause unpredictable result, e.g. "[[Object]]123", which is unexpected, so I think ts should not allow unknown types to concat with strings.

[jack-williams]

Flagging an error here would be correct from the POV that symbol is a subtype of unknown, and you are not allowed to apply + to symbols.

[hronro]

Sorry I don't understand what symbol means here. The problem now is unknown type + number would get an error, but unknown type + string won't get an error.

[jack-williams]

Ah sorry, I mean the symbol type here. Currently you are allowed to concat many things to a string without error, but symbol is not one.

declare const w: Function;
declare const x: { x: number };
declare const y: boolean;
declare const z: symbol;

w + "world";
x + "world";
y + "world";
z + "world"; // Error

So + with a string is very permissive, but not in the case of symbol. Given that something of type unknown could be a symbol, then the safe thing is to flag an error as you suggest. With symbol you will get a runtime error.