Typescript: Identify implicit symbol to string coercion errors

[kitsonk]

TypeScript Version: 2.6.1

Code

Symbols cannot be implicitly coerced to strings, unlike other types. They can only be coerced explicitly. The last two lines of code throw runtime errors like TypeError: Cannot convert a Symbol value to a string:

var symbol = Symbol('foo');

console.log(symbol);
console.log(String(symbol));
console.log("" + 6);
console.log("" + /foo/);
console.log(`${symbol}`); // Runtime Error
console.log("" + symbol); // Runtime Error

When down emitting, the string literal is emitted as the last line, so the run-time behaviour is consistent between ES6 and down emitted code, but in both cases, TypeScript could identify them at errors at design time and prevent implicit coercion.

Expected behavior:

That TypeScript would error on the last two lines, replicating the run-time error, thereby statically identifying the error.

Actual behavior:

No errors.

[aluanhaddad]

I had no idea that some of these were errors. The second to last line being an error freaks me out... I definitely would value the compiler's help here.

[kitsonk]

@aluanhaddad yeah, we had been using string literals for explicit implicit coercion (if that is a thing) and were about to do it for a symbol, but double checked at run-time and realised that it didn't work!

[DanielRosenwasser]

The same behavior would be expected for BigInt as well, so there's that to consider.

[mhegazy]

looks like an error indeed. we should flag this while checking

[mhegazy]

To be specific, nothing else changes, only symbols for now; we will handle other builtin types like BigInt as they come.

[mhegazy]

PRs are welcomed.