[Jack Williams (jack-williams)]

Fixes #19666

Check for symbol types in template expressions:

let symbol!: symbol;
const templateStr = `hello ${symbol}`;
// error TS2352: Type 'symbol' cannot be converted to type 'string'.

The following cases were already handled (but have been added to the tests):

let symbol!: symbol;
let str = "hello ";
const appendStr = "hello " + symbol;
str += symbol;

This PR does not deal with tagged templates.

Question: Should the type of the checked expression be any when encountering an error?

Mohamed Hegazy (@mhegazy)

[Daniel Rosenwasser (DanielRosenwasser)]

I'm not huge on the test file name here, but it's fine. Can you add a case where you have a value of type symbol | number (or something in a union)?

[Jack Williams (jack-williams)]

Will do. I'm open to changing the name. Perhaps templateStringWithSymbolExpressionErrors.ts ?

[Daniel Rosenwasser (DanielRosenwasser)]

I think you should just leave the type of the string literal to be a string; the type of the expression itself isn't observed otherwise. The only time we are concerned with the expression type is a tagged template.

Which reminds me! Can you add a test with the following contents? 😄

// taggedTemplateStringWithSymbolExpression01.ts

declare function foo(template: any, val: symbol): number;
let x!: symbol;

let result: number = foo `${x}`;

Ideally, the test output won't have any errors.

[Jack Williams (jack-williams)]

Update: Daniel Rosenwasser (@DanielRosenwasser)

• Add union types to test with errors.
• Add test case for tagged template with symbol param and number return type (no errors).

To consider:
Rename test noImplicitSymbolToString.ts ==> templateStringWithSymbolExpressionErrors.ts

[Jack Williams (jack-williams)]

ping Daniel Rosenwasser (@DanielRosenwasser) (just incase this got lost under a pile somewhere)