[isolatedDeclarations] Type of const variable declared with template literal expression containing only literals still requires explicit annotation

🔎 Search Terms

isolated declarations template literal expression

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?isolatedDeclarations=true&ts=5.6.0-dev.20240705#code/KYDwDg9gTgLgBAG2PAZhCcC8cAGASAbwHIiBfHAbgCgrRJY4BjCAOwGd4AjAQyi10Ily1WuGioAri0YwAlqzg8AXgAoAlHAJU4OuFGQSoLAcTKUqpIA

💻 Code

export let foo = `${''}`;

export const bar = `${''}`;

export function baz() {
    return `${''}`;
}

🙁 Actual behavior

bar has a type error:

Variable must have an explicit type annotation with --isolatedDeclarations.(9010)
input.tsx(3, 14): Add a type annotation to the variable bar.

foo would also have a type error if as const was added to the expression.

🙂 Expected behavior

No type error

Additional information about the issue

fun fact: I accidentally discovered this behavior when I found that oxc-transform has type errors for both const, let, and return types.

Start with the linked TypeScript Playground and reproduce the isolatedDeclarations diagnostic for the exported const, then compare it with the let and function examples in the issue. Trace the compiler path responsible for inferring types for these declarations; done means the literal-only template expression no longer requires an explicit annotation and the existing cases remain correct.