TypeScript allows invalid intersection type with index signature

[srmagura]

Bug Report

🔎 Search Terms

intersection type, index signature

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ

Tested in TS Playground for version 3.9 up to 4.6 nightly.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Grammar = {
    rest: 'rest' // Error: "rest" is not assignable to "value"
    [key: string]: 'value'
}

// No error - shouldn't this behave the same way?
type Grammar2 = {
    rest: 'rest'
} & {
    [key: string]: 'value'
}

// Compile error - Grammar2 type is essentially unusable
const grammar2: Grammar2 = {
    rest: 'rest',
    myKey: 'value'
}

🙁 Actual behavior

• Only Grammar produces a compile error even though it is equivalent to Grammar2.
• 'rest' cannot be assigned to the rest property of Grammar2.

🙂 Expected behavior

I'm not sure how this should behave.

Additional Context

Posting this issue due to a problem with @types/prismjs. A Prism Grammar has a [key: string]: GrammarValue index signature AND special properties like rest which are not assignable to GrammarValue. How can I fix this in @types/prismjs?

[jcalz]

Duplicate #37809

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.