René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"extends causing string literal can't be used as index","articleBody":"# Bug Report\r\n\r\n\u003c!--\r\n  Please fill in each section completely. Thank you!\r\n--\u003e\r\n\r\n### 🔎 Search Terms\r\n\r\ncannot be used as an index type\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\n- This changed between versions 4.9.5 and 5.0.4\r\n\r\n### ⏯ Playground Link\r\n\r\n\u003c!--\r\n  A link to a TypeScript Playground \"Share\" link which shows this behavior.\r\n  This should have the same code as the code snippet below, and use whichever settings are relevant to your report.\r\n\r\n  As a last resort, you can link to a repo, but these will be slower for us to investigate.\r\n--\u003e\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgCLAWZBvAsAKGWRgHsSAuZARgKICM4pKb8BfAgsATwAcUBBZAF40GMAG0A5KRKSAusgD0i5AHcSUANYBnDvm59kAMWHJpZSZ14oAQqfSZxRhcrUadegyiMmR52cgQAB6QIAAm2sgAFP6SyAA+ZgxQkgCUyAD8ZjJxlCAQAG7QSiqgyGAAFhAaXAA0xr5CIkZWhgDC9mJOziXIICRY6lqgAOb1ACrWZgBEMtNxCHAg-Vh0KACu2hBhyHCRS8igYcHl1gB0UQBMAKwAzAAcqUA)\r\n\r\n### 💻 Code\r\n\r\n\u003c!-- Please post the relevant code sample here as well. This code and the Playground code should be the same, do not use separate examples --\u003e\r\n```ts\r\ninterface Dict {\r\n  foo: 1\r\n  bar: 1\r\n}\r\n\r\ntype A = Dict['foo'] // works\r\n\r\ntype F = 'foo'\r\ntype B = Dict[F] // works\r\n\r\ntype FF = 'foo' extends ('foo' | 'bar') ? 'foo' : never // in theory, FF === F\r\ntype C = Dict[FF] // not working, Type '\"foo\"' cannot be used as an index type.(2538)\r\n```\r\n\r\n### 🙁 Actual behavior\r\n\r\n\u003c!-- What happened, and why it was wrong --\u003e\r\n`type C` throws error\r\n\r\n### 🙂 Expected behavior\r\n\r\n\u003c!-- What you expected to happen instead, and why --\u003e\r\n\r\nno error","author":{"url":"https://github.com/troy351","@type":"Person","name":"troy351"},"datePublished":"2023-07-05T08:16:03.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/54886/TypeScript/issues/54886"}