Template literal type resolution

[Treen11]

🔎 Search Terms

mapped type template literal

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about current version

⏯ Playground Link

https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgCoE8AOECydPIDeAUMsnAFzIDkAgtacgEZXUBCDAvscQCYQIANnCgoEAexABnMMhhUAPKmQQAHpBC8pNONWQAfGk2oA+ABSqqAAwAkhVJysBKZAF4TaLLnwBtVAF0eMC9kADEARjdkJRV1CE1tal0DI1MLazsHZzcPDGw8TD9AiWlZGHCqCKiYZAB6WuQAdwALdGRecQhtMGbgbWgocSgAfiCQ0IAmKJi1DS0dPUNqY3NLZFt7Rxd3T3zfDaz-Ooa4TSbW9s7tAFs4AGtQAHNkHpRRMABXKBAXkLgXiDXTDCSDIQTASBQOCCOTAVTICGjEoyOQTSpTVxyIA

💻 Code

The unexpected error is marked by a comment in the code bellow. A second comment highlights a case, where this issue does not occur, even though the two cases should be identical, as far as i understand. Thank you Matt Kantor (@mkantor) for the minimalistic example. The issue has been discussed on the TS discord in various channels. The original discussion was an "ask for help" here - https://discord.com/channels/508357248330760243/1139519033809575966/1139519033809575966

interface TypeMap {
 a: 'A'
 b: 'B'
}

declare const f: <T extends 'a' | 'b'>(x: `${T}`) => TypeMap[T]

type F1 = <T extends 'a' | 'b'>(x: `${T}`) => TypeMap[T]
const f1: F1 = f // why does this error?

type F2 = <T extends 'a' | 'b'>(x: `${T}`) => TypeMap[`${T}`] // and why does making the return type a template literal fix it?
const f2: F2 = f

🙁 Actual behavior

On line 9 occurs -

Type '<T extends "a" | "b">(x: `${T}`) => TypeMap[T]' is not assignable to type 'F1'.
 Type 'TypeMap[`${T}`]' is not assignable to type 'TypeMap[T]'.
 Type 'TypeMap[`${T}`]' is not assignable to type 'never'.
 Type 'string' is not assignable to type 'never'.
 Type 'string' is not assignable to type 'never'.(2322)

🙂 Expected behavior

No error on line 9