Intersection type in template literal is not reduced to its bare type

🔎 Search Terms

template literal, intersection type

🕗 Version & Regression Information

• This changed between versions 5.0 and 5.1
• This changed in commit or PR #48034 #52345 #54648

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAysBOBLAdgcwCrggHgNJQgA9gJkATAZygoRVQD4oBeKfIk8qmpNKAMigoAZhHhQAqgCgAkAH4oAbVwAaCQF0CxUpUXjVuNTPkBvAL6aOO44oAKg5FAAGAEmO5TjtQC4oAV2QA1sgA9gDuDqZGUADkJAC2YAA2AIYkADKIJPDJidEyPtHcdBlZOXnSBUVo5T7IEABuopKSoJBQAEoAjMywtGiYkNhVDJIA9KNQkwB6si1YHQBMPXA8GFhDfaj8UNbJPt2m9GMT07Nzbe0AzMubAzjRyQBGAMbRR+OTUDPn0O0ALDdVndsA8XtFtrt9lBDsdPt8fh0AKyAujAlzGZC+OKPUQed4nL6zVq-ABsKP663RmOxuMcEKgeygB3xcLOUCAA

💻 Code

type StringType<K extends string> = K extends string & infer U
	? [K, U] extends [U, K]
	? {} extends { [P in `${K}`]: unknown }
	? 'templateLiteral'
	: 'stringLiteral'
	: 'string'
	: never

type R1 = StringType<string>
//   ^? type R1 = "string"
type R2 = StringType<string & { a: 1 }>
//   ^? type R2 = "string"

type R3 = StringType<'abc'>
//   ^? type R3 = "stringLiteral"
type R4 = StringType<'abc' & { a: 1 }>
//   ^? type R4 = "templateLiteral" <-- should be "stringLiteral"

type R5 = StringType<`${number}`>
//   ^? type R5 = "templateLiteral"
type R6 = StringType<`${number}` & { a: 1 }>
//   ^? type R6 = "templateLiteral"

🙁 Actual behavior

type R = `${'abc' & { a: 1 }}`
// did not reduce => `${'abc' & { a: 1 }}`

🙂 Expected behavior

type R = `${'abc' & { a: 1 }}`
// should reduce to => `${'abc'}`
// => `abc`

Additional information about the issue

I mentioned this in #54648 after it is closed. It is limiting our ability to write the types that works with string literal and template literal and there is no alternative way to workaround that.
I'm suggesting this issue should be fixed and restore the behavior in 5.0.

Here is my original comment:

This behavior is causing a few types in type-plus to fail (e.g. IsTemplateLiteral, IsStringLiteral, Omit, IsNegative, etc) https://github.com/unional/type-plus/issues/429.

In term of soundness, IMO it does make sense that ${string & { a: 1 }} to be reduced to ${string}.

in JS, it would be:

const extendedStr = Object.assign('abc', { a: 1 })
console.log(`${extendedStr}`) // 'abc'

the reasoning being the toString(): string remains unchanged thus the resulting type should be safe to reduce.

Reproduce the regression in the linked TypeScript Playground, comparing the shown R4 and R6 results across versions 5.0 and 5.1. Use the cited commits and pull requests (#48034, #52345, and #54648) as the starting history; done means R4 reduces to the string-literal result while the other examples retain their expected classifications.