Problem with infer two types from string at same time

🔎 Search Terms

If i want infer two types from string in same time with infer U extends Subtype, I get different results depending on the input string

type Test<T extends string> =  T extends `${infer Rest}${infer D extends Digit}`
  ? `${Rest}[${D}]`
  : T

It can be solved with two consecutive conditions, but it has its price ( in example you can see three different implementations with different result)

🕗 Version & Regression Information

• This code does not work in versions earlier than 4.7
• This changed between versions 4.7 and 4.8
• The result given in the comments to the code corresponds to versions 4.8-5.3

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20240102#code/C4TwDgpgBAIglgczsKBeKAGKAfKBGHKAJkIGZCAWQgVkIDZCB2QgDkIE4AoTgeh6jgA7AGYQATlAA2AQwDOKACaJkUAGRQxEeVAD2wqPLFCEAwbsHQAxjsFLgcG51CQoAFS3AA+ngA8rqBAAHsAQtrJQAAYAJADeggCuALYARuIAvhEAfGhuAcGhCuHRMUKiEgBKHmmxpeKweSFhsMrAGZwAkAD8UADalfIANLAAuh0AXFAWAG51fFB+DQVFsQkp6ZkRUGBiOiGWIQpQ8eGpwjqa3HO1EpraegbARoIIQ8AAFqGmZVJyii0GEGstigQLsDkETnA0Hc8k8RAWQUahUiKySqTEGWy6H8iKWKJKIjq-VasXgSFaEQ63Rx+SaxWJ1QJ3xgiyaZOQbXaXV6xKGMFGXIm01m-GEcDE2lByHBWx2ewORxOEDOF3aQogMwkcwRtORxVW6LSG1lu0BCuOUFO5wgl341x+2iU5NeHzM9tuKHuhmMANBIJsYMczmhHk8pB1SOWcTR6SyORpkfx3uejPtLNxbJanOprL1NUJFSqpM53L6Hj5ArVkw1IqgYolKCl9hsJvlEEOFqtqvVmqg2oTeP1MYxxu2pv27cVluV1u4wagIVhCmkwGk3hyAHI8Bg8BvIS5F15l6u4Zvt3v54fPMe1+R0FuL1CF6Gb54qPeMI+Dy+V2vaPev2gK9bniSQvAIdAei3T8hjwUZL1DECwNPSCHyGDB4KfYCtFArw716DcN1gzDv1hJCvHfAiiMwEigMQnDkP-atNTnLDQzwCC3HYnwr1fPBsjmeILECSAJwUHtxH3OjYTwEhsW43jfzhAT+B0ABrCYoN3dDaOfGT8JhcCeJ-E9SBUo5hNEg4JLEKS9PAyjDO8YylyUihzKEoIrPbGzeB4AABYBZAAWi8s1QrEHZbIQmSmKc3xFJPahzPU1jSK8IhOKc+FErXfi+1UjSCPPYi7KvIg5K42EcpMtciBSoqtOojCytDIgDLalyjyUsyCt0RrCNKmKMsczrcrfBrNMGmi-MCkKwv2CKota6q4rG2rPGSvrUpWvCstDcNxvywTLLNHzipg5pyV0q9SEqpzDo2+rtoG7SZuGsMOthR7XNMjzTrEqbqPZYAboO0bvq6683P+hbrKovkWlGPg5tCkTwvEZaPtINbIfGra5h2uY7IAUXR-YIzxR54ggLFOCgBmoAACj8TImYASjQbIByafxugICYiE5jNCnpxnWY5rnchF8JqegfmoEFqBujlxW62kSRZBtUmAEd4g1nwAEEhgAISxKAxZZ1w2c51BudzcJDeV-A1aFh3LYl237ZlqATedgXiE5lWxBptXhA1rW0qsOQtByHoxbJqyfBJvWDavDjXno2RcO8TJMgGBPyeAZPU8kKHZMzsiGIyvOC4ZxOzRL-Wy-T0hK68ciw1rwuk5T5vy4odvPE79z857xu+7T9jqCHzvkrH+ui6bqfqrwWfq9zheoAbinJ5btqiHX7PkPqred+Lveofao+c7Ms+l8v8rB-s4eN9Huvt4f0ur5nl+5+7xevdv63TXn-De-F75AP7rdQ+YDj410gRPYBB025wNvgAz+UCV54WftheBE1EG72Qd9X+eCc7zwLqMIAA

💻 Code

type Digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

// infer last digit & rest of string in one condition
type Test_1<T extends `${number}`> = T extends `${infer Rest}${infer D extends Digit}`
	? [Rest, D]
	: never // <T extends `${number}>` protected us before

// infer rest of string, then infer last digit second condition
type Test_2<T extends `${number}`> = T extends `${infer Rest}${Digit}`
	? T extends `${Rest}${infer D extends Digit}`
		? [Rest, D]
		: never
	: never // <T extends `${number}>` protected us before

// infer last digit, then infer rest of string second condition
type Test_3<T extends `${number}`> = T extends `${string}${infer D extends Digit}`
	? T extends `${infer Rest}${D}`
		? [Rest, D]
		: never
	: never // <T extends `${number}>` protected us before

🙁 Actual behavior

type test_11 = Test_1<'101'> // unexpected: never
type test_12 = Test_1<'10'> // ok: ['1', 0]
type test_13 = Test_1<'1'> // unexpected: never
type test_14 = Test_1<'0'> // unexpected: never
//@ts-expect-error
type test_15 = Test_1<''> // ok

type test_21 = Test_2<'101'> // ok: ['10', 1]
type test_22 = Test_2<'10'> // ok: ['1', 0]
type test_23 = Test_2<'1'> // ok: ['', 1]
type test_24 = Test_2<'0'> // ok: ['', 0]
//@ts-expect-error
type test_25 = Test_2<''> // ok

type test_31 = Test_3<'101'> // unexpected: ['10', Digit]
type test_32 = Test_3<'10'> // ok: ['1', 0]
type test_33 = Test_3<'1'> // unexpected: ['', Digit]
type test_34 = Test_3<'0'> // unexpected: ['', Digit]
//@ts-expect-error
type test_35 = Test_3<''> // ok

🙂 Expected behavior

Test_1, Test_2 & Test_3 has equal behaviour

Additional information about the issue

No response