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Description
🔎 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
💻 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