Crash: RangeError: Invalid string length in addSpans during instantiation of recursive template literal types

🔎 Search Terms

RangeError: Invalid string length

addSpans crash

getTemplateLiteralType recursion

Template literal exponential growth

🕗 Version & Regression Information

Crashes in: 5.7.3, 5.8.3, 5.9.3, Nightly.

⏯ Playground Link

https://www.typescriptlang.org/zh/play/?target=7&ts=6.0.0-dev.20260319#code/C4TwDgpgBAIhDGAeAclCAPYEB2ATAzlNgK4C2ARhAE4B8UAvFAFBStSoZZ6ECsUA-FAAsUAFzs0mHAWECoAZjETO0wosEAmJSzYcp3KFsEBDbCG1tl+mQEY5ABiX2A3EyahIUAEoJiVFJJcMiQU1AA0UADKgapQ+MBUAJbYAOZ0jDqsekGEjoLR4j7wfohwSMg0EQAGACQA3pEAvgD69U1VNK7u4NAAouhgADYA9rjQjHWZUADaAApQyd6+-jwRAETGa3TGhLV1sy0A1hAgVQC64qYgro3OQA

💻 Code

type Dec<N extends number> = 
    N extends 5 ? 4 : N extends 4 ? 3 : N extends 3 ? 2 : 
    N extends 2 ? any : 
    N extends 1 ? 0 : 0;

type Recur<N extends number, S extends string> = 
    N extends 0 ? S : Recur<Dec<N>, `${S}_${S}`>;

type Explode = {
    [P in Recur<5, "a"> as `${P}_key`]: any;
};

🙁 Actual behavior

node built/local/tsc.js -noEmit  specializedSignatureIsNotSubtypeOfNonSpecializedSignature_filled_1_unvalidated_mutated_batch12.ts
D:\github\TypeScript\built\local\_tsc.js:123721
      throw e;
      ^

RangeError: Invalid string length
    at addSpans (D:\github\TypeScript\built\local\_tsc.js:62120:19)
    at getTemplateLiteralType (D:\github\TypeScript\built\local\_tsc.js:62095:10)
    at instantiateTypeWorker (D:\github\TypeScript\built\local\_tsc.js:63780:14)
    at instantiateTypeWithAlias (D:\github\TypeScript\built\local\_tsc.js:63736:20)
    at instantiateType (D:\github\TypeScript\built\local\_tsc.js:63703:29)
    at getMappedType (D:\github\TypeScript\built\local\_tsc.js:63383:67)
    at D:\github\TypeScript\built\local\_tsc.js:62756:73
    at map (D:\github\TypeScript\built\local\_tsc.js:196:19)
    at canTailRecurse (D:\github\TypeScript\built\local\_tsc.js:62756:33)
    at getConditionalType (D:\github\TypeScript\built\local\_tsc.js:62723:17)

Node.js v22.19.0

🙂 Expected behavior

The compiler should not crash.

Additional information about the issue

No response

Reproduce the crash with the provided recursive template-literal example or the specializedSignatureIsNotSubtypeOfNonSpecializedSignature_filled_1_unvalidated_mutated_batch12.ts command. Start from addSpans and getTemplateLiteralType in built/local/tsc.js, following the stack through instantiateTypeWorker. Done means the compiler reports or handles the input without throwing RangeError: Invalid string length.