Crash： when inferring from a tuple with middle rest elements and trailing variadic elements

🔎 Search Terms

Internal Error: Debug Failure

Variadic tuple types crash

infer rest elements crash

tuple mapping crash

🕗 Version & Regression Information

Versions tested: 5.7.3, 5.8.3, 5.9.3, Nightly

⏯ Playground Link

https://www.typescriptlang.org/zh/play/?target=7&noEmit=true&ts=6.0.0-dev.20260117#code/PTAEAEGcBcCcEsDG0Bco4FcCmAoEEA7AewFEBbeVdWbPMWLAB1iNAGIBWARi4GYAOHNACejLKADKGAEYAVDIwBMAJSwwAggQAms2AEN4AG3gEA5gDU9CPVqSKAPLNBYAHtCzbIoDAQDWxAHcCAG0AXQA+UABeUCdXd09QYJxQVNAAOkyAChMAMyxYUABhAEowgBoUtMz0vILQACFnNw8tL2C9AmFyzu7EfUgAC1CcUNAq1IB+JKLymoaxidA0AiwANwKAbhwhUXEpOQUVNWhNHX0jEwsreBs7WRPoyRl5JVUNbV0DYzNLa1tEA5gjU9GgAAwVaRoLjlRBoRQRbZAA

💻 Code

// repro #51138
type SubTup2RestAndTrailingVariadic2<T extends unknown[]> = T extends [
    ...(infer C)[],
    ...infer B extends [any,any,crash]
] 
    ? [C,...B] 
    : never;

type SubTup2RestAndTrailingVariadic2Test = SubTup2RestAndTrailingVariadic2<[...a: 0[],b: 1,c: 2]>;

🙁 Actual behavior

.\ts-versions\nightly\node_modules\.bin\tsc.cmd  .\inferTypesWithFixedTupleExtendsAtVariadicPosition_m.ts
D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:123178
      throw e;
      ^

TypeError: Cannot read properties of undefined (reading 'aliasSymbol')
    at inferFromTypes (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:68512:18)                                                                                                  
    at inferFromObjectTypes (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:68994:21)                                                                                            
    at invokeOnce (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:68704:9)                                                                                                       
    at inferFromTypes (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:68666:11)                                                                                                  
    at inferTypes (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:68500:5)                                                                                                       
    at getConditionalType (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:62537:11)                                                                                              
    at getConditionalTypeInstantiation (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:63526:305)                                                                                
    at instantiateTypeWorker (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:63629:14)                                                                                           
    at instantiateTypeWithAlias (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:63566:20)                                                                                        
    at getTypeAliasInstantiation (D:\do\typeFuzz\ts-versions\nightly\node_modules\typescript\lib\_tsc.js:60132:52)                                                                                       

Node.js v22.19.0

🙂 Expected behavior

The compiler should report a syntax error and not crash.

Additional information about the issue

No response

先從 Playground 重現和提供的元組推論範例開始，接著追蹤編譯器呼叫堆疊，從 inferFromTypes 經過 inferFromObjectTypes 到 getConditionalType。使用列出的 TypeScript 版本重現當機，並找出相關的編譯器回歸測試位置。當此輸入回報語法錯誤而不是拋出內部 TypeError 時，即表示完成。