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

[na7ure-a]

🔎 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

[RyanCavanaugh]

🤖 Thank you for your issue! I've done some analysis to help get you started. This response is automatically generated; feel free to 👍 or 👎 this comment according to its usefulness.

Similar Issues

Here are the most similar issues I found

• (68%) microsoft/typescript#43226: Compiler crashes
• (67%) microsoft/typescript#51623: Cannot read properties of undefined (reading 'length')
• (66%) microsoft/typescript#48026: Inferring within mapped tuple's type parameter raises error unexpectedly
• (66%) microsoft/typescript#47487: Leading/Middle Rest Elements in Tuple Types with generic function derive unknown
• (66%) microsoft/typescript#50193: Object.isPartOfTypeNode crashes with TypeError: Cannot read properties of undefined (reading 'kind')
• (65%) microsoft/typescript#56970: error in Variadic Tuple Types , if i reverse the partial function
• (65%) microsoft/typescript#45515: API: Exception in TypeChecker.typeToTypeNode
• (65%) microsoft/typescript#59062: `getTypeListId` crashes with `TypeError: Cannot read properties of undefined (reading 'id')`
• (65%) microsoft/typescript#52017: Broken double inference step in variadic tuple types when rest elements come into play

If your issue is a duplicate of one of these, feel free to close this issue. Otherwise, no action is needed.

Possibly Working as Intended

It looks like this behavior might be intentional. Here's what a computer had to say about it.

The pattern ...(infer C)[] is not legal tuple‐pattern syntax. A rest element may be written as ...infer C (optionally with a constraint such as ...infer C extends unknown[]), but it cannot be followed by []. The compiler should therefore issue a normal syntax or type error. Instead it dereferences an internal object and crashes, which is an internal compiler error. So the observed behavior is a compiler bug, not the correct or expected diagnostics.