Crash on async generator that yields a thenable and another value #65378

🔎 Search Terms

async generator, yield, then, TS1321, crash, nil pointer

🕗 Version & Regression Information

Crashes in 7.0.2 and 7.1.0-dev.20261106.1 (also 5.9.3, with TypeError: Cannot read properties of undefined (reading 'flags'))

⏯ Playground Link

https://www.typescriptlang.org/play/v2/?ts=6.0.0-dev.20260416#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-R8vDD3uxNgQFHK0ZZkAwYzEKCG1DGyf6DgdJfOqADWEKOfEEcgBB6qCH4gjYgJm5gFgjMCbkDVlgp66aVBuedYLVsABGAAGfRYn3IhlBuWwtzIAAmID4NA-QNHYaCQDpSAoCQ19k3QNDRARdgsP4fRcwIVBEAjHIwEg4dFG1dgKBXU8yIohAqJIGiEDopgdmYl9WMQHik3fPDPQlABfCT+FwQhSEoahaHoRhWA4bgxHAMhwjKAACQRSnKRQACodOUAAKABKHTgB08IYAxFhrJ0qAsiYSzrIknSJIAbls+zMR0gDfIk3QmBACSBAhdBDLC8QQCYsg+kUNlv0MWKAIEBKkqYC0EWSQwmSVLBYMijFslGElIygFKy3S0rooq8CoFyhB8sKohioikBECIRKtEwWCJKAA

💻 Code

async function* g() { yield { then() {} }; yield 1; }

🙁 Actual behavior

tsc --noEmit --target es2020 a.ts crashes:

panic: runtime error: invalid memory address or nil pointer dereference
...
internal/checker.(*Checker).getUnionTypeEx
internal/checker.(*Checker).getReturnTypeFromBody

🙂 Expected behavior

TS1321 on the first yield, like with a single yield { then() {} }.

Additional information about the issue

With only one yield (or with a return type annotation) there is no crash.