Crash: optional chain + tagged template panics the optional-chain transform

🔎 Search Terms

flattenChain tagged template, KindTaggedTemplateExpression, "Unhandled case in Node.Expression", "isOptionalChain", optional chain tagged template crash, e?.``(

🕗 Version & Regression Information

• This is a crash
• This is the behavior in every version I tried: TypeScript 6.0.3 (JS) and 7.1.0-dev (main, Go)
• I have reviewed the FAQ for entries about optional chaining and tagged templates. Those entries do not cover this compiler crash.

⏯ Playground Link

Not useful here. The crash needs ES2020 optional-chain emit. Use the CLI command below.

💻 Code

e?.``(

🙁 Actual behavior

panic: Unhandled case in Node.Expression: KindTaggedTemplateExpression [recovered, repanicked]

goroutine 13 [running]:
sync.(*WaitGroup).Go.func1.1()
        sync/waitgroup.go:251 +0x48
panic({0x10417d8e0?, 0x2b3e77a92750?})
        runtime/panic.go:860 +0x12c
github.com/microsoft/TypeScript/tsc/internal/ast.(*Node).Expression(...)
        github.com/microsoft/TypeScript/tsc/internal/ast/ast.go:383 +0x718
github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms.flattenChain(...)
        github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms/optionalchain.go:143 +0x1c4
github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms.(*optionalChainTransformer).visitOptionalExpression(...)
        github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms/optionalchain.go:151 +0x38
github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms.(*optionalChainTransformer).visitCallExpression(...)
        github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms/optionalchain.go:37 +0x190
github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms.(*optionalChainTransformer).visit(...)
        github.com/microsoft/TypeScript/tsc/internal/transformers/estransforms/optionalchain.go:20 +0xd0
github.com/microsoft/TypeScript/tsc/internal/ast.(*NodeVisitor).VisitNode(...)
        github.com/microsoft/TypeScript/tsc/internal/ast/visitor.go:51 +0x30
github.com/microsoft/TypeScript/tsc/internal/ast.(*ExpressionStatement).VisitEachChild(...)
        github.com/microsoft/TypeScript/tsc/internal/ast/ast_generated.go:1592 +0x30
... 74 lines truncated ...
github.com/microsoft/TypeScript/tsc/internal/compiler.(*emitter).emitJSFile(...)
        github.com/microsoft/TypeScript/tsc/internal/compiler/emitter.go:200 +0x198
github.com/microsoft/TypeScript/tsc/internal/compiler.(*emitter).emit(...)
        github.com/microsoft/TypeScript/tsc/internal/compiler/emitter.go:50 +0xec
github.com/microsoft/TypeScript/tsc/internal/compiler.(*Program).Emit.func2()
        github.com/microsoft/TypeScript/tsc/internal/compiler/program.go:1851 +0x1c0

🙂 Expected behavior

The compiler should not crash 😄

Additional information about the issue

Disclosure: This crash was found with a custom mutation fuzzer and AI assistance (Kimi K3). The input was minimized and then verified by hand against TypeScript 7.1.0-dev (main) and TypeScript 6.0.3.

Start in tsc/internal/transformers/estransforms/optionalchain.go, especially flattenChain and visitOptionalExpression, then inspect the Node.Expression handling in tsc/internal/ast/ast.go. Run the minimized e?.``( example through the CLI with ES2020 optional-chain emit. Done means the compiler no longer panics on this input.