{"record":{"id":"6774149536f3b490","repo":"apache/beam","slug":"unable-to-infer-the-types-of-uppername","errorCode":null,"errorMessage":"Unable to infer the types of {{$upperName}}","messagePattern":"Unable to infer the types of (.+?)\\}","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"sdks/go/pkg/beam/register/register.tmpl","lineNumber":34,"sourceCode":"{{$numParams := $processElementMaxIn}}\nfunc build{{$upperName}}Wrapper{{(genericTypingRepresentation $numParams 0 true)}}(doFn any) func(any) reflectx.Func {\n    {{$lowerName}}In := -1\n\t{{$lowerName}}Out := -1\n\t{{$lowerName}}Method := reflect.ValueOf(doFn).MethodByName(\"{{$upperName}}\")\n\tif !{{$lowerName}}Method.IsValid() {\n        return nil\n    }\n    {{$lowerName}}In = {{$lowerName}}Method.Type().NumIn()\n    {{$lowerName}}Out = {{$lowerName}}Method.Type().NumOut()\n    switch {\n    {{range $funcIn := upto $startFinishBundleMaxIn}}\n    case {{$lowerName}}In == {{$funcIn}}:\n            switch { {{range $funcOut := upto 2}}{{$possibleCombos := (possibleBundleLifecycleParameterCombos $funcIn $numParams)}}{{if $possibleCombos}}\n            case {{$lowerName}}Out == {{$funcOut}}:\n    {{$first := true}}{{range $funcCombo := $possibleCombos}}{{if $first}}{{$first = false}}                {{else}} else {{end}}if _, ok := doFn.({{$lowerName}}{{$funcIn}}x{{$funcOut}}{{if (or $funcIn $funcOut)}}[{{(join $funcCombo \", \")}}{{if $funcOut}}{{if $funcIn}}, {{end}}error{{end}}]{{end}}); ok {\n                    return register{{$upperName}}{{$funcIn}}x{{$funcOut}}FuncAndMakeStructWrapper{{if (or $funcIn $funcOut)}}[{{(join $funcCombo \", \")}}{{if $funcOut}}{{if $funcIn}}, {{end}}error{{end}}]{{end}}()\n                } {{end}}{{end}}else {\n                    panic(\"Unable to infer the types of {{$upperName}}\")\n                }{{end}}\n            default:\n                panic(\"Invalid signature for {{$upperName}}\")\n            }\n    {{end}}\n    default:\n        panic(\"Invalid signature for {{$upperName}}\")\n    }\n}\n{{end}}{{define \"BuildWrapper_SetupTeardown\"}}\n{{$lowerName := \"unknown\"}}{{$upperName := \"unknown\"}}{{if (eq .func \"setup\")}}{{$lowerName = \"setup\"}}{{$upperName = \"Setup\"}}{{end}}{{if (eq .func \"teardown\")}}{{$lowerName = \"teardown\"}}{{$upperName = \"Teardown\"}}{{end}}\nfunc build{{$upperName}}Wrapper(doFn any) func(any) reflectx.Func {\n    if _, ok := doFn.({{$lowerName}}0x0); ok {\n        {{$lowerName}}Caller := func(fn any) reflectx.Func {\n            f := fn.(func())\n            return &caller0x0{fn: f}\n        }\n        reflectx.RegisterFunc(reflect.TypeOf((*func())(nil)).Elem(), {{$lowerName}}Caller)","sourceCodeStart":16,"sourceCodeEnd":52,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/go/pkg/beam/register/register.tmpl#L16-L52","documentation":"This panic comes from Apache Beam Go's generated wrapper-building code (register.tmpl, rendered into register.go). When a DoFn's StartBundle/FinishBundle method exists with a recognized arity (NumIn), the framework tries to type-assert the DoFn against every known generic interface instantiation (e.g. startBundle1x1[T0, error]) derived from the ProcessElement type parameters. If none of the assertions succeed, it panics because it cannot map the lifecycle method to a concrete typed wrapper. The library throws it because Beam Go needs statically-known types to build a fast, reflection-free caller for the method.","triggerScenarios":"Registering (directly or via beam.ParDo/register.DoFn) a DoFn whose StartBundle or FinishBundle method's input/output arity matches a case, but whose concrete type parameters do not match any of the interface instantiations generated from possibleBundleLifecycleParameterCombos of the ProcessElement signature — e.g. lifecycle method types that don't line up with the ProcessElement type parameters, or a non-generic/pointer mismatch that makes the type assertion fail.","commonSituations":"Custom DoFns where StartBundle/FinishBundle use types different from ProcessElement's; passing a DoFn by value when the lifecycle methods are on the pointer receiver (or vice versa); upgrading Beam versions where generated interface instantiations changed; defining ProcessElement with generics whose parameters aren't reflected in the lifecycle methods.","solutions":["Make the StartBundle/FinishBundle method signatures use exactly the same type parameters as ProcessElement (e.g. ProcessElement(ctx, T0) (T1, error) with StartBundle(T0-ish) consistent with generated combos).","Ensure you register/parDo the same value kind the methods are defined on: pass &MyDoFn{} if methods use pointer receivers.","Check that the lifecycle method's input count is within the supported max (startFinishBundleMaxIn) and output count is 0 or 1; otherwise you hit the sibling 'Invalid signature' panic instead.","If using a plain (non-generic) DoFn, verify it implements one of the documented lifecycle interfaces (e.g. beam.StartBundle with func(context.Context) error style) exactly.","Update/verify the Apache Beam Go version; regenerate any vendored code so register.go matches the template, and consult Beam docs on supported DoFn signatures."],"exampleFix":"// before: lifecycle method types don't match ProcessElement\ntype Sum struct{}\nfunc (fn *Sum) ProcessElement(ctx context.Context, x int) (int, error) { return x, nil }\nfunc (fn *Sum) StartBundle(ctx context.Context, acc string) error { return nil } // mismatched param type\n\n// after: use context-only or matching types\ntype Sum struct{}\nfunc (fn *Sum) ProcessElement(ctx context.Context, x int) (int, error) { return x, nil }\nfunc (fn *Sum) StartBundle(ctx context.Context) error { return nil }","handlingStrategy":"validation","validationCode":"// Before registering, check lifecycle method signature shape at startup:\nfunc validateLifecycle(fn any, method string) error {\n    m := reflect.ValueOf(fn).MethodByName(method)\n    if !m.IsValid() {\n        return nil // no lifecycle method is fine\n    }\n    t := m.Type()\n    if t.NumIn() > 2 || t.NumOut() > 1 { // rough pre-check; combos must also match ProcessElement type params\n        return fmt.Errorf(\"%s signature %v not registrable\", method, t)\n    }\n    return nil\n}","typeGuard":"func hasValidStartBundle(fn any) bool {\n    _, ok1 := fn.(interface{ StartBundle() error })\n    _, ok2 := fn.(interface{ StartBundle(context.Context) error })\n    return ok1 || ok2 // extend with combos matching your ProcessElement type params\n}","tryCatchPattern":"func safeRegister(fn any) (err error) {\n    defer func() {\n        if r := recover(); r != nil {\n            err = fmt.Errorf(\"doFn registration failed: %v\", r)\n        }\n    }()\n    register.DoFn2x1(fn)\n    return nil\n}","preventionTips":["Keep StartBundle/FinishBundle signatures aligned with ProcessElement's type parameters.","Always register the same value kind the lifecycle methods are defined on (pointer vs value receiver).","Write a unit test that registers every production DoFn so signature drift fails in CI, not at pipeline runtime.","Consult the DoFn signature table in Beam Go docs before adding parameters or return values to lifecycle hooks."],"tags":["go","apache-beam","panics","generics","type-inference"],"backgroundTag":"type-mismatch","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T11:17:12.474Z"}