Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 2 inputs and 2 outputs

What it means

ToFunc2x2 adapts a generic reflectx Func into a Func2x2 (2 inputs, 2 outputs). It panics when the wrapped function's reflect type does not have exactly 2 inputs and 2 outputs, surfacing the mismatch when the wrapper is created.

Solutions

Make the function take 2 inputs and return exactly 2 values.
Use the ToFunc/MakeFuncNxM variant matching the real arity.
Validate NumIn()/NumOut() before adapting.

Example fix

// before
reflectx.MakeFunc2x2(func(a, b int) int { return a + b }) // panics: 1 output
// after
reflectx.MakeFunc2x2(func(a, b int) (int, int) { return a + b, a - b })

Defensive patterns

Strategy: validation

Validate before calling

t := c.Type()
if t.NumIn() != 2 || t.NumOut() != 2 {
    return fmt.Errorf("func %v has %d ins/%d outs; ToFunc2x2 needs 2 ins/2 outs", t, t.NumIn(), t.NumOut())
}

Type guard

func isFunc2x2(c reflectx.Func) bool {
    return c.Type().NumIn() == 2 && c.Type().NumOut() == 2
}

Try / catch

func safeToFunc2x2(c reflectx.Func) (f reflectx.Func2x2) {
    defer func() {
        if r := recover(); r != nil {
            log.Fatalf("ToFunc2x2 arity mismatch: %v", r)
        }
    }()
    return reflectx.ToFunc2x2(c)
}

Prevention

Match inputs and outputs counts explicitly when writing 2x2 functions.
Update variant selection whenever a return value is added or removed.
Include wrapper construction in CI tests.

When it happens

Trigger: Calling reflectx.ToFunc2x2 with a Func whose Type().NumIn() != 2 or Type().NumOut() != 2.

Common situations: Binary functions updated to return an extra value; copy-pasted adapter calls between arity variants; merge functions whose signature changed during refactoring.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/905880f6ef73aaea. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:557

}

func (c *shimFunc2x2) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc2x2) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc2x2) Call2x2(arg0, arg1 any) (any, any) {
	ret := c.inner.Call([]any{arg0, arg1})
	_ = ret
	return ret[0], ret[1]
}

func ToFunc2x2(c Func) Func2x2 {
	if c.Type().NumIn() != 2 || c.Type().NumOut() != 2 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 2 inputs and 2 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func2x2); ok {
		return sc
	}
	return &shimFunc2x2{inner: c}
}

func MakeFunc2x2(fn any) Func2x2 {
	return ToFunc2x2(MakeFunc(fn))
}

type Func2x3 interface {
	Func
	Call2x3(any, any) (any, any, any)
}

type shimFunc2x3 struct {
	inner Func

View on GitHub (pinned to 12126d8942)