Go: unexported interface methods "seal" the interface support when embedding types

main.go

// This code demonstrates how you can embed a type,
// and have it satisfy an interface (`ProblemDetailer`)
// but only for the embedded type.
// This works because the interface uses an unexported method,
// that is part of the embedded type's package.
// https://go.dev/play/p/7lbikIyPHi1
//
// IMPORTANT: If the `isProblem` inteface method was exported (i.e `IsProblem`),
//            then our `Fake` struct would pass the `.(errorsx.ProblemDetailer)` type assertion
//            e.g. https://go.dev/play/p/pPD3OgtiyHx
package main

import (
	"fmt"

	"play.ground/errorsx"
)

// Embeds Problem — should satisfy ProblemDetailer via promoted method.
type OrderProblem struct {
	errorsx.Problem
	OrderID string
}

// Independently defines isProblem() — should NOT satisfy ProblemDetailer
// because unexported methods are package-scoped.
type Fake struct{}

func (Fake) isProblem() {}

func main() {
	// 1. Pointer to Problem
	var p any = &errorsx.Problem{}
	_, ok := p.(errorsx.ProblemDetailer)
	fmt.Printf("*Problem      satisfies ProblemDetailer: %v\n", ok)

	// 2. Value Problem
	var v any = errorsx.Problem{}
	_, ok = v.(errorsx.ProblemDetailer)
	fmt.Printf(" Problem      satisfies ProblemDetailer: %v\n", ok)

	// 3. Pointer to embedding type
	var op any = &OrderProblem{}
	_, ok = op.(errorsx.ProblemDetailer)
	fmt.Printf("*OrderProblem satisfies ProblemDetailer: %v\n", ok)

	// 4. Value embedding type
	var ov any = OrderProblem{}
	_, ok = ov.(errorsx.ProblemDetailer)
	fmt.Printf(" OrderProblem satisfies ProblemDetailer: %v\n", ok)

	// 5. Fake with its own isProblem() — different package identity
	var f any = Fake{}
	_, ok = f.(errorsx.ProblemDetailer)
	fmt.Printf(" Fake         satisfies ProblemDetailer: %v\n", ok)

	// 6. Pointer to Fake
	var fp any = &Fake{}
	_, ok = fp.(errorsx.ProblemDetailer)
	fmt.Printf("*Fake         satisfies ProblemDetailer: %v\n", ok)
}
-- go.mod --
module play.ground

go 1.21
-- errorsx/errorsx.go --
package errorsx

// ProblemDetailer identifies RFC 7807 Problem Details payloads.
// Unexported method seals the interface — only types embedding
// Problem (or defined in this package) can satisfy it.
type ProblemDetailer interface {
	isProblem()
}

type Problem struct {
	Title  string
	Detail string
}

// Value receiver: both Problem and *Problem satisfy ProblemDetailer.
func (Problem) isProblem() {}