Why Does a Type Switch with Multiple Cases in Go Cause 'undefined' Variable Errors, and How Can This Be Resolved?

Type Switch with Multiple Cases in Go

When using a type switch with multiple cases, one may encounter an error stating that a variable in a case with multiple types is undefined. This behavior stems from the Go language specification, which dictates that a type switch guard may include a short variable declaration.

In such cases, the variable has the same type as the type listed in single-type cases. However, in cases with a listing of multiple types, the variable has the type of the expression in the type switch guard.

To illustrate this, consider the following code:

type A struct {
  a int
}

func (this *A) test() {
  fmt.Println(this)
}

type B struct {
  A
}

var foo interface{}
foo = A{}

switch a := foo.(type) {
  case B, A:
    a.test()
}

Running this code will result in the error "a.test undefined (type interface {} is interface with no methods)." This is because the variable a has the type interface{}, not the type of the specific case.

To resolve this issue, one can assert that the type switch guard expression has the expected method. For instance:

type tester interface {
  test()
}

func main() {
  var foo interface{}
  foo = &B{}

  if a, ok := foo.(tester); ok {
    fmt.Println("foo has test() method")
    a.test()
  }
}

This code first checks if foo has the test() method, and if it does, it assigns the value of foo to a and calls the test() method.

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