V8025. The case in a type switch is unreachable. A previous case already covers this type.

The analyzer has detected a case clause that will never be executed in a type switch construct. One of the previous case clauses has already handled the type expected by this case clause.

The example:

type Value interface {
  GetValue() string
}

type ConcreteValue struct {
  value string
}

func (v *ConcreteValue) GetValue() string {
  return v.value
}

....

func foo(x any) {
  switch x.(type) {
    case Value:
      ....
    case *ConcreteValue: // <=
      ....
  }
}

The first case clause checks whether x implements the Value interface, and the second one checks whether x has the *ConcreteValue type. The Go specification states that the appropriate execution block is determined by traversing the list of types in the case clauses left-to-right and top-to-bottom. Upon the first match, the control flow will pass to the corresponding block. Since the *ConcreteValue type implements the Value interface, objects of this type will be intercepted by the first case clause, rather than the second, as developers intended.

To avoid type-masking issues, always use specific types (or pointers to them) instead of general interfaces:

func foo(x any) {
  switch x.(type) {
    case *ConcreteValue:
      ....
    case Value:
      ....
  }
}

This diagnostic is classified as:

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