How Can Multiple Cases in Go Type Switches Lead to Unexpected Behavior?

Multiple Cases in Go Type Switches

When assigning an interface value to a type-switched variable, the resulting type depends on the case list structure. Multiple case listings in a switch-case statement may cause unexpected behavior.

In the code snippet you provided:

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

The error arises because the type of the variable a is interface{}, not A or B. This is because the case list contains multiple types, so the type of a remains the same as the type of the interface expression (foo).

To resolve this, the case list should contain only a single type:

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

By limiting the case list to a specific type, the variable a will have the expected type, allowing the method call to succeed.

Alternatively, you can explicitly assert the interface type using an assertion expression:

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

In this case, the variable a will only have the expected type if the assertion is successful (i.e., ok is true).

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