Why Can't I Use Interfaces with Type Constraints in Type Conversions in Go?

Interface Limitations and Type Constraints

Go interfaces offer type safety and code flexibility, but certain limitations apply regarding their usage. One common issue that developers encounter is the error "interface contains type constraints: cannot use interface in conversion" when attempting to use interfaces with type elements.

Understanding Type Constraints

Type constraints refer to the restrictions placed on the type of values that can implement an interface. In Go, only basic interfaces (those containing only methods) are allowed as type parameters or components of other interfaces. Interfaces that embed comparable types or other non-basic interfaces are considered non-basic themselves.

Reason for the Error

The provided code declares an interface Number that contains a union of int, int64, and float64 types. Since Number is non-basic due to its type constraints, it cannot be used in type conversions or as a slice element type.

Acceptable Usage of Interfaces with Type Constraints

While interfaces with type constraints cannot be used directly, they can still be utilized for their intended purpose: restricting the types that can implement a generic type or function. For example, the following code defines a generic struct and function using type constraints:

type Coordinates[T Number] struct {
    x, y T // T must be a type that satisfies the Number interface
}

func sum[T Number](a, b T) T { // T must be a type that satisfies the Number interface
    return a + b
}

Conclusion

In Go, understanding the limitations of interfaces, particularly those involving type constraints, is crucial for effective code development. Remembering that non-basic interfaces cannot be used in direct type conversions or as slice element types helps prevent errors and ensures code integrity.

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