Why Can't I Use a Go Interface with Type Constraints in a Conversion?

Understanding Type Constraints in Go Interfaces

Go interfaces are powerful tools for abstracting data types and promoting code reuse. However, the use of interfaces is constrained by certain rules, one of which is the prohibition of interfaces containing type elements.

In the given code snippet, the error "interface contains type constraints: cannot use interface Number in conversion" arises due to this restriction. The Number interface, defined as type Number interface { int | int64 | float64 }, defines a union of three specific types. This makes Number a non-basic interface, meaning it cannot be used as the type of a value or variable.

The language specification disallows using non-basic interfaces in type conversions or as elements of other types. The a variable cannot be initialized as []Number{Number(1), Number(2), Number(3), Number(4)} because Number cannot be used to construct a new value.

To overcome this limitation, you can use the interface{} type, which can hold values of any type. The aa variable is successfully created as []interface{}{interface{}(1), interface{}(2), interface{}(3), 4} because the elements are not restricted by the type constraints of Number.

Alternatively, you can define an interface without type constraints, such as NNumber, and use it to create type-safe slices. The aaa variable is initialized as []NNumber{NNumber(1), NNumber(2), NNumber(3), 4} without errors because NNumber does not contain type restrictions.

By understanding the concepts of basic and non-basic interfaces and the limitations surrounding them, you can effectively utilize Go interfaces while avoiding potential issues related to type constraints.

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