How to Avoid IncompatibleAssign Errors When Using Mixed Type Constraints in Go Generics?

Managing IncompatibleAssignErrors in Generic Types with Mixed Value Constraints

In Go, generics allow for the creation of types with specific constraints on their fields. However, assigning value literals to struct fields can lead to IncompatibleAssign errors when mixing different type groups in the constraints.

Consider this example:

type constraint interface {
    ~float32 | ~float64
}

type foo[T constraint] struct {
    val T
}

func (f *foo[float64]) setValToPi() {
    f.val = 3.14
}

This code assigns the literal 3.14 to the val field of a foo[float64] type. This is acceptable because 3.14 is a valid float64 value.

However, the error arises when the constraint is extended to include int types:

type constraint interface {
    ~float32 | ~float64 | ~int
}

type foo[T constraint] struct {
    val T
}

func (f *foo[float64]) setValToPi() {
    f.val = 3.14 // IncompatibleAssign: cannot use 3.14 (untyped float constant) as float64 value in assignment
}

This error originates from the method declaration:

func (f *foo[float64]) setValToPi() {
    // ...
}

Here, float64 is treated as a type parameter name rather than the intended constraint. Consequently, the compiler cannot determine the specific type of the val field and cannot verify that 3.14 is compatible with all possible constraint instances.

To resolve this, it is essential to use a generic method parameter to accept a value of the type parameter type instead:

func (f *foo[T]) SetValue(val T) {
    f.val = val
}

This solution prevents assigning fixed values like 3.14, which is an inappropriate practice in this context. Alternative approaches involve using any/interface{} as the field type.

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