How to copy data from one type to another that has identical structures?

In the below code:

type ProductEntity struct {
	ID          int     `json:"id"`
	Name        string  `json:"name"`
	Description string  `json:"description"`
	Price       float32 `json:"price"`
	SKU         string  `json:"sku"`
	CreatedOn   string  `json:"-"`
	UpdatedOn   string  `json:"-"`
	DeletedOn   string  `json:"-"`
}

type ProductEntityList []*ProductEntity

type PostRequestModel struct {
	ID          int     `json:"id"`
	Name        string  `json:"name"`
	Description string  `json:"description"`
	Price       float32 `json:"price"`
	SKU         string  `json:"sku"`
	CreatedOn   string  `json:"-"`
	UpdatedOn   string  `json:"-"`
	DeletedOn   string  `json:"-"`
}

type RequestBody []*PostRequestModel

func convertModelToEntity(modelList RequestBody) ProductEntityList {

	// return entity.ProductEntityList(modelList) // type conversion error 

}

How to copy data from one type to another, that has identical structures? Because RequestBody and ProductEntityList are two different type definitions

If those type are truly equal, use type alias:

type PostRequestModel = ProductEntity

If you do so, you can simply convert from ProductEntityList to RequestBody (try it on the Go Playground):

func convertModelToEntity(modelList RequestBody) ProductEntityList {
    return ProductEntityList(modelList) // Works!!
}

If you can't use type alias, then you can't convert from one slice to another. You have to create a new slice. Note that you can convert the individual slice elements because the pointed struct types have identical fields. This is possible because Spec: Conversions:

> A non-constant value x can be converted to type T in any of these cases:
> - [...]
> - ignoring struct tags (see below), x's type and T are pointer types that are not defined types, and their pointer base types have identical underlying types.

So *ProductEntity is convertible to *PostRequestModel (and vice versa) because the underlying type of ProductEntity and PostRequestModel is the "same" struct type.

Try it on the Go Playground:

func convertModelToEntity(modelList RequestBody) ProductEntityList {
	r := make(ProductEntityList, len(modelList))
	for i, m := range modelList {
		r[i] = (*ProductEntity)(m)
	}
	return r
}

Also note that if RequestBody and ProductEntityList have identical memory layout (they do in your example), you may use package unsafe to simply convert them, but I'd rather avoid it (try it on the Go Playground):

func convertModelToEntity(modelList RequestBody) ProductEntityList {
	return *(*ProductEntityList)(unsafe.Pointer(&modelList))
}

Why to avoid this? Using package unsafe your app may become non-portable and the Go 1 compatibility guarantees may not apply to it. E.g. you may add a field to ProductEntity only but not to PostRequestModel. As a result, your app will continue to compile without errors, but may blow up at any time. Always look at package unsafe as the last resort.