This isn't permitted, because it would require the type to have a method GreaterOrEqual(other SortableComparable) bool. But since SortableComparable embeds comparable, it may only be used as a type constraint, not as an ordinary type. So no type can ever have such a method.

But the error message could be better. CC @griesemer.

As an aside, the two programs you (@voldyman) are comparing are not the same: The first one has a different interface type for the other parameter than the second one.

Got it, in that case the error message while trying to create a implementation is also not clear.

https://go2goplay.golang.org/p/-knY7KsfF67

type sortable interface {
	GreaterOrEqual(other sortable) bool
}

type SortableComparable interface {
	comparable
	sortable
}

type Num struct {
	num int
}

func (n Num) GreaterOrEqual(s sortable) bool {
	other, ok := s.(Num)
	if !ok {
		return false
	}
	return n.num >= other.num
}

var _ SortableComparable = Num{2}

type checking failed for main
prog.go2:29:7: interface is (or embeds) comparable

related question: Is there a way to express "the type should have methods a & b, and also be allowed in the key of a map " with the current generics proposal?
(https://go2goplay.golang.org/p/MbM5zhGwMS9)

@voldyman Interfaces that contain type lists or embed comparable cannot be used as type of a variable (incl. parameter, result, field, etc.); it can only be used as a constraint. But map keys can be interfaces even if they don't embed comparable (that's always been the case). Taking your example and modifying it a bit shows how a SortableComparable can be used as a map key; though there is no way to define your own equality function that then would be used for the map. The latter would require a custom (generic) map, but that will be possible with generics.

Thanks for the detailed answer!

(I'll leave this issue open for the compiler error message but feel free to close it if not required)

Change https://go.dev/cl/410954 mentions this issue: go/types, types2: better error message for inavlid use of constraint type