How do you write methods on a generic type? What is wrong with func (s Stack[int]) Sum() int?

The receiver must list the type's parameters by name, and you may rename them. You cannot repeat or tighten the constraints in the receiver, and you cannot specialize a method for one type argument.

type Stack[T any] struct{ items []T }

func (s *Stack[T]) Push(v T) { s.items = append(s.items, v) }
func (s *Stack[E]) Len() int { return len(s.items) } // renaming is allowed

// TRAP: this compiles, but "int" is a NEW type parameter name shadowing
// the predeclared int. It is NOT a specialization for Stack[int].
func (s Stack[int]) Top() int {
    return s.items[len(s.items)-1] // "int" here means T
}

Given Stack[string], Top() returns a string. If you write var total int = 0; total += x inside such a method, you get confusing errors, because int is now a type parameter constrained by any.

If you need behaviour that only exists for some type arguments, such as Sum on numbers, write a separate function with a narrower constraint:

func Sum[T cmp.Ordered | ~complex128](s *Stack[T]) T { var t T; for _, v := range s.items { t += v }; return t }

Interviewers like this question because the shadowing trap tells them who has actually written generic code.