What does the ~ (tilde) token mean in a constraint? What happens without it?

~T means "every type whose underlying type is T". Without the tilde, only the exact type T itself matches. Named types such as type Celsius float64 or time.Duration (underlying int64) are then rejected.

type Exact interface{ int64 }
type Approx interface{ ~int64 }

func A[T Exact](v T) T  { return v }
func B[T Approx](v T) T { return v * 2 }

// A(time.Second)            // error: time.Duration does not satisfy Exact
fmt.Println(B(time.Second))  // 2s  (result is time.Duration, not int64)

type Name string
func Upper[T ~string](s T) T { return T(strings.ToUpper(string(s))) }
fmt.Printf("%T\n", Upper(Name("go"))) // main.Name

Rules and gotchas:

• In ~T, T must be its own underlying type. ~time.Duration is illegal, and so is ~error or any other interface.
• Inside the function you usually need an explicit conversion to call non-generic APIs, as in string(s) above. The result then keeps the caller's named type.
• Library constraints such as cmp.Ordered use ~ everywhere. That is why sorting works on named types.

Interviewers want to hear that you understand the difference between underlying type and named type, and why the result type is preserved.