f := new(big.Float)
	f.SetPrec(ConstPrec)
	switch u := constant.Val(v).(type) {
	case int64:
		f.SetInt64(u)
	case *big.Int:
		f.SetInt(u)
	case *big.Float:
		f.Set(u)
	case *big.Rat:
		f.SetRat(u)
	default:
		base.Fatalf("unexpected: %v", u)
	}
	return f
}

// ConstOverflow reports whether constant value v is too large
// to represent with type t.