Hat tip to Mike Ash.

func 📝<🌈>(😍: 🌈) {print(😍)} // Works
📝("💤")

// U+2764 is outside of the range of the allowed characters for identifiers
// func 📝📝<🌈>(❤️: 🌈) {print(❤️)} // Doesn't 
// 📝📝("💤")