E115: An operator was declared with the wrong shape

An operator function teaches Turf what a symbol like + or [] means for your own type. Each operator has a fixed number of operands, and Turf checked yours against it.

fn operator+(a: Vec, b: Vec, c: Vec): Vec { ... }   // `+` takes two operandsfn operator[](self: Grid): int { ... }              // `[]` takes (self, index)fn operator[]=(self: Grid, i: int): void { ... }    // `[]=` takes (self, index, value)

Binary operators (+ - * / % ** & | ^ << >> == != < > <= >=) take exactly two parameters. A few have only one form: ! and ~ are unary (one parameter), and - can be either. The message tells you what the operator you wrote needs and how many you gave.

Fix the parameter list to match:

fn operator+(a: Vec, b: Vec): Vec { ... }

Run turf --explain E115 in a terminal to read this again.