E097: A generic operator's type parameter has to be tied to its operands

A generic operator is written operator<T> ...: one definition that works for many types. Turf decides which T to use by looking at the operands at each use, so every type parameter has to appear in the operands, and at least one operand has to be a type that this module owns.

A parameter no operand mentions can never be decided:

struct Cell<T> { v: T, n: int }operator<T> [](p: Cell<int>, i: int): T {    // T appears only in the result    return p.v;}

Tie it to an operand:

operator<T> [](p: Cell<T>, i: int): T {    return p.v;}

And an operator whose operands are all bare T would redefine + for every type in the program, including ones this module doesn’t own, so that is rejected too:

operator<T> +(a: T, b: T): T { return a; }   // for which types?

Make at least one operand a type you declared – Cell<T>, Vec<T>, your own struct.

Run turf --explain E097 in a terminal to read this again.