E4079

Cannot derive trait: method is already defined.

This error occurs when you try to derive a trait for a type that already has a implementation for the trait.

错误示例

impl Show for A with output(self : A, logger : &Logger) -> Unit {
  logger.write_object(self._)
}

type A Int derive(Hash, Show) // Error: Cannot derive trait Show for A: method output is already defined at ...

建议

You can pick either the manual implementation or the derived implementation for the type by removing the other one.

To use the derived implementation:

// Remove the manual implementation
// impl Show for A with output(self : A, logger : &Logger) -> Unit {
//   logger.write_object(self._)
// }

type A Int derive(Hash, Show)

Or to use the manual implementation:

impl Show for A with output(self : A, logger : &Logger) -> Unit {
  logger.write_object(self._)
}

type A Int derive(Hash)