code: func (c *Converter) RegisterGeneratedUntypedConversionFunc(a, b interface{}, fn ConversionFunc) error {
  	return c.generatedConversionFuncs.AddUntyped(a, b, fn)
  }
docs: // RegisterGeneratedUntypedConversionFunc registers a function that converts between a and b by passing objects of those
  // types to the provided function. The function *must* accept objects of a and b - this machinery will not enforce
  // any other guarantee.