to_reduced_units() fails with g*t/kg

[brwe]

In am trying to run the following example:

import pint

ureg = pint.UnitRegistry()
 
ridiculous_factor = 0.5 * ureg.g * ureg.t / ureg.kg
ridiculous_factor.to_reduced_units()

But this fails with:

---------------------------------------------------------------------------
DimensionalityError                       Traceback (most recent call last)
<ipython-input-211-e3a1dda5eff7> in <module>
      4 
      5 ridiculous_factor = 0.5 * ureg.g * ureg.t / ureg.kg
----> 6 ridiculous_factor.to_reduced_units()

~/.local/lib/python3.9/site-packages/pint/quantity.py in to_reduced_units(self)
    800         # can we make this more efficient?
    801         newq = copy.copy(self)
--> 802         newq.ito_reduced_units()
    803         return newq
    804 

~/.local/lib/python3.9/site-packages/pint/quantity.py in ito_reduced_units(self)
    790                         break
    791 
--> 792         return self.ito(newunits)
    793 
    794     def to_reduced_units(self) -> Quantity[_MagnitudeType]:

~/.local/lib/python3.9/site-packages/pint/quantity.py in ito(self, other, *contexts, **ctx_kwargs)
    699         other = to_units_container(other, self._REGISTRY)
    700 
--> 701         self._magnitude = self._convert_magnitude(other, *contexts, **ctx_kwargs)
    702         self._units = other
    703 

~/.local/lib/python3.9/site-packages/pint/quantity.py in _convert_magnitude(self, other, *contexts, **ctx_kwargs)
    678                 return self._REGISTRY.convert(self._magnitude, self._units, other)
    679 
--> 680         return self._REGISTRY.convert(
    681             self._magnitude,
    682             self._units,

~/.local/lib/python3.9/site-packages/pint/registry.py in convert(self, value, src, dst, inplace)
   1001             return value
   1002 
-> 1003         return self._convert(value, src, dst, inplace)
   1004 
   1005     def _convert(self, value, src, dst, inplace=False, check_dimensionality=True):

~/.local/lib/python3.9/site-packages/pint/registry.py in _convert(self, value, src, dst, inplace)
   1915                 value, src = src._magnitude, src._units
   1916 
-> 1917         return super()._convert(value, src, dst, inplace)
   1918 
   1919     def _get_compatible_units(self, input_units, group_or_system):

~/.local/lib/python3.9/site-packages/pint/registry.py in _convert(self, value, src, dst, inplace)
   1516 
   1517         if not (src_offset_unit or dst_offset_unit):
-> 1518             return super()._convert(value, src, dst, inplace)
   1519 
   1520         src_dim = self._get_dimensionality(src)

~/.local/lib/python3.9/site-packages/pint/registry.py in _convert(self, value, src, dst, inplace, check_dimensionality)
   1034             # then the conversion cannot be performed.
   1035             if src_dim != dst_dim:
-> 1036                 raise DimensionalityError(src, dst, src_dim, dst_dim)
   1037 
   1038         # Here src and dst have only multiplicative units left. Thus we can

DimensionalityError: Cannot convert from 'gram * metric_ton / kilogram' ([mass]) to 'dimensionless' (dimensionless)

If I replace the g with kg all works as expected.
Version is v0.18

Any pointer appreciated.

[jules-ch]

It is a bug related to to_reduced_units it does not handle well 3 units with the same dimensionality it seems.

[jules-ch]

Duplicate #774