Add check for delta unit to convert

CHANGES

@@ -4,8 +4,7 @@ Pint Changelog
 0.24 (unreleased)
 -----------------
 
-- Nothing changed yet.
-
+- Fix detection of invalid conversion between offset and delta units. (PR #1905)
 
 0.23 (2023-12-08)
 -----------------

pint/facets/nonmultiplicative/registry.py

@@ -120,7 +120,7 @@ def _is_multiplicative(self, unit_name: str) -> bool:
         Raises
         ------
         UndefinedUnitError
-            If the unit is not in the registyr.
+            If the unit is not in the registry.
         """
         if unit_name in self._units:
             return self._units[unit_name].is_multiplicative
@@ -250,6 +250,7 @@ def _convert(
                 src, dst, extra_msg=f" - In destination units, {ex}"
             )
 
+        # convert if no offset units are present
         if not (src_offset_unit or dst_offset_unit):
             return super()._convert(value, src, dst, inplace)
 
@@ -263,13 +264,17 @@ def _convert(
 
         # clean src from offset units by converting to reference
         if src_offset_unit:
+            if any(u.startswith("delta_") for u in dst):
+                raise DimensionalityError(src, dst)
             value = self._units[src_offset_unit].converter.to_reference(value, inplace)
             src = src.remove([src_offset_unit])
             # Add reference unit for multiplicative section
             src = self._add_ref_of_log_or_offset_unit(src_offset_unit, src)
 
         # clean dst units from offset units
         if dst_offset_unit:
+            if any(u.startswith("delta_") for u in src):
+                raise DimensionalityError(src, dst)
             dst = dst.remove([dst_offset_unit])
             # Add reference unit for multiplicative section
             dst = self._add_ref_of_log_or_offset_unit(dst_offset_unit, dst)

pint/testsuite/test_unit.py

@@ -978,6 +978,8 @@ class TestConvertWithOffset(QuantityTestCase):
         (({"degC": 2}, {"kelvin": 2}), "error"),
         (({"degC": 1, "degF": 1}, {"kelvin": 2}), "error"),
         (({"degC": 1, "kelvin": 1}, {"kelvin": 2}), "error"),
+        (({"delta_degC": 1}, {"degF": 1}), "error"),
+        (({"delta_degC": 1}, {"degC": 1}), "error"),
     ]
 
     @pytest.mark.parametrize(("input_tuple", "expected"), convert_with_offset)