Better unit test around at usage

Better error message.

Author: FlorianDeconinck

src/gt4py/cartesian/frontend/gtscript_frontend.py

@@ -1163,9 +1163,13 @@ def visit_Subscript(self, node: ast.Subscript):
                 field_axes = self.fields[result.name].axes
                 if index is not None:
                     if len(field_axes) != len(index):
+                        ro_field_message = ""
+                        if len(field_axes) == 0:
+                            ro_field_message = f"Did you mean .at{index}?"
                         raise GTScriptSyntaxError(
                             f"Incorrect offset specification detected. Found {index}, "
-                            f"but the field has dimensions ({', '.join(field_axes)})"
+                            f"but the field has dimensions ({', '.join(field_axes)}). "
+                            f"{ro_field_message}"
                         )
                     result.offset = {axis: value for axis, value in zip(field_axes, index)}
             elif isinstance(node.value, ast.Subscript) or _is_absolute_indexing_node(node):

tests/cartesian_tests/unit_tests/frontend_tests/test_gtscript_frontend.py

@@ -1477,16 +1477,47 @@ def at_write(
             with computation(PARALLEL), interval(...):
                 global_field.at[1, 0, 2] = in_field
 
-        parse_definition(data_dims, name=inspect.stack()[0][3], module=self.__class__.__name__)
+        def ROField_access_as_IJK(
+            out_field: gtscript.Field[gtscript.IJK, np.int32],
+            global_field: gtscript.ROField[(np.int32, (3, 3, 3))],
+        ):
+            with computation(PARALLEL), interval(...):
+                out_field = global_field[1, 0, 2]
+
+        def data_dims_with_at(
+            out_field: gtscript.Field[gtscript.IJK, np.int32],
+            global_field: gtscript.Field[(np.int32, (3, 3, 3))],
+        ):
+            with computation(PARALLEL), interval(...):
+                out_field = global_field.at[1, 0, 2]
 
+        # Check .at on read with a Field with data dimensions
+        parse_definition(
+            data_dims_with_at, name=inspect.stack()[0][3], module=self.__class__.__name__
+        )
+
+        # Check .at on read
         parse_definition(at_read, name=inspect.stack()[0][3], module=self.__class__.__name__)
 
+        # Check classic data dimensions are working
+        parse_definition(data_dims, name=inspect.stack()[0][3], module=self.__class__.__name__)
+
+        # Can't write to the field
         with pytest.raises(
             gt_frontend.GTScriptSyntaxError,
             match=r".*writing to a ROField \('at' global indexation\) is forbidden.*",
         ):
             parse_definition(at_write, name=inspect.stack()[0][3], module=self.__class__.__name__)
 
+        # Can't index cartesian style
+        with pytest.raises(
+            gt_frontend.GTScriptSyntaxError,
+            match="Incorrect offset specification detected. Found .* but the field has dimensions .* Did you mean .at",
+        ):
+            parse_definition(
+                ROField_access_as_IJK, name=inspect.stack()[0][3], module=self.__class__.__name__
+            )
+
 
 class TestNestedWithSyntax:
     def test_nested_with(self):