[FXC-2300] Improved error message when using legacy output field names (#1366) (#1367)

Co-authored-by: Ben <106089368+benflexcompute@users.noreply.github.com>

Author: github-actions[bot]

flow360/component/simulation/user_code/core/types.py

@@ -529,7 +529,9 @@ def check_value_is_not_legacy_variable(cls, v):
 
         all_field_names = set(AllFieldNames.__args__)
         if v in all_field_names:
-            raise ValueError(f"'{v}' is a reserved (legacy) output field name.")
+            raise ValueError(
+                f"'{v}' is a reserved (legacy) output field name. It cannot be used in expressions."
+            )
         return v
 
     @pd.field_validator("name", mode="after")

tests/simulation/test_expressions.py

@@ -874,9 +874,18 @@ def test_unique_dimensions():
         ("template", "'template' is a reserved keyword."),
         ("temperature", "'temperature' is a reserved solver side variable name."),
         ("area", "'area' is a reserved solver side variable name."),
-        ("velocity", "'velocity' is a reserved (legacy) output field name."),
-        ("mut", "'mut' is a reserved (legacy) output field name."),
-        ("pressure", "'pressure' is a reserved (legacy) output field name."),
+        (
+            "velocity",
+            "'velocity' is a reserved (legacy) output field name. It cannot be used in expressions.",
+        ),
+        (
+            "mut",
+            "'mut' is a reserved (legacy) output field name. It cannot be used in expressions.",
+        ),
+        (
+            "pressure",
+            "'pressure' is a reserved (legacy) output field name. It cannot be used in expressions.",
+        ),
     ],
 )
 def test_invalid_names_raise(bad_name, expected_msg):