refactor[cartesian]: Proper error messaging for functions in ifs

src/gt4py/cartesian/frontend/gtscript_frontend.py

@@ -1339,9 +1339,17 @@ def visit_If(self, node: ast.If) -> list:
             self.decls_stack[-2].extend(self.decls_stack[-1])
             self.decls_stack.pop()
 
+        try:
+            condition = self.visit(node.test)
+        except KeyError as e:
+            raise GTScriptSyntaxError(
+                message="Using function calls in the condition of an if is not allowed,"
+                + " the function needs to be assigned to a variable outside the condition.",
+                loc=nodes.Location.from_ast_node(node),
+            ) from e
         result.append(
             nodes.If(
-                condition=self.visit(node.test),
+                condition=condition,
                 loc=nodes.Location.from_ast_node(node),
                 main_body=nodes.BlockStmt(stmts=main_stmts, loc=nodes.Location.from_ast_node(node)),
                 else_body=(

tests/cartesian_tests/unit_tests/frontend_tests/test_gtscript_frontend.py

@@ -17,6 +17,7 @@
 import gt4py.cartesian.definitions as gt_definitions
 from gt4py.cartesian import gtscript
 from gt4py.cartesian.frontend import gtscript_frontend as gt_frontend, nodes
+from gt4py.cartesian.frontend.exceptions import GTScriptSyntaxError
 from gt4py.cartesian.gtscript import (
     __INLINED,
     FORWARD,
@@ -1719,6 +1720,26 @@ def func(field: gtscript.Field[np.float64]):
             )
 
 
+@gtscript.function
+def boolean_return(a):
+    return a == 1
+
+
+class TestFunctionIfError:
+    def test_function_if_error(self):
+        def func(field: gtscript.Field[np.float64]):  # type: ignore
+            with computation(PARALLEL), interval(...):
+                field = 0
+                if boolean_return(field):
+                    field = 1
+
+        with pytest.raises(
+            GTScriptSyntaxError,
+            match="Using function calls in the condition of an if is not allowed",
+        ):
+            parse_definition(func, name=inspect.stack()[0][3], module=self.__class__.__name__)
+
+
 class TestAnnotations:
     @staticmethod
     def sumdiff_defs(