fix(dialect): don't parse @@ system variables as macros

sqlmesh/core/dialect.py

@@ -218,6 +218,11 @@ def _parse_macro(self: Parser, keyword_macro: str = "") -> t.Optional[exp.Expr]:
     if self._prev.text != SQLMESH_MACRO_PREFIX:
         return self._parse_parameter()
 
+    # @@name is a system variable, e.g. T-SQL's @@DATEFIRST or BigQuery's @@query_label,
+    # and can't be a macro reference, so parse it like the dialect would
+    if self._curr and self._curr.text == SQLMESH_MACRO_PREFIX and self._match(TokenType.PARAMETER):
+        return self.expression(exp.Parameter(this=self._parse_parameter()))
+
     comments = self._prev.comments
     index = self._index
     field = self._parse_primary() or self._parse_function(functions={}) or self._parse_id_var()

tests/core/test_dialect.py

@@ -1324,6 +1324,38 @@ def test_macro_parse():
     )
 
 
+@pytest.mark.parametrize(
+    "dialect, sql",
+    [
+        ("tsql", "SELECT @@DATEFIRST AS x"),
+        ("bigquery", "SELECT @@query_label AS x"),
+        ("mysql", "SELECT @@session.time_zone AS x"),
+    ],
+)
+def test_double_at_system_variable_is_not_a_macro(dialect: str, sql: str):
+    query = parse_one(sql, read=dialect)
+    assert not list(query.find_all(d.MacroVar))
+    assert query.sql(dialect) == sql
+
+
+def test_double_at_system_variable_in_model():
+    model = load_sql_based_model(
+        parse(
+            """
+            MODEL (name db.m, dialect tsql);
+            SELECT (DATEPART(WEEKDAY, d) + @@DATEFIRST - 2) % 7 + 1 AS iso_weekday, @x AS x FROM t
+            """,
+            default_dialect="tsql",
+        ),
+        dialect="tsql",
+        variables={"x": 1},
+    )
+    assert model.render_query_or_raise().sql("tsql") == (
+        "SELECT (DATEPART(WEEKDAY, [d]) + @@DATEFIRST - 2) % 7 + 1 AS [iso_weekday], 1 AS [x] "
+        "FROM [t] AS [t]"
+    )
+
+
 def test_conditional_statement():
     q = parse_one(
         """