fix: Swap parser override order for macro IF under T-SQL (Issue #5823) (#5828)

Signed-off-by: Jagan Nalla <jagannalla1@gmail.com>

Author: jagannalla

sqlmesh/core/dialect.py

@@ -556,6 +556,10 @@ def _parse_if(self: Parser) -> t.Optional[exp.Expr]:
     # to parse a statement / command to support the macro @IF(condition, statement)
     index = self._index
     try:
+        if self.dialect == "tsql":
+            if not (self._index >= 2 and self._tokens[self._index - 2].text == "@"):
+                return self.__parse_if()  # type: ignore
+            return Parser.__parse_if(self)  # type: ignore
         return self.__parse_if()  # type: ignore
     except ParseError:
         self._retreat(index)
@@ -1133,8 +1137,8 @@ def extend_sqlglot() -> None:
     _override(Parser, _parse_value)
     _override(Parser, _parse_lambda)
     _override(Parser, _parse_types)
-    _override(TSQL.Parser, Parser._parse_if)
     _override(Parser, _parse_if)
+    _override(TSQL.Parser, Parser._parse_if)
     _override(Parser, _parse_id_var)
     _override(Parser, _parse_interval_span)
     _override(Parser, _warn_unsupported)

tests/core/test_dialect.py

@@ -709,6 +709,20 @@ def test_conditional_statement():
     q = parse_one("@IF(cond, VACUUM ANALYZE);", read="postgres")
     assert q.sql(dialect="postgres") == "@IF(cond, VACUUM ANALYZE)"
 
+    # Verify that the original error case from issue #5823 (Required keyword: 'true' missing) is resolved.
+    # It must be parsed as a macro function containing an Anonymous expression rather than exp.If.
+    q = parse_one("@IF(1 = 1, ALTER TABLE x ADD y INT);", read="tsql")
+    assert q.sql(dialect="tsql") == "@IF(1 = 1, ALTER TABLE x ADD y INTEGER)"
+    assert isinstance(q.this, exp.Anonymous)
+    assert q.this.name == "IF"
+
+    # Note: SQLGlot's fallback Command parser strips quotes from string literal tokens when parsing unparsed commands
+    q = parse_one("@IF(cond, PRINT 'hello');", read="tsql")
+    assert q.sql(dialect="tsql") == "@IF(cond, PRINT hello)"
+
+    q = parse_one("@IF(@runtime_stage = 'evaluating', SELECT 1);", read="tsql")
+    assert q.sql(dialect="tsql") == "@IF(@runtime_stage = 'evaluating', SELECT 1)"
+
 
 def test_model_name_cannot_be_string():
     with pytest.raises(ParseError) as parse_error: