Fixed #36606 -- Optimized QuerySet.values_list(flat=True) without fields.

Author: adamchainz

django/db/models/query.py

@@ -1416,11 +1416,14 @@ def values(self, *fields, **expressions):
     def values_list(self, *fields, flat=False, named=False):
         if flat and named:
             raise TypeError("'flat' and 'named' can't be used together.")
-        if flat and len(fields) > 1:
-            raise TypeError(
-                "'flat' is not valid when values_list is called with more than one "
-                "field."
-            )
+        if flat:
+            if len(fields) > 1:
+                raise TypeError(
+                    "'flat' is not valid when values_list is called with more than one "
+                    "field."
+                )
+            elif not fields:
+                fields = [self.model._meta.concrete_fields[0].attname]
 
         field_names = {f: False for f in fields if not hasattr(f, "resolve_expression")}
         _fields = []