  x Optional longer message describing the error.
   ,-[4:12]
 3 | 
 4 | def foo(x: blah) -> None:
   :            ^^|^
   :              `-- Primary label giving some source-dependent context
 5 |     print('function')
 6 |     x = bar() + baz
   :         ^^|^^
   :           `-- Sub-diagnostic label for additional context
   `----
  help: Additional sub-diagnostic message without span