Allow type: ignore error codes filtering

Author: davidhalter

conformance/tests/directives_type_ignore.py

@@ -11,7 +11,7 @@
 y: int = ""  # type: ignore - additional stuff
 
 # The following type violation should be suppressed.
-z: int = ""  # type: ignore[additional_stuff]
+z: int = ""  # type: ignore[assignment]
 
 # > In some cases, linting tools or other comments may be needed on the same
 # > line as a type comment. In these cases, the type comment should be before

docs/spec/directives.rst

@@ -73,6 +73,13 @@ other comments and linting markers:
 
   # type: ignore # <comment or other marker>
 
+Any text following ``# type: ignore`` is ignored, provided there is at least one
+whitespace character after ``# type: ignore``. The form of
+``# type: ignore[error_code1, error_code2]`` can be used to filter error codes
+that may vary across type checkers. Type checkers may ignore the bracketed
+codes and treat this form as equivalent to ``# type: ignore``.
+
+
 .. _`cast`:
 
 ``cast()``