Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core)
+ homeassistant/util/async_.py:158: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
+ homeassistant/util/async_.py:164: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
+ homeassistant/helpers/frame.py:112: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]

Arguably these are true positives: line can clearly be None

Diff from mypy_primer, showing the effect of this PR on open source code:

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/debug/tbtools.py:378: error: Unsupported operand types for - ("None" and "int")  [operator]
+ src/werkzeug/debug/tbtools.py:378: note: Left operand is of type "Optional[int]"

pyp (https://github.com/hauntsaninja/pyp)
+ pyp.py:641: error: Argument 1 to "code_for_line" has incompatible type "Optional[int]"; expected "int"

core (https://github.com/home-assistant/core)
+ homeassistant/util/async_.py:158: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
+ homeassistant/util/async_.py:164: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
+ homeassistant/helpers/frame.py:112: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]

@AlexWaygood any idea under what circumstances lineno can be None?

edit: I got misled by something, removed some noise

In practice it looks like lineno's in traceback come from https://github.com/python/cpython/blob/b27195332e91e932501f16cf9877761b218a9c99/Lib/traceback.py#L329

which we have as an int

https://github.com/python/cpython/blob/b27195332e91e932501f16cf9877761b218a9c99/Lib/traceback.py#L305 looks pretty reachable to me, but I'll confess I don't know what real-life code ends up with us getting there :)

So should we be changing TracebackType.tb_lineno?

I don't know. I'm not an exception-handling expert, and the language reference doesn't shed much light here. Feel free to revert the int -> int | None changes if you're confident I'm incorrect here :)

This might have something to do with #6769. Presumably traceback line numbers come from stack frames.