At some point in 3.9 Python appears to have stopped accepting source that starts with an indent, then a '\', then the indented statement. From the lexical analysis [1] "Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation."

Running the attached program under 3.8.12 I get:
```
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
```

But running under 3.10.0 I get:
```
  File "/Users/jeremyp/tmp/nodent.py", line 3
    """Print a Fibonacci series up to n."""
    ^
IndentationError: expected an indented block after function definition on line 1
```

Running under 3.9.9 also gives an IndentationError, both with and without -X oldparser. So this doesn't seem directly related to the new parser, but seems likely it is fall out from the general grammar restructuring.

IMHO it isn't a particularly nice feature for the language to have. Especially since not all lines like '    \' behave the same. But it was there and documented for many years, so should probably be put back. Does a core developer agree? That the implementation is not following the spec?

[1]: https://docs.python.org/3/reference/lexical_analysis.html#indentation

This is a side effect on the fix in:

https://bugs.python.org/issue40847

Unfortunately I am not sure if this is going to be easier to retrofit for the reasons exposed in the issue. I will try to take a look, but if you have some cycles to spare, a PR would be welcomed.

nevermind, I am convinced is a bug. I filed a PR for it

Wow, this was a fast turnaround! I was going to spin some cycles on this, but would have not seen the solution in 50m.

New changeset a0efc0c1960e2c49e0092694d98395555270914c by Pablo Galindo Salgado in branch 'main':
bpo-46091: Correctly calculate indentation levels for whitespace lines with continuation characters (GH-30130)
https://github.com/python/cpython/commit/a0efc0c1960e2c49e0092694d98395555270914c

New changeset 3fc8b74ace033a17346a992f661928ba619e61e8 by Pablo Galindo Salgado in branch '3.10':
[3.10] bpo-46091: Correctly calculate indentation levels for whitespace lines with continuation characters (GH-30130). (GH-30898)
https://github.com/python/cpython/commit/3fc8b74ace033a17346a992f661928ba619e61e8