TabError and IndentationError from line continuation

[ays684]

Bug report

Bug description:

The documentation states:

Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.

When this rule applies to some, but not all, lines in a block, a TabError is sometimes unexpectedly raised:

L1 = 'if 1:\n'
L2 = '    1\n'
L3 = '    \\\n'
L4 = '        1\n'
exec(L1 + L2 + L3 + L4)
# works as expected

L1 = 'if 1:\n'
L2 = '\t1\n'
L3 = '\t\\\n'
L4 = '\t\t1\n'
exec(L1 + L2 + L3 + L4)
# TabError: inconsistent use of tabs and spaces in indentation

Also, when a line containing only a backslash is explicitly joined with an indented line, an IndentationError is unexpectedly raised:

exec('\\\n    1')
# IndentationError: unexpected indent

CPython versions tested on:

3.10, 3.11, 3.12, 3.13, 3.14

Operating systems tested on:

macOS

[picnixz]

I think the last point is actually the expected behavior, at least to me. The continuation does not continue anything amd then we have a newline and some spaces so the indentation is wrong.

[picnixz]

As for the mixed tabs error it does look like an issue.

[picnixz]

cc @pablogsal