Add a regression test for #15979, and fix linecount-report for Windows (#20111)

#16019, which fixed this issue (for
linux) was not accompanied by a regression test. Thus, nobody noticed
that it doesn't work on Windows!

This fixes an Internal Error where namespace packages were not supported
properly. This fix was inspired by @sterliakov noticing that
#18128 was very similar to #19843,
which has a similar fix. Note that we use `os.path.isdir(tree.path)`
instead of trying to catch an `IsADirectoryError` exception because of a
bug on Windows which causes it to throw a `PermissionError` instead in
[the relevant
situation](https://discuss.python.org/t/permissionerror-errno-13-permission-denied-python-2023/22360/8),
which makes `except IsADirectoryError` unreliable. (We also can't just
`except (IsADirectoryError, PermissionError)` because what if there is
an actual permission error?)

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ivan Levkivskyi <[email protected]>

Author: 3 people

mypy/report.py

@@ -169,11 +169,10 @@ def on_file(
     ) -> None:
         # Count physical lines.  This assumes the file's encoding is a
         # superset of ASCII (or at least uses \n in its line endings).
-        try:
+        if not os.path.isdir(tree.path):  # can happen with namespace packages
             with open(tree.path, "rb") as f:
                 physical_lines = len(f.readlines())
-        except IsADirectoryError:
-            # can happen with namespace packages
+        else:
             physical_lines = 0
 
         func_counter = FuncCounterVisitor()

test-data/unit/reports.test

@@ -548,6 +548,13 @@ namespace_packages = True
 </body>
 </html>
 
+[case testLinecountReportCrashOnNamespacePackages]
+# cmd: mypy --linecount-report report -p folder
+-- Regression test for https://github.com/python/mypy/issues/15979
+[file folder/subfolder/something.py]
+class Something:
+    pass
+
 [case testReportIsADirectoryErrorCrashOnNamespacePackages]
 # cmd: mypy --linecoverage-report report -p folder
 -- Regression test for https://github.com/python/mypy/issues/18128