Fix IsADirectoryError for namespace packages when using --linecoverage-report (#20109)

Fixes #18128. This fixes an Internal Error where namespace packages were
not supported properly. This fix was inspired by @sterliakov noticing
that this bug 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?) Anyway, we just early-return — which, based
on my manual testing and reading the code, seems to be the right thing
to do here.

---------

Co-authored-by: Ivan Levkivskyi <[email protected]>

Author: wyattscarpenter

mypy/report.py

@@ -421,6 +421,9 @@ def on_file(
         type_map: dict[Expression, Type],
         options: Options,
     ) -> None:
+        if os.path.isdir(tree.path):  # can happen with namespace packages
+            return
+
         with open(tree.path) as f:
             tree_source = f.readlines()
 

test-data/unit/reports.test

@@ -548,6 +548,14 @@ namespace_packages = True
 </body>
 </html>
 
+[case testReportIsADirectoryErrorCrashOnNamespacePackages]
+# cmd: mypy --linecoverage-report report -p folder
+-- Regression test for https://github.com/python/mypy/issues/18128
+-- "IsADirectoryError for namespace packages when using --linecoverage-report"
+[file folder/subfolder/something.py]
+class Something:
+    pass
+
 [case testReportCoberturaCrashOnNamespacePackages]
 # cmd: mypy --cobertura-xml-report report -p folder
 -- Regression test for https://github.com/python/mypy/issues/19843