From 444e54deedeb96ed0a8bc4c5926b209d5b4908ee Mon Sep 17 00:00:00 2001
From: Stephan Loyd <stephanloyd9@gmail.com>
Date: Wed, 11 May 2022 20:31:21 +0800
Subject: [PATCH] Suppress a possible relative_to() failure, fixes #9894

---
 AUTHORS                        | 1 +
 changelog/9894.improvement.rst | 1 +
 src/_pytest/pathlib.py         | 8 ++++++--
 testing/test_pathlib.py        | 5 +++++
 4 files changed, 13 insertions(+), 2 deletions(-)
 create mode 100644 changelog/9894.improvement.rst

diff --git a/AUTHORS b/AUTHORS
index 86a814a13..cdd84b9b5 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -312,6 +312,7 @@ Stefan Scherfke
 Stefan Zimmermann
 Stefano Taschini
 Steffen Allner
+Stephan Loyd
 Stephan Obermann
 Sven-Hendrik Haase
 Sylvain Marié
diff --git a/changelog/9894.improvement.rst b/changelog/9894.improvement.rst
new file mode 100644
index 000000000..551bbb9bd
--- /dev/null
+++ b/changelog/9894.improvement.rst
@@ -0,0 +1 @@
+Suppress a potential pathlib relative_to() error, which is possible to be raised from ExceptionInfo.getexpr() when running Python from double-slashed path (like //usr/bin/python3) and single-slashed tests path.
diff --git a/src/_pytest/pathlib.py b/src/_pytest/pathlib.py
index c5a411b59..5d48543ac 100644
--- a/src/_pytest/pathlib.py
+++ b/src/_pytest/pathlib.py
@@ -707,8 +707,12 @@ def bestrelpath(directory: Path, dest: Path) -> str:
     # Can be the case for a relative path and an absolute path.
     if not base:
         return str(dest)
-    reldirectory = directory.relative_to(base)
-    reldest = dest.relative_to(base)
+    try:
+        reldirectory = directory.relative_to(base)
+        reldest = dest.relative_to(base)
+    except ValueError:
+        # GH 9894
+        return str(dest)
     return os.path.join(
         # Back from directory to base.
         *([os.pardir] * len(reldirectory.parts)),
diff --git a/testing/test_pathlib.py b/testing/test_pathlib.py
index c901dc6f4..9d15fc39b 100644
--- a/testing/test_pathlib.py
+++ b/testing/test_pathlib.py
@@ -413,6 +413,11 @@ def test_bestrelpath() -> None:
     assert bestrelpath(curdir, curdir.parent) == ".."
     assert bestrelpath(curdir, Path("hello")) == "hello"
 
+    # GH 9894
+    directory = Path("/var/tmp/pytest-issue")
+    dest = Path("//usr/lib/python3.9/importlib/__init__.py")
+    assert bestrelpath(directory, dest) == str(dest)
+
 
 def test_commonpath() -> None:
     path = Path("/foo/bar/baz/path")