main: improve message on `pytest path/to/a/directory::mytest`

The path part of a `<path>::part1::part2` style collection argument must
be a file, not a directory.

Previously this crashed with an uncool assert "invalid arg".

1 files changed, 8 insertions, 0 deletions

diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index 2abfffba0..7ff362c34 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -808,6 +808,7 @@ def resolve_collection_argument(
     found module.
 
     If the path doesn't exist, raise UsageError.
+    If the path is a directory and selection parts are present, raise UsageError.
     """
     strpath, *parts = str(arg).split("::")
     if as_pypath:
@@ -821,4 +822,11 @@ def resolve_collection_argument(
             else "file or directory not found: {arg}"
         )
         raise UsageError(msg.format(arg=arg))
+    if parts and fspath.is_dir():
+        msg = (
+            "package argument cannot contain :: selection parts: {arg}"
+            if as_pypath
+            else "directory argument cannot contain :: selection parts: {arg}"
+        )
+        raise UsageError(msg.format(arg=arg))
     return py.path.local(str(fspath)), parts