FileManager.copyItems(atPath:toPath:) produces incorrect file names when dest doesn't have a trailing slash

[CraigSiemens]

When calling FileManager.copyItems(atPath:toPath:) with a source folder and a destination folder without a trailing slash, the copied files in the source directory will be renamed to have the destination directory name prepended to the file/directory names.

This is occurring on macOS 15.4.1 (24E263) and it didn’t occur in macOS 15.3 and earlier.

swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Target: arm64-apple-macosx15.0

Filed feedback FB17323332

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Script to reproduce the issue

mkdir -p a/b/c z
touch a/b/foo.txt
echo 'import Foundation; try FileManager.default.copyItem(atPath: "a/b/", toPath: "z/b")' | swift -

It will start with the following folder structure before preforming the copy

.
├── a
│   └── b
│       ├── c
│       └── foo.txt
└── z

After the copy, the resulting folder structure should be

.
├── a
│   └── b
│       ├── c
│       └── foo.txt
└── z
    └── b
        ├── c
        └── foo.txt

But it is actually the following, prepending the contained item names with the destination directory name.

.
├── a
│   └── b
│       ├── c
│       └── foo.txt
└── z
    ├── b
    ├── bc
    └── bfoo.txt

Adding a trailing slash to the toPath to become "z/b/" works around the issue.