Curl attempts to write file to "/index.html" instead of "index.html"
In fact, when merging the output-dir and the actual file name curl explicitly uses / as a path separator. This isn't universally portable, some platforms may use different path separator character (however / is used in most).
I expected the following
More consistent and platform agnostic behaviour. Perhaps it would be more portable to actually chdir to the --output-dir rather than trying to construct the path manually.
AmigaOS and compatibles could also be problematic in some cases as "foo/bar" is "foo/bar" but "foo//bar" is "foo/../bar" ... this normally is no issue except when something assumes that multiple // are "ignored". This can become problematic if code makes assumptions about this. Technically this specific use case isn't affected by this, I think.