Basis spec incompatibility for OS.Path.joinDirFile?

According to my reading of the basis spec, evaluating this expression 
should raise InvalidArc:
    OS.Path.joinDirFile {dir = "a", file = "b/c"}

However, in SML/NJ 110.60, it acts like OS.Path.concat, returning the 
two arguments separated by "/".  InvalidArc is raised as expected in MLton.

The online Basis manual says:

joinDirFile {dir, file}
    creates a whole path out of a directory and a file by extending the 
path dir with the arc file. If the string file does not correspond to an 
arc, raises InvalidArc. The exception Size is raised if the resulting 
string would have size greater than String.maxSize.

Is this a bug in SML/NJ's implementation?

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