Proposed resolution:

This wording is relative to N4727.

1. Edit [fs.op.create_directory] as indicated:

   bool create_directory(const path& p, const path& existing_p);
   bool create_directory(const path& p, const path& existing_p, error_code& ec) noexcept;
   

   -4- Effects: Establishes the postcondition by attempting to createCreates the directory p resolves to, with attributes copied from directory existing_p. The set of attributes copied is operating system dependent. Creation failure because p resolves to an existing directory shall not be treated asalready exists is not an error. [Note: For POSIX-based operating systems, the attributes are those copied by native API stat(existing_p.c_str(), &attributes_stat) followed by mkdir(p.c_str(), attributes_stat.st_mode). For Windows-based operating systems, the attributes are those copied by native API CreateDirectoryExW(existing_p.c_str(), p.c_str(), 0). — end note]

   -5- Postconditions: is_directory(p).

   […]

[ 2018-06 Rapperswil: Adopted ]

[ 2018-03-27 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

LWG 2935 clarified that create_directory is not supposed to report an error if exists(p), even if p is not a directory. However, the P/R there missed the existing_p overloads.