Proposed resolution:

This wording is relative to N4762.

1. Change [time.syn], header <chrono> synopsis, as indicated:

   […]
   // [time.clock.file], classtype file_clock
   classusing file_clock = see below;
   […]
   

2. Change [time.clock.file] as indicated:

   23.17.7.5 ClassType file_clock [time.clock.file]

3. Change [time.clock.file.overview], class file_clock synopsis, as indicated:

   namespace std::chrono {
     using file_clock = see below;
     class file_clock {
     public:
       using rep = a signed arithmetic type;
       using period = ratio<unspecified, unspecified>;
       using duration = chrono::duration<rep, period>;
       using time_point = chrono::time_point<file_clock>;
       static constexpr bool is_steady = unspecified;
       
       static time_point now() noexcept;
       
       // Conversion functions, see below
     };
   }
   

   -1- The clock file_clock is an alias for a type meeting the Cpp17TrivialClock requirements ([time.clock.req]), and using a signed arithmetic type for file_clock::rep. file_clock is used to create the time_point system used for file_time_type ([filesystems]). Its epoch is unspecified, and noexcept(file_clock::now()) is true. [Note: The type that file_clock denotes may be in a different namespace than std::chrono, such as std::filesystem. — end note]

4. Change [time.clock.file.members] as indicated:

   static time_point now();
   

   -1- Returns: A file_clock::time_point indicating the current time.

   -2- The type file_clock shalltype denoted by file_clock provides precisely one of the following two sets of static member functions: […]

[ 2018-11, Adopted in San Diego ]

[ 2018-08-23 Batavia Issues processing: Minor wording changes, and status to "Tentatively Ready". ]

[ 2018-07-30 Priority set to 1 after reflector discussion; wording updates based on several discussion contributions. ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4762.

1. Change [time.syn], header <chrono> synopsis, as indicated:

   […]
   // [time.clock.file], classtype file_clock
   classusing file_clock = see below;
   […]
   

2. Change [time.clock.file] as indicated:

   23.17.7.5 ClassType file_clock [time.clock.file]

3. Change [time.clock.file.overview], class file_clock synopsis, as indicated:

   namespace std::chrono {
     using file_clock = see below;
     class file_clock {
     public:
       using rep = a signed arithmetic type;
       using period = ratio<unspecified, unspecified>;
       using duration = chrono::duration<rep, period>;
       using time_point = chrono::time_point<file_clock>;
       static constexpr bool is_steady = unspecified;
       
       static time_point now() noexcept;
       
       // Conversion functions, see below
     };
   }
   

   -1- The clock file_clock is an alias for a type meeting the TrivialClock requirements ([time.clock.req]), which uses a signed arithmetic type for file_clock::rep. file_clock is used to create the time_point system used for file_time_type ([filesystems]). Its epoch is unspecified, and noexcept(file_clock::now()) is true. [Note: The type file_clock denotes may be in a different namespace than std::chrono, such as std::filesystem. — end note]

4. Change [time.clock.file.members] as indicated:

   static time_point now();
   

   -1- Returns: A file_clock::time_point indicating the current time.

   -2- The class file_clock shalltype denoted by file_clock provides precisely one of the following two sets of static member functions: […]

It was pointed out in one of Eric's changes to libc++ here that P0355 adds file_clock, which is intended to be the clock used for std::filesystem::file_time_type's clock.

Unfortunately, this is an ABI break for implementations that are already shipping C++17 filesystem that did not call their clock type std::file_clock. For example, MSVC++'s is called std::filesystem::_File_time_clock.

We can keep much the same interface of P0355 by making file_clock a typedef for an unspecified type. This technically changes the associated namespaces for expressions using that clock for the sake of ADL, but I can't imagine a user who cares, as clocks aren't generally called in ADL-able expressions, durations and time_points are.

Previous resolution [SUPERSEDED]:

This wording is relative to N4750.

1. Change [time.syn], header <chrono> synopsis, as indicated:

   […]
   // [time.clock.file], classtype file_clock
   classusing file_clock = unspecified;
   […]
   

2. Change [time.clock.file] as indicated:

   23.17.7.5 ClassType file_clock [time.clock.file]

3. Change [time.clock.file.overview], class file_clock synopsis, as indicated:

   namespace std::chrono {
     using file_clock = see below;
     class file_clock {
     public:
       using rep = a signed arithmetic type;
       using period = ratio<unspecified, unspecified>;
       using duration = chrono::duration<rep, period>;
       using time_point = chrono::time_point<file_clock>;
       static constexpr bool is_steady = unspecified;
       
       static time_point now() noexcept;
       
       // Conversion functions, see below
     };
   }
   

   -1- The clock file_clock is an alias for a type meeting the TrivialClock requirements ([time.clock.req]), uses a signed arithmetic type for file_clock::rep, and is used to create the time_point system used for file_time_type ([filesystems]). Its epoch is unspecified. [Note: The type file_clock denotes may be in a different namespace than std::chrono, such as std::filesystem. — end note]

4. Change [time.clock.file.members] as indicated:

   static time_point now();
   

   -1- Returns: A file_clock::time_point indicating the current time.

   -2- The class file_clock shalltype denoted by file_clock provides precisely one of the following two sets of static member functions: […]