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Related to questions How do I check for C++11 support? and What is the value of __cplusplus for C++17?

How can I inquire whether the compiler can handle / is set up to use C++20? I know that it is in principle possible to inquire the C++ version by:

#if __cplusplus > ???

  // C++20 code here

#endif

What should ??? be for C++20?

It's too early for that.

Until the standard replaces it, use:

#if __cplusplus > 201703L

  // C++20 code

#endif

since the predefined macro of C++20 is going to be larger than the one of C++17.

As @SombreroChicken's answer mentions, [cpp.predefined] (1.1) specifies (emphasis mine):

__cplusplus

The integer literal 201703L. [Note: It is intended that future versions of this International Standard will replace the value
of this macro with a greater value.]

The macros used, as of Nov 2018, are:

• GCC 9.0.0: 201709L for C++2a. Live demo
  


• Clang 8.0.0: 201707L. Live demo
  


• VC++ 15.9.3: 201704L (as @Acorn's answer mentions).

PS: If you are interested in specific features, then [cpp.predefined] (1.8) defines corresponding macros, which you could use. Notice though, that they might change in the future.

The new value will be available at some point at [cpp.predefined]p1.1:

_­_­cplusplus

The integer literal 201703L. [ Note: It is intended that future versions of this International Standard will replace the value of this macro with a greater value. — end note ]

The current values used in the major compilers are, as of 2018-11-30:

• 
  gcc: 201709L godbolt
  


• 
  clang: 201707L godbolt
  


• 
  msvc: 201704L godbolt (requires /Zc:__cplusplus)


• 
  icc: unsupported godbolt (for the moment)

Since all of them are already higher than C++17's 201703L, you can already use:

#if __cplusplus > 201703L

    // C++20 code

#endif

There's no known __cplusplus version yet because C++20 is still in development. There are only drafts for C++20.

The latest draft N4788 still contains:

__cplusplus

The integer literal 201703L. [Note: It is intended that future versions of this International Standard will replace the value
of this macro with a greater value. —end note]

As for checking it, I would use @gsamaras answer.