title: enable c++17 intellisense open folder visual studio ninja-clang
body: <p>Using "<code>-Xclang -std=c++17</code>" I can build the executable, however I can't find what activates c++17 intellisense. I've tried many combinations as shown below and none seem to work</p>
  
  <p>CMakeLists.txt</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.9.2)
  set(CMAKE_CXX_STANDARD 17)
  project(myapp)
  add_compile_options("-Xclang" "-std=c++17")
  add_executable(myapp main.cpp)
  set_target_properties(myapp PROPERTIES CXX_STANDARD 17)
  target_compile_features(myapp PRIVATE cxx_std_17)
  </code></pre>
  
  <p>main.cpp</p>
  
  <pre><code>#include &lt;tuple&gt;
  namespace test1::test2 // red [qualified name is not allowed]
  //       ^^^^^^^^^^^^^
  {}
  
  int main()
  {
      auto[a, b] = std::pair&lt;int, int&gt;();
      //  ^^^^^^
      return 0;
  }
  </code></pre>
  
  <p>CMakeSettings.json</p>
  
  <pre><code>{
    // See https://go.microsoft.com//fwlink//?linkid=834763 for more information about this file.
    "configurations": [
      {
        "name": "x64-Debug",
        "generator": "Ninja",
        "configurationType": "Debug",
        "inheritEnvironments": [ "msvc_x64_x64" ],
        "buildRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\build\\${name}",
        "installRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\install\\${name}",
        "cmakeCommandArgs": "",
        "buildCommandArgs": "-v",
        "ctestCommandArgs": "",
        "variables": [
          {
            "name": "CMAKE_CXX_COMPILER",
            "value": "clang-cl"
          },
          {
            "name": "CMAKE_C_COMPILER",
            "value": "clang-cl"
          },
          {
            "name": "CMAKE_SYSTEM_NAME",
            "value": "Windows"
          }
        ]
      }
    ]
  }
  </code></pre>