[SOLVED] cmake keeps regenerating cache [cmake + visual studio open folder + ninja + clang-cl]

Issue

When I edit the CMakeLists.txt file, the first time it works and uses the cache, but the second time I always get

1> The existing cache contains references to non-existing MSVC compiler. Deleting cache and regenerating.

This only happens If I use the clang compiler. If I use the default msvc compiler the cache is always used

// main.cpp

int main()
{
    return 0;
}

#CMakeLists.txt
cmake_minimum_required(VERSION 3.0)
add_executable(app main.cpp)

//CMakeSettings.json
{
  // See https://go.microsoft.com//fwlink//?linkid=834763 for more information about this file.
  "configurations": [
    {
      "name": "x86-Debug",
      "generator": "Ninja",
      "configurationType": "Debug",
      "inheritEnvironments": [ "msvc_x86" ],
      "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"
        }
      ]
    }
  ]
}

Solution

Turning my comments into an answer

You have to give full paths for your compilers when you use ninja with visual-studio-2017:

CMakeSettings.json

...
"variables": [
     {
         "name": "CMAKE_CXX_COMPILER",
         "value": "C:\\Program Files\\LLVM\\bin\\clang-cl.exe"
     },
     {
         "name": "CMAKE_C_COMPILER",
         "value": "C:\\Program Files\\LLVM\\bin\\clang-cl.exe"
     },
     {
         "name": "CMAKE_SYSTEM_NAME",
         "value": "Windows"
     }
]
...

Here is what I think is happening:

• CMake always translates compiler paths to absolute paths (your first call)
• In the second call VS2017 does again override the CMAKE_CXX_COMPILER variables to CMake
  • Which is not intended since it overwrites/forces the values again, but CMake does remember that it already searched for the compiler
  • Et voila, you get an error message about a changed compiler

I think this being a bug in VS2017's usage of CMake.

References

• How do I tell CMake to use Clang on Windows?
• Building a x86 application with CMake, Ninja and Clang on x64 Windows
• Visual C++ Team Blog: Customizing your Environment with Visual C++ and Open Folder

Answered By - Florian

Answer Checked By - Pedro (WPSolving Volunteer)