Clang Kit scanning will accidentally use GCC for C++

[ProfFan]

Brief Issue Summary

The Clang kit scan is broken with current LLVM 11. It will show as

and then call CMake with only the CMAKE_C_COMPILER set. This leads to the mixing of GCC and Clang toolchains

This is very subtle and causes a lot of confusion.

Expected:

1. Kits without a C++ compiler should not show if the project is C++. Or such kits should have a different style to warn users.
2. LLVM 11.1 Kits should be properly detected

CMake Tools Log

[kit] Found Kit: GCC for aarch64-linux-gnu 10.2.0
[kit] Found Kit: GCC for aarch64-linux-gnu 10.2.0
[kit] Found Kit: GCC for arm-none-eabi 10.2.0
[kit] Found Kit: GCC for arm-none-eabi 10.2.0
[kit] Found Kit: Clang 11.1.0
[kit] Found Kit: Clang 11.1.0
[kit] Found Kit: GCC 10.2.0
[kit] Found Kit: GCC 7.4.1
[kit] Found Kit: GCC 8.4.0
[kit] Found Kit: GCC 9.3.0
[kit] Found Kit: GCC for riscv64-linux-gnu 10.2.0
[kit] Found Kit: GCC for riscv64-linux-gnu 10.2.0
[kit] Found Kit: GCC for riscv64-unknown-elf 8.2.0
[kit] Found Kit: GCC for riscv64-unknown-elf 8.2.0
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 10.2.0
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 10.2.0
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 7.4.1
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 7.4.1
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 8.4.0
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 8.4.0
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 9.3.0
[kit] Found Kit: GCC for x86_64-pc-linux-gnu 9.3.0
[kit] Found Kit: GCC 10.2.0
[kit] Successfully loaded 15 kits from /home/fan/.local/share/CMakeTools/cmake-tools-kits.json

Platform and Versions

• Operating System: Arch Linux
• CMake Version: 3.20.1
• VSCode Version: 1.55.1
• CMake Tools Extension Version: 1.6.0
• Compiler/Toolchain: LLVM 11.1

[bobbrow]

Thank you for reporting this. We may not get to it immediately, but we are always open to accept a PR if someone has a chance to investigate this before we do.

[andreeis]

Your first suggestion (to not show kits that are missing CXX compiler when being within a C++ project) sounds useful and we will consider it in the future.
Regarding the second expectation (to detect a complete clang 11.1.0 kit), can you confirm if you have clang++-11 in /usr/bin? If not, maybe your installation of clang 11.1 is corrupt and you can try a reinstall. If yes, can you copy paste the output from "clang++-11 --version" and "clang-11 --version"?

[ProfFan]

Sure!

> /usr/bin/clang++ --version
clang version 11.1.0
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
> /usr/bin/clang --version
clang version 11.1.0
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[andreeis]

Ok, weird. How does the llvm kit looks to be written in cmake-tools-kits.json? To open this kits file and copy paste the kit definition for us, you can use the command "CMake: Edit User-Local CMake Kits" in the palette.
Also, worth trying, delete the LLVM kit from that file and run the command "CMake: Scan for kits" and see how the LLVM 11.1 kit definition is written (probably still wrong).
Then, until we figure out how to fix this, to get unblocked you can write the missing CXX line yourself in the LLVM 11.1 kit definition in the cmake-tools-kits.json file.
After getting more info from you, we may split this report in 2: one about not including a kit that does not have CXX compiler when we detect a C++ project and the other about why we don't detect the ++ portion of the clang llvm 11.1 kit.

[ProfFan]

I tried removing and rescanning, it's the same result.

The JSON file (sorry I have too many compilers :) )

  {
    "name": "Clang 11.1.0",
    "compilers": {
      "C": "/usr/bin/clang-11"
    }
  },
  {
    "name": "Clang 11.1.0 x86_64-pc-linux-gnu",
    "compilers": {
      "C": "/usr/bin/clang-11"
    }
  },
  {
    "name": "GCC 10.1.0",
    "compilers": {
      "C": "/bin/gcc",
      "CXX": "/bin/g++"
    }
  },
  {
    "name": "GCC 10.2.0",
    "compilers": {
      "C": "/opt/cuda/bin/gcc",
      "CXX": "/opt/cuda/bin/g++"
    }
  },
  {
    "name": "GCC 10.2.0 riscv64-linux-gnu",
    "compilers": {
      "C": "/usr/bin/riscv64-linux-gnu-gcc-10.2.0"
    }
  },
  {
    "name": "GCC 10.2.0 x86_64-pc-linux-gnu",
    "compilers": {
      "C": "/opt/cuda/bin/gcc",
      "CXX": "/opt/cuda/bin/g++"
    }
  },
  {
    "name": "GCC 10.3.0 aarch64-linux-gnu",
    "compilers": {
      "C": "/usr/bin/aarch64-linux-gnu-gcc-10.3.0"
    }
  },
  {
    "name": "GCC 10.3.0 arm-none-eabi",
    "compilers": {
      "C": "/usr/bin/arm-none-eabi-gcc-10.3.0"
    }
  },
  {
    "name": "GCC 7.4.1",
    "compilers": {
      "C": "/usr/bin/gcc-7",
      "CXX": "/usr/bin/g++-7"
    }
  },
  {
    "name": "GCC 7.4.1 x86_64-pc-linux-gnu",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-7.4.1"
    }
  },
  {
    "name": "GCC 8.2.0 riscv64-unknown-elf",
    "compilers": {
      "C": "/usr/bin/riscv64-unknown-elf-gcc-8.2.0"
    }
  },
  {
    "name": "GCC 8.2.1",
    "compilers": {
      "C": "/bin/gcc",
      "CXX": "/bin/g++"
    }
  },
  {
    "name": "GCC 8.4.0",
    "compilers": {
      "C": "/usr/bin/gcc-8",
      "CXX": "/usr/bin/g++-8"
    }
  },
  {
    "name": "GCC 8.4.0 x86_64-pc-linux-gnu",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-8.4.0"
    }
  },
  {
    "name": "GCC 9.3.0",
    "compilers": {
      "C": "/usr/bin/gcc-9",
      "CXX": "/usr/bin/g++-9"
    }
  },
  {
    "name": "GCC 9.3.0 x86_64-pc-linux-gnu",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-9.3.0"
    }
  },
  {
    "name": "GCC for riscv64-linux-gnu 10.2.0",
    "compilers": {
      "C": "/usr/bin/riscv64-linux-gnu-gcc-10.2.0"
    }
  },
  {
    "name": "GCC for riscv64-unknown-elf 8.2.0",
    "compilers": {
      "C": "/usr/bin/riscv64-unknown-elf-gcc-8.2.0"
    }
  },
  {
    "name": "GCC for x86_64-pc-linux-gnu 10.2.0",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-10.2.0"
    }
  },
  {
    "name": "GCC for x86_64-pc-linux-gnu 7.4.1",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-7.4.1"
    }
  },
  {
    "name": "GCC for x86_64-pc-linux-gnu 8.4.0",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-8.4.0"
    }
  },
  {
    "name": "GCC for x86_64-pc-linux-gnu 9.3.0",
    "compilers": {
      "C": "/usr/bin/x86_64-pc-linux-gnu-gcc-9.3.0"
    }
  }
]

[andreeis]

Ok I'll install llvm 11.1 on one of my ubuntus and I hope to reproduce this.

[minhducsun2002]

Can confirm identical behaviour with VS Code 1.55.2, CMake Tools 1.7.1, and similar LLVM & CMake version on Arch Linux.

[riri]

I get the same, clang++ is not detected and g++ is used for C++

[andreeis]

@riri, @minhducsun2002, @ProfFan, did you have the same llvm installed for a while and the problem started to manifest with a CMake Tools update (like 1.7.0 or 1.7.1) or with an LLVM update?

@bobbrow, is it possible that your fix for #1821 to address this as well?

[bobbrow]

The original report in this issue is for version 1.6.0 and #1821 is a 1.7.0 regression, so I'm not sure they're related. But folks on this thread can try installing my patch here to see if it fixes the problem.

1. Download from here: https://github.com/microsoft/vscode-cmake-tools/suites/2617691844/artifacts/57356650
2. Unzip the VSIX.
3. Use the "Install from VSIX" command in VS Code to install it.
4. Run the "Reload Window" command to ensure that it was installed.
5. Run the "Scan For Kits" command again to see if it fixed the problem.