Intellisense not recognizing predefined macros with CMake, Clang and -nostdinc

Bug type: Language Service

Describe the bug

• OS and Version: Ubuntu 21.10
• VS Code Version:
  Version: 1.63.2
  Commit: 899d46d82c4c95423fb7e10e68eba52050e30ba3
  Date: 2021-12-15T09:39:46.686Z
  Electron: 13.5.2
  Chromium: 91.0.4472.164
  Node.js: 14.16.0
  V8: 9.1.269.39-electron.0
  OS: Linux x64 5.13.0-39-generic
• C/C++ Extension Version: 1.9.7
• Other extensions you installed (and if the issue persists after disabling them):
  ms-vscode.cmake-tools
• Workspace: Single project, trivial project with just one trivial source.

C++ compilers provide some predefined macros, like __INT_FAST32_TYPE__ and Intellisense should be able to know that definition. I don't know how Intellisense gets to know the predefined macros; the cmake-tools extension may provide them, the cmake-tools extension may provide the compiler path and the list of arguments used to invoke it, it may even have nothing to do with the cmake-tools extension and Intellisense just does something to get them. One way to get predefined macros is to invoke the compiler, like c++ -dM -E foo.h with a dummy foo.h.

I tested and found out that both Clang 13.0.0 and GCC 11.2.0 provide the same predefined macros for types (except a couple exceptions). Passing the flags -nostdinc and -nostdlib had no effect on this.

However, passing the flag -nostdinc for some target, on the CMakeLists.txt, when using clang, resulted in the predefined macros not being recognized by Intellisense.

The offending CMakeLists.txt:

cmake_minimum_required(VERSION 3.20.0)
project(scratch VERSION 0.1.0)

add_executable(scratch main.cpp)
target_compile_features(scratch PRIVATE cxx_std_20)
target_compile_options(scratch PRIVATE -nostdinc)
# target_link_options(scratch PRIVATE -nostdlib)

main.cpp is a trivial source.

Steps to reproduce

1. Create a new project, maybe with command Cmake: Quick Start
2. Add compile option -nostdinc in CMakeLists.txt (like in the code snippet above)
3. Run vscode command Cmake: Delete Cache and Reconfigure (not sure if necessary)
4. Run vscode command Developer: Reload Window (not sure if necessary)
5. On reload, the predefined macros will be highlighted as undefined on the source

Expected behavior
Predefined macros should be recognized, despite passing the -nostdinc option

NOTE: Not sure if this is a bug with this extension or with the cmake-tools extension.

Reproduce the report using the shown CMakeLists.txt and trivial main.cpp, with Clang and the -nostdinc option, then run the listed CMake Tools cache/reload commands. Trace how the CMake configuration reaches IntelliSense and determine whether the behavior belongs to CMake Tools or the C/C++ extension. Done means predefined macros remain recognized under this configuration.