To reproduce:

• Create a compile_commands.json file with the following contents. (The other referenced filenames don't necessarily need to exist).

[
  {
    "directory": "/",
    "file": "main.cpp",
    "output": "main.o",
    "arguments": [
      "g++",
      "-o",
      "main.o",
      "-c",
      "-std=c++17",
      "-DDEFINEA=\"/path/to/main.xml\"",
      "-DDEFINEB=\"/path/to/other.xml\"",
      "main.cpp"
    ]
  }
]

• Open compile_commands.json in QtCreator to create a project.
• Ask QtCreator to convert the project back into a compile_commands.json file (via Build -> Generate Compilation Database)

Observed behaviour:

QtCreator and clangd will behave as though the `DEFINEA` macro was never defined. This can be directly observed by looking at QtCreator's output compile_commands.json file, which for me contains this:

[
{
    "arguments": [
        "/usr/bin/gcc",
        "-c",
        "--target=x86_64-linux-gnu",
        "-m64",
        "-std=c++17",
        "-DDEFINEB=\"/path/to/other.xml\"",
        "-x",
        "c++",
        "/main.cpp"
    ],
    "directory": "/tmp/QtCreator-lSGjam",
    "file": "/main.cpp"
}
]

I believe this behaviour is related to the filterFromFileName function in compilationdatabaseutils.cpp, which is filtering out all flags that contain the file's basename.