File with non-standard extension causes editor's Language Mode to keep reverting back to C

[pjyoung-ibm]

Environment

• OS and Version: macOS 14.5 and RHEL 8.6
• VS Code Version: 1.91.1 (macOS) and 1.90.1 (RHEL)
• C/C++ Extension Version: 1.20.5 (both)
• If using SSH remote, specify OS of remote machine: N/A

Bug Summary and Steps to Reproduce

Bug Summary:
Our development environment involves opening files that are symlinks to files with "odd" file extensions. One contrived example:

/path/to/file/Test.C: symbolic link to /other/path/to/file/cur,Test.C,1.1^123

When opening either the symlink or the actual file, VSCode starts with a Language Mode of C++. But after some time automatically (or after making a small edit to the file), it will revert to a Language Mode of C. Even if I manually change the Language Mode back to C++, it will still revert to C later. These files are C++ source files, so the C++ start is correct. Reverting to C causes incorrect syntax highlighting and false positives for compiler errors. Making a copy of the file to a different place without the odd part at the end of the file causes the Language Mode of the editor to stay on C++. So there is something odd about the extension of the actual file.

I have tried adding the following to my user settings to see if that would make a difference, but while it did get VSCode to recognize both files a C++ files without manual intervention initially, it still ended up reverting back to C at some point:

{
    "files.associations": {
        "*.C": "cpp",
        "*.C,*": "cpp"
    }
}

Changing the state of C_Cpp.autoAddFileAssociations did not seem to make any difference. Also, this issue did not occur with the "C_Cpp.intelliSenseEngine": "Tag Parser" setting (it stayed on C++ Language Mode for both the symlink and the actual file). So it appears to be something with the default IntelliSense engine.

Steps to reproduce:

1. Open a stock VSCode instance with only the C/C++ extension installed, and default settings for everything.
2. Create a file like Test.C,1.1^123, make it a simple "Hello World" like the below:

#include <iostream>

int main() {
    std::cout << "Hello World" << std::endl;
    return 0;
}

3. If the Language Mode is not C++, manually change it to C++.
4. After a bit of time, you should see the Language Mode change to C. If not, make a small edit to the file.
5. The Language Mode should now be reverted to C. This issue should repeat indefinitely even if manually changed back to C++.

Expected behavior:
The Language Mode should not automatically revert to C, even after manually changed by the user to C++.

Configuration and Logs

NO C_CPP_PROPERTIES.JSON FILE EXISTS, DEFAULTS USED.

C/C++: LOG DIAGNOSTICS OUTPUT (macOS)
-------------------------------------
-------- Diagnostics - 7/11/2024, 5:50:09 PM
Version: 1.20.5
Current Configuration:
{
    "name": "Mac",
    "includePath": [
        "/Users/parker/vscode/cppbugtest/user/**"
    ],
    "defines": [],
    "macFrameworkPath": [
        "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks"
    ],
    "compilerPath": "/usr/bin/clang",
    "cStandard": "c17",
    "cppStandard": "c++17",
    "intelliSenseMode": "macos-clang-x64",
    "compilerPathInCppPropertiesJson": "/usr/bin/clang",
    "intelliSenseModeIsExplicit": false,
    "cStandardIsExplicit": false,
    "cppStandardIsExplicit": false,
    "mergeConfigurations": false,
    "compilerPathIsExplicit": false,
    "browse": {
        "path": [
            "/Users/parker/vscode/cppbugtest/user/**",
            "${workspaceFolder}"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
cpptools version (native): 1.20.5.0
Translation Unit Mappings:
[ /Users/parker/vscode/cppbugtest/user/cur,Test.C,1.1^123 - source TU]:
Translation Unit Configurations:
[ /Users/parker/vscode/cppbugtest/user/cur,Test.C,1.1^123 ]:
    Process ID: 25880
    Memory Usage: 11 MB
    Compiler Path: /usr/bin/clang
    Includes:
        /usr/local/include
        /Library/Developer/CommandLineTools/usr/lib/clang/15.0.0/include
        /Library/Developer/CommandLineTools/SDKs/MacOSX14.4.sdk/usr/include
        /Library/Developer/CommandLineTools/usr/include
    Frameworks:
        /Library/Developer/CommandLineTools/SDKs/MacOSX14.4.sdk/System/Library/Frameworks
    Standard Version: c17
    IntelliSense Mode: macos-clang-x64
    Other Flags:
        --clang
        --clang_version=160000
Total Memory Usage: 11 MB

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 10262



C/C++: LOG DIAGNOSTICS OUTPUT (RHEL)
------------------------------------
-------- Diagnostics - 7/11/2024, 5:52:35 PM
Version: 1.20.5
Current Configuration:
{
    "name": "Linux",
    "includePath": [
        "${workspaceFolder}/**"
    ],
    "defines": [],
    "compilerPath": "/bin/clang",
    "cStandard": "c17",
    "cppStandard": "c++14",
    "intelliSenseMode": "linux-clang-x64",
    "compilerPathInCppPropertiesJson": "/bin/clang",
    "intelliSenseModeIsExplicit": false,
    "cStandardIsExplicit": false,
    "cppStandardIsExplicit": false,
    "mergeConfigurations": false,
    "compilerPathIsExplicit": false,
    "browse": {
        "path": [
            "${workspaceFolder}/**"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
cpptools version (native): 1.20.5.0
Translation Unit Mappings:
[ /tmp/Test.C,1.1^123 - source TU]:
Translation Unit Configurations:
[ /tmp/Test.C,1.1^123 ]:
    Process ID: 2560643
    Memory Usage: 16 MB
    Compiler Path: /bin/clang
    Includes:
        /usr/lib64/clang/15.0.7/include
        /usr/local/include
        /usr/include
    Standard Version: c17
    IntelliSense Mode: linux-clang-x64
    Other Flags:
        --clang
        --clang_version=150007
Total Memory Usage: 16 MB

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 5202

Other Extensions

N/A

Additional context

Screen.Recording.2024-07-11.at.6.02.21.PM.mov

[Colengms]

Hi @pjyoung-ibm . Thank you for reporting this issue. I can reproduce it.

[sean-mcmanus]

@pjyoung-ibm Fixed with https://github.com/microsoft/vscode-cpptools/releases/tag/v1.22.0