using operator auto is giving false error

[msftrncs]

Environment

Version: 1.108.1 (user setup)
Commit: 585eba7c0c34fd6b30faac7c62a42050bfbc0086
Date: 2026-01-14T14:55:44.241Z
Electron: 39.2.7
ElectronBuildId: 12953945
Chromium: 142.0.7444.235
Node.js: 22.21.1
V8: 14.2.231.21-electron.0
OS: Windows_NT x64 10.0.26100

C/C++ extension version 1.30.1

Bug Summary and Steps to Reproduce

Bug Summary:

When a class declares a conversion function using operator auto() IntelliSense will later be confused on references to that member of that class from other classes with a using statement. The code compiles fine, but IntelliSense will report it with an error squiggle.

Steps to reproduce:

1. Use this source file

   struct a {
       operator auto() { return char(8); }
   };
   
   struct b : a {
      using a::operator auto;
      using a::operator char;
   };

2. See error

class "a" has no member "operator char"

Expected behavior:

I would have expected using operator a::auto to be the error, and using a::operator char; to not be flagged as an error.

Configuration and Logs

There is no _c_cpp_properties.json_ in use for this example.

-------- Diagnostics - 1/15/2026, 8:47:45 PM
Version: 1.30.1
Current Configuration:
{
    "name": "Win32",
    "includePath": [
        "${workspaceFolder}/**"
    ],
    "defines": [
        "_DEBUG",
        "UNICODE",
        "_UNICODE"
    ],
    "intelliSenseModeIsExplicit": false,
    "cStandardIsExplicit": false,
    "cppStandardIsExplicit": false,
    "mergeConfigurations": false,
    "recursiveIncludes": {},
    "recursiveIncludesReduceIsExplicit": false,
    "recursiveIncludesPriorityIsExplicit": false,
    "recursiveIncludesOrderIsExplicit": false,
    "compilerPath": "",
    "compilerPathIsExplicit": true,
    "browse": {
        "limitSymbolsToIncludedHeaders": true
    }
}
Modified Settings:
{
    "C_Cpp.autocompleteAddParentheses": true,
    "C_Cpp.default.compilerPath": ""
}
Additional Tracked Settings:
{
    "editorTabSize": 4,
    "editorInsertSpaces": true,
    "editorAutoClosingBrackets": "languageDefined",
    "filesEncoding": "utf8",
    "filesAssociations": {
        "*.Ps1xml": "xml",
        "*.H86": "hex",
        "*.kwaconfig": "json",
        "*.tcc": "cpp",
        "*.xslt": "xsl",
        "*.rstc": "ini"
    },
    "filesExclude": {
        "**/.git": true,
        "**/.svn": true,
        "**/.hg": true,
        "**/.DS_Store": true,
        "**/Thumbs.db": true
    },
    "filesAutoSaveAfterDelay": false,
    "editorInlayHintsEnabled": true,
    "editorParameterHintsEnabled": true,
    "searchExclude": {
        "**/node_modules": true,
        "**/bower_components": true,
        "**/*.code-search": true
    },
    "workbenchSettingsEditor": "ui"
}
cpptools version (native): 1.30.1.0
Current database path: C:\USERS\CMORRIS\APPDATA\LOCAL\TEMP\{6C345B41-3F76-4648-B7E6-2DB157100C44}\.BROWSE.VC.DB
Translation Unit Mappings:
[ C:\Users\cmorris\test.cpp - source TU]:
Translation Unit Configurations:
[ C:\Users\cmorris\test.cpp ]
    Process ID: 4588
    Memory Usage: 60 MB
    Include paths:
    Defines:
        _DEBUG
        UNICODE
        _UNICODE
    Standard Version: ms_c++17
    IntelliSense Mode: windows-msvc-x64
Total Memory Usage: 60 MB

Other Extensions

No response

Additional context

No response

[msftrncs]

Using an online C++ compiler, I would get this error with the above example:

ERROR!
/tmp/8E8wuzIuVw/main.cpp:6:22: error: 'operator auto' has not been declared in 'struct a'
    6 |    using a::operator auto;
      |                      ^~~~


=== Code Exited With Errors ===

[sean-mcmanus]

@msftrncs I see cl.exe and clang give an error for the operator char and gcc give an error for the operator auto. So the bug would be that the EDG parser we use doesn't mimic the gcc behavior when passed the --g++ flag. But if you're using cl.exe then it wouldn't be a bug in your case.

I've filed an internal bug tracking this (2684099).