IntelliSense reports error when template default argument is decltype(lambda) (C++20)

Type: LanguageService

IntelliSense reports error with this example:

template <class U = decltype([]() {})>
void foo() {}

int main() {
    foo(); // <-- error here: no instance of function template "foo" matches the argument list
}

While this code perfectly compiles with GCC and C++20: https://godbolt.org/z/b5a7hj

It won't compile with C++17 tho, so I believe the problem lies in C++20 compatibility.

Describe the bug

• OS and Version: Windows 10 2004, on WSL2 Ubuntu 20.04
• VS Code Version: 1.52.1
• C/C++ Extension Version: v1.2.0-insiders2
• Other extensions you installed (and if the issue persists after disabling them): issue persists without any other extension
• Does this issue involve using SSH remote to run the extension on a remote machine?: WSL2
• A clear and concise description of what the bug is, including information about the workspace (i.e. is the workspace a single project or multiple projects, size of the project, etc). Reproduceable with single file

Steps to reproduce

The code above with configuration:

{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [
                "${workspaceFolder}/**"
            ],
            "defines": [],
            "compilerPath": "/usr/bin/gcc",
            "cStandard": "gnu17",
            "cppStandard": "c++20", // gnu++20 results in same issue
            "intelliSenseMode": "linux-gcc-x64"
        }
    ],
    "version": 4
}

Expected behavior

For IntelliSense to not report false error when using C++20.

-------- Diagnostics - 1/21/2021, 1:56:40 PM
Version: 1.2.0-insiders2
Current Configuration:
{
    "name": "Linux",
    "includePath": [
        "${workspaceFolder}/**"
    ],
    "defines": [],
    "compilerPath": "/usr/bin/gcc",
    "cStandard": "c17",
    "cppStandard": "c++20",
    "intelliSenseMode": "linux-gcc-x64",
    "compilerArgs": [],
    "intelliSenseModeIsExplicit": true,
    "cStandardIsExplicit": true,
    "cppStandardIsExplicit": true,
    "compilerPathIsExplicit": true,
    "browse": {
        "path": [
            "${workspaceFolder}/**"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
Translation Unit Mappings:
[ /home/oliver/pro/lulu/poc/poc.cpp ]:
    /home/oliver/pro/lulu/poc/poc.cpp
Translation Unit Configurations:
[ /home/oliver/pro/lulu/poc/poc.cpp ]:
    Process ID: 15806
    Memory Usage: 16 MB
    Compiler Path: /usr/bin/gcc
    Includes:
        /usr/include/c++/10
        /usr/include/x86_64-linux-gnu/c++/10
        /usr/include/c++/10/backward
        /usr/lib/gcc/x86_64-linux-gnu/10/include
        /usr/local/include
        /usr/include/x86_64-linux-gnu
        /usr/include
    Standard Version: c++20
    IntelliSense Mode: linux-gcc-x64
    Other Flags:
        --g++
        --gnu_version=100200
Total Memory Usage: 16 MB

------- Workspace parsing diagnostics -------
Number of folders and files enumerated: 3585
Number of files discovered (not excluded): 3424

Screenshots

Iniziate con l’esempio C++20 in un singolo file e con la configurazione c_cpp_properties.json mostrata nell’issue, quindi riproducete la diagnosi errata di IntelliSense confrontando il risultato di GCC. Il lavoro è concluso quando IntelliSense accetta il template con un argomento predefinito di decltype(lambda) senza l’errore di function-template segnalato.