Unable to deduce type in template metaprogram using concepts

[davidBronicki]

Bug type: Language Service

The Issue
Intellisense unable to deduce member type of constrained template (via c++20 concepts).

The following code encapsulates the error, important lines have a comment preceding them:

    #include <type_traits>
    #include <stddef.h>
    
    template<typename... Ts>
    struct Bundle;
    
    template<typename First, typename... Rest>
    struct Bundle<First, Rest...>
    {
        static constexpr size_t size = 1 + sizeof...(Rest);
    
        typedef First FirstType;
    
        typedef Bundle<Rest...> Bundle_Rest;
    };
    
    template<>
    struct Bundle<>
    {
        static constexpr size_t size = 0;
    };
    
    template<typename T>
    struct Is_Bundle : std::integral_constant<bool, false>{};
    
    template<typename... Ts>
    struct Is_Bundle<Bundle<Ts...>> : std::integral_constant<bool, true>{};
    
    template<typename T>
    concept Bundle_T = Is_Bundle<T>::value;
    
    template<typename T>
    concept Has_Type = requires (typename T::T){true;};
    
    template<Bundle_T B1, Bundle_T B2>
    struct Concat {};
    
    template<typename... T1s, typename... T2s>
    struct Concat<Bundle<T1s...>, Bundle<T2s...>>
    {
        typedef Bundle<T1s..., T2s...> T;
    };
    
    template<typename Type>
    struct TestExpression
    {
        typedef bool T;
    };
    
    template<Bundle_T B, template<typename> typename Expression>
    //if this requires line is removed, the issues disappear (but concepts are cool so I want it)
    requires Has_Type<Expression<typename B::FirstType>> || (B::size == 0)
    struct Template_For
    {
        typedef typename Concat<Bundle<typename Expression<typename B::FirstType>::T>,
	        typename Template_For<typename B::Bundle_Rest, Expression>::T>::T T;
    };
    
    template<template<typename> typename Expression>
    struct Template_For<Bundle<>, Expression>
    {
        typedef Bundle<> T;
    };
    
    template<Bundle_T T>
    struct TheEnd {};
    
    int main()
    {
        //Intellisense label Type0 as <error-type>
        typedef Template_For<Bundle<int>, TestExpression>::T Type0;
    
        //due to the Bundle_T concept, this shows a red squiggle error
        TheEnd<Type0> a;
    }

Expected Behavior

'Type0' should deduce to 'Bundle', and therefor the line after should not have a red squiggle.

System Used

• OS and Version: Ubuntu 20.04
• VS Code Version: 1.59.1
• C/C++ Extension Version: 1.6.0
• Other extensions you installed (and if the issue persists after disabling them): Jupyter, Pylance, Python (persists upon disabling all)

c_cpp_properties.json

{
"configurations": [
{
"name": "Linux",
"includePath": [
"${workspaceFolder}/**"
],
"defines": [],
"compilerPath": "/bin/clang++",
"cStandard": "c11",
"intelliSenseMode": "linux-clang-x64",
"cppStandard": "c++20"
}
],
"version": 4
}

[michelleangela]

I've filed an internal bug (1398106) against our shared Visual Studio (VS) IntelliSense process.

[sean-mcmanus]

Fixed with https://github.com/microsoft/vscode-cpptools/releases/tag/v1.19.0 (pre-release)