Lambda copying a reference to a lambda reference VS2017 compile error

Question

Compiling the following code in Visual Studio 2017:

#include "pch.h"
#include <iostream>

int main()
{
    int test = 5;

    auto cb1 = [test]()
    {
        auto cb2 = [&]()
        {
            auto cb3 = [test]()
            {
                std::cout << test;
            };
            cb3();
        };
        cb2();
    };
    cb1();
}

Gives the compiler error

test.cpp(17): error C2440: '<function-style-cast>': cannot convert from 'const int' to 'main::<lambda_80fd0d4feae1377a5d8b8955e10105ab>::()::<lambda_38fc83ae6a7bd6540ebe1721869db4f1>'
test.cpp(17): note: No constructor could take the source type, or constructor overload resolution was ambiguous
test.cpp(18): error C3536: 'cb2': cannot be used before it is initialized
test.cpp(18): error C2064: term does not evaluate to a function taking 0 arguments

Does anybody know why Visual Studio gives this error? (it seem to compile ok on clang) You can get it to compile by replacing auto cb2 = [&]() with auto cb2 = [&test]() why does that fix the errors?

Even more interesting adding std::cout << test; or const int &ref = test; to the body of cb2 fixes the compiler error.

Answer 1

这只是旧版 Visual Studio 编译器中的一个错误。通过实验可以看到，该错误一直存在到MSVC v16.10，并在MSVC v16.11中修复。幸运的是，包括 Visual Studio 2019 在内的现代编译器接受您的程序。演示： https: //gcc.godbolt.org/z/rYG7Ma8n9