Why can't a const mutable lambda with an auto& parameter be invoked?

December 10, 2021

#include <type_traits>

int main()
{
    auto f1 = [](auto&) mutable {};
    static_assert(std::is_invocable_v<decltype(f1), int&>); // ok

    auto const f2 = [](auto&) {};
    static_assert(std::is_invocable_v<decltype(f2), int&>); // ok

    auto const f3 = [](auto&) mutable {};
    static_assert(std::is_invocable_v<decltype(f3), int&>); // failed
}

See demo

Why can’t a const mutable lambda take a reference argument?

>Solution :

You get an error for this for the very same reason:

struct foo {
    void operator()(){}
};

int main() {
    const foo f;
    f();
}

The error is:

<source>:7:5: error: no matching function for call to object of type 'const foo'
    f();
    ^
<source>:2:10: note: candidate function not viable: 'this' argument has type 'const foo', but method is not marked const
    void operator()(){}
         ^

Because you cannot call a non-const method on a const instance. Lambdas got the default constness right, so without mutable the operator() is const. With mutable the operator() is a non-const method that you cannot call on a const f3;