Wrong error message when mutate const object capturing by reference

[vovchikzd]

When trying to create a mutable lambda with const object captured by reference and then mutate that object

int main() {
    const int a = 69;
    auto lambda = [&r = a](int x) mutable {
        r += x;
        return r;
    };
}

you got incorrect error message

<source>:4:11: error: cannot assign to a variable captured by copy in a non-mutable lambda
    4 |         r += x;
      |         ~ ^

but r is captured by reference and lambda is mutable
godbolt example: https://godbolt.org/z/sKd3bh6aM