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d3ac9a5790
There is a bug in `expr::visit`. When trying to return a reference from a visitor it actually returns a reference to some temporary location.
So trying to do something like:
```c++
const expression e = new_bind_variable(123);
const bind_variable& ref = visit(overloaded_functor {
[](const bind_variable& bv) -> const bind_variable& { return bv; },
[](const auto&) -> const bind_variable& { throw std::runtime_error("Unreachable"); }
}, e);
std::cout << ref << std::endl;
```
Would actually print a random stack location instead of the value inside of `e`.
Additionally trying to return a non-const reference doesn't compile.
Current implementation of `expr::visit` is:
```c++
auto visit(invocable_on_expression auto&& visitor, const expression& e) {
return std::visit(visitor, e._v->v);
}
```
For reference, `std::visit` looks like this:
```c++
template<typename _Res, typename _Visitor, typename... _Variants>
constexpr _Res
visit(_Visitor&& __visitor, _Variants&&... __variants)
{
return std::__do_visit<_Res>(std::forward<_Visitor>(__visitor),
std::forward<_Variants>(__variants)...);
}
```
The problem is that `auto` can evaluate to `int` or `float`, but not to `int&`.
It has now been changed to `decltype(auto)`, which is able to express references.
I also added a missing `std::forward` on the visitor argument.
The new version looks like this:
```c++
template <invocable_on_expression Visitor>
decltype(auto) visit(Visitor&& visitor, const expression& e) {
return std::visit(std::forward<Visitor>(visitor), e._v->v);
}
```
I added some tests of `expr::visit` in `boost/expr_test`, but sadly they are not as throughout as they could be, Ideally I could return a refernce from `std::visit` and `expr::visit` and then check that they both point to the same address in memory.
I can't do this because it would require to access a private field of `expression`.
Some test pass before the fix, even though they shouldn't, but I'm not sure how to make them better without making field of expression public.
I played around with some code, it can be found here: https://github.com/cvybhu/attached-files/blob/main/visit/visit_playground.cpp
Closes #10073
* github.com:scylladb/scylla:
cql3: expr: Add a test to show that std::forward is needed in expr::visit
cql3: expr: add std::forward in expr::visit
cql3: expr: Add tests for expr::visit
cql3: expr: Fix expr::visit so that it works with references