performance-unnecessary-value-param false positive with iterator as template parameter

[firewave]

#include <list>

template<class Iterator>
void func(Iterator it)
{
}

void f()
{
    std::list<int> l;
    func(l.rbegin());
}

<source>:4:20: warning: the parameter 'it' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]
    4 | void func(Iterator it)
      |                    ^
      |           const   &

https://godbolt.org/z/611TeePzP

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Oliver Stöneberg (firewave)

```cpp #include <list>

template<class Iterator>
void func(Iterator it)
{
}

void f()
{
std::list<int> l;
func(l.rbegin());
}

<source>:4:20: warning: the parameter 'it' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]
4 | void func(Iterator it)
| ^
| const &

https://godbolt.org/z/611TeePzP
</details>

[vbvictor]

I think that this is not false-positive if we strictly follow check conditions:

The check is only applied to parameters of types that are expensive to copy which means they are not trivially copyable or have a non-trivial copy constructor or destructor.

std::reverse_iterator which is returned from rbegin is not trivially copyable and trivially copy constructible
https://godbolt.org/z/5KK7zs171

[firewave]

Thanks for pointing that out.

Interestingly it is both when using libc++ - https://godbolt.org/z/bq5971onb.
Same with MSVC - https://godbolt.org/z/1zE96cYa8.