[clang-tidy] request: forbid constructing reference_wrapper from reference parameter

[mwoehlke-kitware]

We (CMake) recently ran into a bug that can be summarized thusly:

struct Foo {
  Foo(std::string const& ref) : Ref{ref} {}
  void bar() { /* do something with Ref */ }

  std::reference_wrapper<std::string const> Ref;
};

int main()
{
  Foo foo{{}}; // uh-oh
  foo.bar(); // crash
}

Passing the reference through a T [const]& function parameter bypassed std::reference_wrapper's protection against being constructed from a temporary. Correct code would have Foo's ctor also taking a std::reference_wrapper<T> rather than a T&. This seems like the sort of thing clang-tidy ought to be able to spot and... discourage.

I propose that clang-tidy should flag construction of a std::reference_wrapper<T> from (at least) a T&, if the latter is a function parameter, and should recommend changing the parameter type from T& to std::reference_wrapper<T>. (Ideally this would cover convertible types, but that might be difficult for some potential conversions. Possibly it could at least cover U& where U is a subclass of T.)