[clang-tidy] False positive `cppcoreguidelines-pro-type-static-cast-downcast` when base type is not polymorphic

[ChrisThrasher]

https://godbolt.org/z/ddE4bYnse

struct Base
{
    int i;
};

struct Derived : Base
{
    float f;
};

int main()
{
    Derived d1;
    Base& base = d1;
    Derived& d2 = static_cast<Derived&>(base);
}

cppcoreguidelines-pro-type-static-cast-downcast emits a warning even when the base class is not polymorphic. In the above case the static_cast is necessary to cast from base to derived since a dynamic_cast is not allowed.

[PiotrZSL]

Then make that class polymorphic.

[ChrisThrasher]

It's not a class I control. I'm working with 3rd party types. I have no control over the definition of the base nor derived classes.

[PiotrZSL]

I could add a "StrictMode" option to a check, with 'true' would check all casts, with false would check only on polymorphic types. Could be ?

[ChrisThrasher]

My thought was that the check would detect non-polymorphic base classes and omit the diagnostic in such cases. Is that feasible?

[PiotrZSL]

Yes, but I would put this under config option.

[ChrisThrasher]

What's the use case for the example I shared above? If the base class is not polymorphic how else are you supposed to cast from base to derived? That doesn't seem like a useful diagnostic unless there is an alternative way of doing that cast.

Ultimately I trust your judgment. Having a config option for this would satisfy my needs.

[PiotrZSL]

Simply you don't, proper solution would be to design code so cast wouldn't be needed.