[clang] [llvm] [Clang] Protect ObjCMethodList assignment operator against self-assignment (PR #97933)

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smanna12 wrote:

> > As far as I can tell, `ObjCMethodList` is already self-assignment safe so long as `PointerIntPair` is. It looks like the latter might not be though since its copy assignment operator (as implemented via `PunnedPointer`) uses `memcpy` and it does not guard against self-assignment. I wonder if a change should instead be made there. Can you share more details from the static analysis report?
> 
> Thank you @tahonermann for reviews and feedbacks.
> 
> This is what static analyzer tool complains. It does not provide much details:
> 
> ```
> In clang::​ObjCMethodList::​operator =(clang::​ObjCMethodList const &): Assignment operator does not check for self-assignment
>  ObjCMethodList &operator=(const ObjCMethodList &L) {
>   	
>  //Unsafe assignment operator (SELF_ASSIGN)
> self_assign: No protection against the object assigning to itself.
>     MethodAndHasMoreThanOneDecl = L.MethodAndHasMoreThanOneDecl;
>     NextAndExtraBits = L.NextAndExtraBits;
>     return *this;
>   }
> ```

I have updated patch. @tahonermann, could you please revisit the patch? Thank you

https://github.com/llvm/llvm-project/pull/97933