[cfe-dev] How to lookup a name from a specific expression's location?

[Vassil Vassilev via cfe-dev]

On 10/3/20 3:40 PM, Oleksandr Koval via cfe-dev wrote:
> Hi, I'm writing a check that detects and removes redundant `this->` 
> usage. Finding explicit `this->` was simple, now I want to check 
> whether it's safe to remove it. My main concern is such case:
> struct  X{
>      void  f(int  x){
>          // without `this->` will use parameter
>          this->x++;
>
>          // can remove `this->` because
>          // local `y` is not visible yet
>          this->y++;
>          y++;
>
>          int  y;
>          y++;
>      }
>      int  x;
>      int  y;
> };
> I need to know whether a specific name(function or variable) will 
> resolve to the same declaration without `this->` part. My matcher is 
> /memberExpr(has(cxxThisExpr()))/, I bind  MemberExpr and CXXThisExpr, 
> I can rewrite matcher to also bind enclosing CXXMethodDecl. As I 
> understand, simple enumeration of parameter/local variable names won't 
> work(as in case with `y`) because of lookup/visibility rules. Is there 
> some `lookup()` function whose result I can compare against the 
> declaration of original function/variable? I found Sema::LookupName() 
> but can't figure out how to get Scope of the found expression. Can you 
> give me an example please?


   The Scope is a parser thing which contains an opaque ptr to the 
DeclContext. In principle, you can create a fake Scope and attach the 
relevant DeclContext via setEntity. I think just passing Sema::TUScope 
may work. If you want some pseudocode:

   Scope *fakeS = new Scope(...);
   fakeS->setEntity(DC);
   // you may need to do some work for the variable shadowing
   LookupResult R(Sema, Var->getName(), SourceLocation(), 
Sema::LookupOrdinaryName, Sema::ForRedeclaration);
   Sema->LookupName(R, fakeS);
   delete fakeS;

   Note that is an approximation as we do not attach the scope to the 
chain of scopes like the parser does. It some extra work that is also 
possible.


>
> -- 
> Regards,
> Oleksandr Koval.
>
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