[cfe-dev] Explicit this expression in class template is dependent?

[James Dennett]

On Fri, Aug 22, 2014 at 12:56 PM, Jason Haslam <jason.haslam at gmail.com> wrote:
> Hi all,
>
> I find that explicit this expressions in class templates create a CXXDependentScopeMemberExpr in the AST while the equivalent implicit this is a MemberExpr. For example code like this:
>
> template <typename T> struct A {
>   int i;
>   int getImplicit() { return i; } // MemberExpr
>   int getExplicit() { return this->i; } // CXXDependentScopeMemberExpr
> };
>
> Why is explicit dependent? Is it technical (per the language standard) or an implementation detail? I’d like both expressions to resolve to the member for static analysis/refactoring/etc.

$ cat ct.cc

template <typename T> struct A {
  int getExplicit() { return this->i; } // might compile
  int getImplicit() { return i; } // must not compile
};

$ clang++ -std=c++11 -c ct.cc
ct.cc:4:30: error: use of undeclared identifier 'i'
  int getImplicit() { return i; } // MemberExpr
                             ^
1 error generated.

Lookup in "return i;" is done in phase one, when the template is
parsed.  `i` isn't there, so it's an error.  Lookup in "return
this->i;" is done in phase two, at template instantiation time.  If
there were dependent base classes, "return this->i" could find things
in them, while "return i" would not.

There is a rule that allows Clang to give an error for `getExplicit`
also -- the rule that says that a template (/temploid) is ill-formed
(no diagnostic required) if there is no possible well-formed
instantiation of it.

It would also be convenient for tools if Clang became a little more
aggressive in noticing that "return this->i" isn't dependent in any
meaningful way in the code above, given that there are no dependent
base classes.