[PATCH] Introducing initial UseAuto transform

[Vane, Edwin]

>   It can be done easier, I think.  We know that all functions that return iterators have a return value type with correct sugar.  That is, `vector<T>::begin()` return value is always `vector<T>::iterator`.
>
> [REV] This isn't always true. When dealing with for-loops I found from dumping the AST that you don't get vector<T>::iterator but whatever vector<T>::iterator typedefs to which is quite long and complex with libstdc++. It was this realization that prompted the comparison of canonical types.

I guess there is something not entirely correct in that test, because this is what I see in my copy of libstdc++ 4.7:

      iterator
      begin() _GLIBCXX_NOEXCEPT /*...*/

[REV] I put this:

llvm::errs() << D->getType().getAsString() << " = " << E->getType().getAsString() << "\n";

right before the cananonical type equality test. Here's the output when running the iterator.cpp test:

class MyIterator = class MyIterator
class DummyIterator = class DummyIterator
std::vector<int>::iterator = iterator
int_iterator = std::vector<int>::iterator
std::vector<int>::iterator = iterator
std::vector<int>::reverse_iterator = reverse_iterator
std::vector<int>::iterator = iterator
std::vector<int>::iterator = class __gnu_cxx::__normal_iterator<int *, class std::vector<int, class std::allocator<int> > >
int_iterator = iterator
int_iterator = iterator

Notice how the RHS type, even though not user-provided, varies quite a lot. This is with libstdc++4.7. I'm not entirely sure this matters. From what you're suggesting about unsugaring user types it sounds like we do the canonical type test and then do an extra bit of work to make sure the user-provided type is no less sugared than the types allowed by the spec? How do you propose we deal with custom iterators? The canonical type may then be the LHS type.