Question

I was merily experimenting with the new trailing return types, where I hit a problem with this (simplified) code

#include <list>

class MyContainer{
  std::list<int> ints;

  auto begin( ) -> decltype(ints.begin())
  {
    return ints.begin();
  }

  auto begin( ) const -> decltype(ints.begin())
  {
    return ints.begin();
  }
};

Ignore the fact of how pointless this code is. The important part is the compiler error generated when using GCC 4.6.1 (with -std=c++0x flag):

In member function 'std::list<int>::iterator MyContainer::begin() const':
error: could not convert '((const MyContainer*)this)->MyContainer::ints.std::list<_Tp, _Alloc>::begin [with _Tp = int, _Alloc = std::allocator<int>, std::list<_Tp, _Alloc>::const_iterator = std::_List_const_iterator<int>]()' from 'std::list<int>::const_iterator {aka std::_List_const_iterator<int>}' to 'std::list<int>::iterator {aka std::_List_iterator<int>}'

In case you’re not of fan of error involving templates, the short story is that in the body of the const version of MyContainer::begin, the expression ints.begin() returns a value of type std::list<int>::const_iterator (since ints is const in such a context). However, decltype(ints.begin()) produces the type std::list<int>::iterator, i.e. decltype ignores the const qualifier of the begin method when deciding the type of the expression. Unsurprisingly, a conflict in types is the result.

This seems to me to be a bug in the GCC compiler. It would only make sense for decltype to honor the const qualifier and produce the const_iterator type. Can anyone confirm or deny (maybe even explain) this? Maybe I am overlooking something in the mechanics of decltype, but this looks like a pretty straightforward scenario.

Note: as far as I can tell, the same behaviour holds not only for std::list<int>, but for any type with member functions overloaded on const-ness which return incompatible types.

Answer 1

You are correct, this is a bug. According to N3291, section 5.1.1, paragraph 3:

If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” between the optional cv-qualifer-seq and the end of the function-definition, member-declarator, or declarator. It shall not appear before the optional cv-qualifier-seq and it shall not appear within the declaration of a static member function (although its type and value category are defined within a static member function as they are within a non-static member function). [Note: this is because declaration matching does not occur until the complete declarator is known. —end note ] Unlike the object expression in other contexts, *this is not required to be of complete type for purposes of class member access (5.2.5) outside the member function body. [Note: only class members declared prior to the declaration are visible. —end note ]

But this was a recent change between the last working draft and N3291. So GCC was right less than 6 months ago; that’s the danger of writing code to a moving specification.