Defect report 1207

Question

I don't understand the reason for this defect report 1207, more specifically in reference to the following sentence (emphasis is mine):

Because the transformation of a member name into a class member access expression (9.3.1 [class.mfct.non-static] paragraph 3) only occurs inside the body of a non-static member function, the type of v in the trailing-return-type is non-const but is const in the return expression, resulting in a type mismatch between the return expression and the return type of the function.

Edit

That is, I don't understand why the type of v in the trailing return type is deduced to be non-const.

Answer 1

vector v;
auto end() const -> decltype(v.begin()) { return v.begin(); }

decltype(v.begin()), in the trailing-return-type, is iterator - because v has the type vector, seen from outside.

But inside the functions body, the const-specifier of the member function end() is considered. The type of v is the type of this->v - which in turn depends on the const-ness of this.

this' pointee is const (because of the afore-mentioned const-specifier), thus the type of this inside the member function is block const*.

Therefore the type of this->v inside the member function is vector const (because of the const access-path), and v.begin() - which is actually (this->v).begin() - calls the const-overload whose return type is const_iterator. On the other hand, v.begin() in the trailing-return-type "calls" the non-const overload that returns iterator.

That type inconsistence is the problem.

Answer 2

It's saying that, when the trailing-return-type is parsed, the member v is looked up to sufficiently make sense of v.begin() .. but the actual access to v in the context of having invoked end() is not considered, so neither is the fact that end() (and therefore its access to v) is const.