COMP: making it compile on VS2017

[dzenanz]

The regression was introduced by 4f30980.

WIP. Reduces number of compile failures on Visual Studio 2017 on My computer. I plan to get back to this later this week.

[N-Dekker]

@dzenanz FYI, I did an similar partial revert for VS2107 on elastix! SuperElastix/elastix@fd523e5

Specifically to work around VS2107 compiler bug "Compile error when using "using declaration" referencing a base class type that refers to itself", EssentiaX - Reported March 12, 2019 [Fixed in version 16.2], https://developercommunity.visualstudio.com/t/486683

The compile error only seemed to appear when the type that the class was "using" was actually a base class.

[dzenanz]

I saw SuperElastix/elastix@fd523e5. I tried that first, but then realized that using X = Superclass::X; works in VS2017 too.

[N-Dekker]

Are you sure all of these changes are necessary, for VS2017? I had the impression that the VS2017 error "C2886: symbol cannot be used in a member using-declaration" only occurred when the "type to be used" itself is a base-class of the class that has the using-declaration. (Compiler bug https://developercommunity.visualstudio.com/t/486683)

[dzenanz]

You are more than welcome to take over this PR, and explore the minimum amount of changes which get it compile on VS2017.

[N-Dekker]

I'm having a look now, and it appears that there's another category of erroneous (!) VS2017 compile errors that we're encountering now after pull request #2567, saying:

error C2244: unable to match function definition to an existing declaration

This VS2017 compile error may be reproduced by:

template <typename T>
class Base
{
public:
  using NestedType = T;
};

template <typename T>
class Derived : public Base<T>
{
  using Superclass = Base<T>;
  using typename Superclass::NestedType;
  NestedType MemberFunction();
};

template <typename T>
typename Derived<T>::NestedType
Derived<T>::MemberFunction()
{  // <==  error C2244: unable to match function definition to an existing declaration

  return NestedType{};
}

And this particular error may be solved by a trailing return type instead:

template <typename T>
auto
Derived<T>::MemberFunction() -> NestedType
{  // OK!

  return NestedType{};
}

And in this particular case (having a nested dependent type as return type), I think it would actually be quite nice to use a trailing return type.

To be continued...

[N-Dekker]

I saw SuperElastix/elastix@fd523e5. I tried that first, but then realized that using X = Superclass::X; works in VS2017 too.

Indeed, VS2017 supports both C++11 using type aliases and C++98 using-declarations. Only limited by compiler bugs 😸

[dzenanz]

Closing in favor of #2780.