Workaround VS2017 compile errors using-declarations

[N-Dekker]

Worked around three categories of VS2017 compile errors, caused by compiler bugs:

error C2653: '...': is not a class or namespace name
error C2886: '...': symbol cannot be used in a member using-declaration
error C2244: unable to match function definition to an existing declaration

The errors were reported by @dzenanz at #2759 "COMP: making it compile on VS2017" , after the merge of pull request #2567 4f30980 "STYLE: Avoid repeating parent aliases"

[dzenanz]

Awesome Niels!

[N-Dekker]

Awesome Niels!

@dzenanz Thanks 😄 Honestly it was more work that I had expected, especially because I wanted to preserve as much as possible from pull request #2567. I did not realize before that trailing return types would help to work around those errors.

[dzenanz]

it was more work that I had expected

I realized that too, and since I was tight with time I made that draft PR with progress (in case someone else is bugged by it too) 😄

[N-Dekker]

...since I was tight with time I made that draft PR with progress (in case someone else is bugged by it too)

@dzenanz It bugged you? Does that mean you actually still use VS2017? Anyway, can you possibly confirm that this PR does indeed make VS2017 happy?

[dzenanz]

I do have VS2017 installed, and a few years ago when I set up nightly build on my machine (Ryzenator) VS2017 was the latest and greatest. So it is still built nightly. I just tried building your branch, and it built successfully! I have switched to VS2019 for daily use long time ago.

[N-Dekker]

Maybe it should be a style guideline to use a trailing return type for any class template member function definition that is defined outside its class template definition (so usually in a *.hxx file), whenever the return type is specified in terms of a dependent member type. What do you think?

[dzenanz]

I agree. That would make specifying those types easier.

[N-Dekker]

I agree. That would make specifying those types easier.

An easier guideline could be something like: Whenever choosing between a trailing return type (as introduced with C++11) and the old (C++98) form of having the return type before the function name, prefer the form that is the shortest, and the simplest.

// Preferred:
int f()

// Not preferred:
auto f() -> int

// Preferred:
auto Template::MemberFun() -> MemberType

// Not preferred:
typename Template::MemberType Template::MemberFun()

OK?

Anyway, most importantly, the coding style should be compatible with VS2017 😸

[thewtex]

Thanks, @N-Dekker !