Compilation fails with gcc5.3.0 due to isnan() ambiguity

[TobiKattmann]

Describe the bug
In PR #1105 an isnan(SU2_TYPE::GetValue(monitor)) check was introduced. I get a compiler error (serial build, no AD DD involved) call of overload isnan(souble&) is ambiguos so the complier is probably confused about the return type of SU2_TYPE::GetValue(monitor) which is passivedouble which itself is hardcoded to be double in datatype_structure.hpp. Or sth else is happening idk

I only have the problem with gcc5.3.0. On the same system I have an unsupported gcc7.4.0 that works. My private machine with gcc10.2.0 works fine as well. So I guess it is a specific compiler problem.

In my I hit the ⌨️ a few times without actually knowing what I am doing:

• use isnan(monitor) ... same error
• use isnan(static_cast<double>(monitor)) ... same error
• use isnan(static_cast<double>(SU2_TYPE::GetValue(monitor))) ... same error
• changed the return type of GetValue to double ...same error

I guess this problem is really specific to my setup. gcc5.3.0 is old but not grandpa kinda old with the release dec 2015 ... well ok maybe it is 👴
I would be happy with some suggestions what to try out maybe, also if I misinterpret the error a pointer in the right direction would help. If @pcarruscag could have a quick look that would be great.

Thanks and warm regards, Tobi

To Reproduce
You probably cant without an old enough compiler

Desktop (please complete the following information):

• OS: RHES 7.6
• C++ compiler and version: g++ (GCC) 5.3.0
• SU2 Version: latest develop

[pcarruscag]

Can you try sticking the good old std:: behind isnan
(I don't have 5.3.0 and 5.4.0 does not complain).

[TobiKattmann]

Dang... that fixes it 🐙 . I close issue then as you already resolve this in PR #1107
Thanks pedro