Not at all.
Compiling your code using MSVC and MinGW ensures that your code is compatible with Microsoft C / C ++ libraries. As far as I understand, you are only talking about code compatibility, but such a thing does not exist. If you push C ++ to gray areas, it is quite possible that the same code will have different results depending on the platform on which you are compiling it.
The best and only way to guarantee full compatibility is to compile and test on both platforms.
Although using GCC with -std=c++0X -Wall -Wextra -pedantic (or any other std version) and getting rid of all warnings will give a good idea of ββthe quality of the code.
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