It appears this library uses the deduced return type feature of C++14. However the Seeedstudio WIO Terminal compiles at C++11 (-std=gnu++11) (see the platform.txt for the Arduino IDE compilation).
You might want to either change the cpp compilation for options in the platform.txt to -std=gnu++14 OR don't use deduced return types.
It appears this library uses the deduced return type feature of C++14. However the Seeedstudio WIO Terminal compiles at C++11 (-std=gnu++11) (see the platform.txt for the Arduino IDE compilation).
You might want to either change the cpp compilation for options in the platform.txt to -std=gnu++14 OR don't use deduced return types.