View Compilation Error

0_0_38705350_10041.cpp:33:29: error: 'constexpr' needed for in-class initialization of static data member 'const double kD_Tree<K>::alpha' of non-integral type [-fpermissive]
   33 |         const static double alpha=0.75;
      |                             ^~~~~
0_0_38705350_10041.cpp: In instantiation of 'class kD_Tree<5>':
0_0_38705350_10041.cpp:155:11:   required from here
0_0_38705350_10041.cpp:33:29: error: 'constexpr' needed for in-class initialization of static data member 'const double kD_Tree<5>::alpha' of non-integral type [-fpermissive]