message: Replace `static constexpr int` with `enum`.
diff: @@ -503,7 +503,7 @@ protected:
  struct InterpConfig
  {
  int config;
  - static constexpr int conforming = -1; // helper value
  + enum { conforming = -1 }; // helper value
  // Conforming face
  InterpConfig() : config(conforming) { }
  