message: Add missing explicit qualifier
diff: @@ -58,7 +58,7 @@ public:
  // -- constructors, destructors, and assignment operators --------------------
  - action(impl_ptr ptr) noexcept : pimpl_(std::move(ptr)) {
  + explicit action(impl_ptr ptr) noexcept : pimpl_(std::move(ptr)) {
  // nop
  }
  