[DF] Use enable_if_t alias wherever possible, for readability

Author: eguiraud

tree/dataframe/inc/ROOT/RDF/ActionHelpers.hxx

@@ -392,28 +392,28 @@ class R__CLING_PTRCHECK(off) FillHelper : public RActionImpl<FillHelper<HIST>> {
    // in c++17 or later
 
    // return unchanged value for scalar
-   template <typename T, typename std::enable_if<!IsDataContainer<T>::value, int>::type = 0>
+   template <typename T, std::enable_if_t<!IsDataContainer<T>::value, int> = 0>
    ScalarConstIterator<T> MakeBegin(const T &val)
    {
       return ScalarConstIterator<T>(&val);
    }
 
    // return iterator to beginning of container
-   template <typename T, typename std::enable_if<IsDataContainer<T>::value, int>::type = 0>
+   template <typename T, std::enable_if_t<IsDataContainer<T>::value, int> = 0>
    auto MakeBegin(const T &val)
    {
       return std::begin(val);
    }
 
    // return 1 for scalars
-   template <typename T, typename std::enable_if<!IsDataContainer<T>::value, int>::type = 0>
+   template <typename T, std::enable_if_t<!IsDataContainer<T>::value, int> = 0>
    std::size_t GetSize(const T &)
    {
       return 1;
    }
 
    // return container size
-   template <typename T, typename std::enable_if<IsDataContainer<T>::value, int>::type = 0>
+   template <typename T, std::enable_if_t<IsDataContainer<T>::value, int> = 0>
    std::size_t GetSize(const T &val)
    {
 #if __cplusplus >= 201703L
@@ -452,15 +452,14 @@ public:
    void InitTask(TTreeReader *, unsigned int) {}
 
    // no container arguments
-   template <typename... ValTypes,
-             typename std::enable_if<!Disjunction<IsDataContainer<ValTypes>...>::value, int>::type = 0>
+   template <typename... ValTypes, std::enable_if_t<!Disjunction<IsDataContainer<ValTypes>...>::value, int> = 0>
    void Exec(unsigned int slot, const ValTypes &...x)
    {
       fObjects[slot]->Fill(x...);
    }
 
    // at least one container argument
-   template <typename... Xs, typename std::enable_if<Disjunction<IsDataContainer<Xs>...>::value, int>::type = 0>
+   template <typename... Xs, std::enable_if_t<Disjunction<IsDataContainer<Xs>...>::value, int> = 0>
    void Exec(unsigned int slot, const Xs &...xs)
    {
       // array of bools keeping track of which inputs are containers
@@ -509,14 +508,14 @@ public:
    }
 
    // if the fObjects vector type is derived from TObject, return the name of the object
-   template <typename T = HIST, typename std::enable_if<std::is_base_of<TObject, T>::value>::type * = nullptr>
+   template <typename T = HIST, std::enable_if_t<std::is_base_of<TObject, T>::value, int> = 0>
    std::string GetActionName()
    {
       return std::string(fObjects[0]->IsA()->GetName()) + "<BR/>" + std::string(fObjects[0]->GetName());
    }
 
    // if fObjects is not derived from TObject, indicate it is some other object
-   template <typename T = HIST, typename std::enable_if<!std::is_base_of<TObject, T>::value>::type * = nullptr>
+   template <typename T = HIST, std::enable_if_t<!std::is_base_of<TObject, T>::value, int> = 0>
    std::string GetActionName()
    {
       return "Fill custom object";