Use if constexpr in smt_function_application_termt::indices

With C++17's `if constexpr` feature we can now remove two alternate
function templates and just put the alternate implementations in line.
This puts the specialisation into the outer function, so that it is
specialised with the correct implementation for the given `functiont`,
rather than dispatching based on the `std::true_type` /
`std::false_type` overloads.

Author: thomasspriggs

src/solvers/smt2_incremental/ast/smt_terms.h

@@ -153,28 +153,15 @@ class smt_function_application_termt : public smt_termt
   {
   };
 
-  /// Overload for when \p functiont does not have indices.
-  template <class functiont>
-  static std::vector<smt_indext>
-  indices(const functiont &function, const std::false_type &has_indices)
-  {
-    return {};
-  }
-
-  /// Overload for when \p functiont has indices member function.
-  template <class functiont>
-  static std::vector<smt_indext>
-  indices(const functiont &function, const std::true_type &has_indices)
-  {
-    return function.indices();
-  }
-
-  /// Returns `function.indices` if `functiont` has an `indices` member function
-  /// or returns an empty collection otherwise.
+  /// Returns `function.indices()` if `functiont` has an `indices` member
+  /// function or returns an empty collection otherwise.
   template <class functiont>
   static std::vector<smt_indext> indices(const functiont &function)
   {
-    return indices(function, has_indicest<functiont>{});
+    if constexpr(has_indicest<functiont>::value)
+      return function.indices();
+    else
+      return {};
   }
 
 public: