Revert "fix nonlinear"

This reverts commit 9e48676.

Author: tarun-mitruka

ikarus/finiteelements/mechanics/nonlinearelastic.hh

@@ -152,11 +152,12 @@ public:
    */
   template <typename ScalarType, int strainDim, bool voigt = true>
   auto materialTangent(const Eigen::Vector<ScalarType, strainDim>& strain) const {
-    if constexpr (Concepts::AutodiffScalar<ScalarType>) {
+    if constexpr (std::is_same_v<ScalarType, double>)
+      return mat_.template tangentModuli<strainType, voigt>(strain);
+    else {
       decltype(auto) matAD = mat_.template rebind<ScalarType>();
       return matAD.template tangentModuli<strainType, voigt>(strain);
-    } else
-      return mat_.template tangentModuli<strainType, voigt>(strain);
+    }
   }
 
   /**
@@ -169,11 +170,12 @@ public:
    */
   template <typename ScalarType, int strainDim>
   auto getInternalEnergy(const Eigen::Vector<ScalarType, strainDim>& strain) const {
-    if constexpr (Concepts::AutodiffScalar<ScalarType>) {
+    if constexpr (std::is_same_v<ScalarType, double>)
+      return mat_.template storedEnergy<strainType>(strain);
+    else {
       decltype(auto) matAD = mat_.template rebind<ScalarType>();
       return matAD.template storedEnergy<strainType>(strain);
-    } else
-      return mat_.template storedEnergy<strainType>(strain);
+    }
   }
 
   /**
@@ -187,11 +189,12 @@ public:
    */
   template <typename ScalarType, int strainDim, bool voigt = true>
   auto getStress(const Eigen::Vector<ScalarType, strainDim>& strain) const {
-    if constexpr (Concepts::AutodiffScalar<ScalarType>) {
+    if constexpr (std::is_same_v<ScalarType, double>)
+      return mat_.template stresses<strainType, voigt>(strain);
+    else {
       decltype(auto) matAD = mat_.template rebind<ScalarType>();
       return matAD.template stresses<strainType, voigt>(strain);
-    } else
-      return mat_.template stresses<strainType, voigt>(strain);
+    }
   }
 
   const Geometry& geometry() const { return *geo_; }