Add construct_at, destroy_at wrappers. Use throughout.

Eigen/src/Core/CoreEvaluators.h

@@ -1723,14 +1723,14 @@ struct evaluator<EvalToTemp<ArgType> >
   EIGEN_DEVICE_FUNC explicit evaluator(const XprType& xpr)
     : m_result(xpr.arg())
   {
-    ::new (static_cast<Base*>(this)) Base(m_result);
+    internal::construct_at<Base>(this, m_result);
   }
 
   // This constructor is used when nesting an EvalTo evaluator in another evaluator
   EIGEN_DEVICE_FUNC evaluator(const ArgType& arg)
     : m_result(arg)
   {
-    ::new (static_cast<Base*>(this)) Base(m_result);
+    internal::construct_at<Base>(this, m_result);
   }
 
 protected:

Eigen/src/Core/Inverse.h

@@ -104,7 +104,7 @@ struct unary_evaluator<Inverse<ArgType> >
   unary_evaluator(const InverseType& inv_xpr)
     : m_result(inv_xpr.rows(), inv_xpr.cols())
   {
-    ::new (static_cast<Base*>(this)) Base(m_result);
+    internal::construct_at<Base>(this, m_result);
     internal::call_assignment_no_alias(m_result, inv_xpr);
   }
 

Eigen/src/Core/ProductEvaluators.h

@@ -109,7 +109,7 @@ struct product_evaluator<Product<Lhs, Rhs, Options>, ProductTag, LhsShape, RhsSh
   explicit product_evaluator(const XprType& xpr)
     : m_result(xpr.rows(), xpr.cols())
   {
-    ::new (static_cast<Base*>(this)) Base(m_result);
+    internal::construct_at<Base>(this, m_result);
 
 // FIXME shall we handle nested_eval here?,
 // if so, then we must take care at removing the call to nested_eval in the specializations (e.g., in permutation_matrix_product, transposition_matrix_product, etc.)

Eigen/src/Core/Ref.h

@@ -199,8 +199,8 @@ protected:
       return false;
     }
 
-    ::new (static_cast<Base*>(this)) Base(expr.data(), rows, cols);
-    ::new (&m_stride) StrideBase(
+    internal::construct_at<Base>(this, expr.data(), rows, cols);
+    internal::construct_at(&m_stride,
       (StrideType::OuterStrideAtCompileTime == 0) ? 0 : outer_stride,
       (StrideType::InnerStrideAtCompileTime == 0) ? 0 : inner_stride );
     return true;

Eigen/src/Core/ReturnByValue.h

@@ -106,7 +106,7 @@ struct evaluator<ReturnByValue<Derived> >
   EIGEN_DEVICE_FUNC explicit evaluator(const XprType& xpr)
     : m_result(xpr.rows(), xpr.cols())
   {
-    ::new (static_cast<Base*>(this)) Base(m_result);
+    internal::construct_at<Base>(this, m_result);
     xpr.evalTo(m_result);
   }
 

Eigen/src/Core/Solve.h

@@ -125,7 +125,7 @@ struct evaluator<Solve<Decomposition,RhsType> >
   EIGEN_DEVICE_FUNC explicit evaluator(const SolveType& solve)
     : m_result(solve.rows(), solve.cols())
   {
-    ::new (static_cast<Base*>(this)) Base(m_result);
+    internal::construct_at<Base>(this, m_result);
     solve.dec()._solve_impl(solve.rhs(), m_result);
   }
 

Eigen/src/Core/util/Memory.h

@@ -1150,6 +1150,38 @@ inline int queryTopLevelCacheSize()
   return (std::max)(l2,l3);
 }
 
+
+
+/** \internal
+ * This wraps C++20's std::construct_at, using placement new instead if it is not available.
+ */
+
+#if EIGEN_COMP_CXXVER >= 20
+using std::construct_at;
+#else
+template<class T, class... Args>
+T* construct_at( T* p, Args&&... args )
+{
+  return ::new (const_cast<void*>(static_cast<const volatile void*>(p)))
+    T(std::forward<Args>(args)...);
+}
+#endif
+
+/** \internal
+ * This wraps C++17's std::destroy_at.  If it's not available it calls the destructor.
+ * The wrapper is not a full replacement for C++20's std::destroy_at as it cannot
+ * be applied to std::array.
+ */
+#if EIGEN_COMP_CXXVER >= 17
+using std::destroy_at;
+#else
+template<class T>
+void destroy_at(T* p)
+{
+  p->~T();
+}
+#endif
+
 } // end namespace internal
 
 } // end namespace Eigen