* default MatrixBase ctor: make it protected, make it a static assert, only do the check when debugging eigen to avoid slowing down compilation for everybody (this check is paranoiac, it's very seldom useful)

* add private MatrixBase ctors to catch cases when the user tries to construct MatrixBase objects directly

Eigen/src/Core/MatrixBase.h

@@ -136,12 +136,6 @@ template<typename Derived> class MatrixBase
           */
     };
 
-    /** Default constructor. Just checks at compile-time for self-consistency of the flags. */
-    MatrixBase()
-    {
-      ei_assert(ei_are_flags_consistent<Flags>::ret);
-    }
-
 #ifndef EIGEN_PARSED_BY_DOXYGEN
     /** This is the "real scalar" type; if the \a Scalar type is already real numbers
       * (e.g. int, float or double) then \a RealScalar is just the same as \a Scalar. If
@@ -627,6 +621,24 @@ template<typename Derived> class MatrixBase
     #ifdef EIGEN_MATRIXBASE_PLUGIN
     #include EIGEN_MATRIXBASE_PLUGIN
     #endif
+
+  protected:
+    /** Default constructor. Do nothing. */
+    MatrixBase()
+    {
+      /* Just checks for self-consistency of the flags.
+       * Only do it when debugging Eigen, as this borders on paranoiac and could slow compilation down
+       */
+#ifdef EIGEN_INTERNAL_DEBUGGING
+      EIGEN_STATIC_ASSERT(ei_are_flags_consistent<Flags>::ret,
+                          INVALID_MATRIXBASE_TEMPLATE_PARAMETERS)
+#endif
+    }
+
+  private:
+    MatrixBase(int);
+    MatrixBase(int,int);
+    template<typename OtherDerived> explicit MatrixBase(const MatrixBase<OtherDerived>&);
 };
 
 #endif // EIGEN_MATRIXBASE_H

Eigen/src/Core/util/StaticAssert.h

@@ -74,6 +74,7 @@
         THIS_METHOD_IS_ONLY_FOR_COLUMN_MAJOR_MATRICES,
         THIS_METHOD_IS_ONLY_FOR_ROW_MAJOR_MATRICES,
         INVALID_MATRIX_TEMPLATE_PARAMETERS,
+        INVALID_MATRIXBASE_TEMPLATE_PARAMETERS,
         BOTH_MATRICES_MUST_HAVE_THE_SAME_STORAGE_ORDER,
         THIS_METHOD_IS_ONLY_FOR_DIAGONAL_MATRIX
       };