Piotr's patch was missing many occurences of size_t. So,

using std::size_t;
This is the only way that we can ensure QCC support in the long term without having to think about it everytime.

Eigen/src/Core/MatrixStorage.h

@@ -40,7 +40,7 @@ template <typename T, int Size, int MatrixOptions,
   ei_matrix_array()
   {
     #ifndef EIGEN_DISABLE_UNALIGNED_ARRAY_ASSERT
-    ei_assert((reinterpret_cast<std::size_t>(array) & 0xf) == 0
+    ei_assert((reinterpret_cast<size_t>(array) & 0xf) == 0
               && "this assertion is explained here: http://eigen.tuxfamily.org/dox/UnalignedArrayAssert.html  **** READ THIS WEB PAGE !!! ****");
     #endif
   }