Q:

Eigen::LLT< Eigen::MatrixXd > has incomplete type

Check out this code. Compiled on Ubuntu ...
MatrixXd A(3,3);
A << 4,-1,2, -1,6,0, 2,0,5;
cout << "The matrix A is" << endl << A << endl;
LLT<MatrixXd> lltOfA(A); // compute the Cholesky decomposition of A

Here's a doctest case:
#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
#include <doctest/doctest.h>
#include <Eigen/Core>

TEST_CASE("llt")
{
  Eigen::MatrixXd A(3,3);
  A<<1,2,3,4,5,6,7,8,9;
  Eigen::LLT<Eigen::MatrixXd> lltof(A);
}

Compilation fails with: 
/src/test/test-proto.cc:40:38: error: variable ‘Eigen::LLT<Eigen::Matrix<double, -1, -1>, 1> lltof’ has initializer but incomplete type
   Eigen::LLT<Eigen::MatrixXd> lltof(A);

What gives? This is reduced from my code to exactly represent the documentation.

A:

Oops. Test case should be:
#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
#include <doctest/doctest.h>
#include "proto.h"
#include <Eigen/Dense> //NOT Eigen/Core

TEST_CASE("llt")
{
  Eigen::MatrixXd A(3,3);
  A<<1,2,3,4,5,6,7,8,9;
  Eigen::LLT<Eigen::MatrixXd> lltof(A);
}

Note the change in #include. 
Dumb mistake, but I'm leaving it up for my future self / google.

