By "cross-product matrix", I mean the scenario where you want to use the matrix strategy on a cross-product of multiple dimensions. For example, say you want to run tests on 2 OS (Linux, Windows) and 2 versions of Python (3.6, 3.7). Currently, you'd need to manually create a matrix with all the combinations:

strategy:
  matrix:
    Linux_Python36:
      VM_IMAGE: 'ubuntu-16.04'
      PYTHON_VERSION: '3.6'
    Linux_Python37:
      VM_IMAGE: 'ubuntu-16.04'
      PYTHON_VERSION: '3.7'
    Windows_Python36:
      VM_IMAGE: 'vs2017-win2016'
      PYTHON_VERSION: '3.6'
    Windows_Python37:
      VM_IMAGE: 'vs2017-win2016'
      PYTHON_VERSION: '3.7'

While this isn't too bad with a small number of dimensions and a small number of variables in each dimension, it can quickly become unmaintainable as the dimensions and variables grow. It would be nice if there was a way to express this more succinctly, something like:

strategy:
  cross-product:
    PYTHON_VERSION: [ '3.6', '3.7' ]
    VM_IMAGE: [ `ubuntu-16.04`, `vs2017-win2016` ]