\overline \mathrm x works in KaTeX and plain LaTeX, and the MathAtom2 test confirms this. However, \overline \mathrm x does not work when \usepackage{amsmath} is in effect. (This quirk is mentioned also here: mathjax/MathJax#677) The trouble is that this quirk causes texcmp to fail on this test. Perhaps we should add an option in the YAML to turn off amsmath in this test?