XeTeX: can not use fontname and filename in one command

[antonistsolomitis]

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Description

can't use with xelatex a system-install font with a TeX instaled font. Say a command such as \setmainfont[ItalicFont=AFontInTeXTree.otf]{Times New Roman} fails. (It works with lualatex) This is necessary when providing complemented glyphs to a commercial installed font. For example, the fontsetup package provides fspMinionGreekSmallCaps.otf and fspPalatinoGreekSmallCaps.otf to complement the commercial MinionPro and LinotypePalatino. So the fsp fonts will be TeX-installed but typically MinionPro or LinotypePalatino are system-installed in, say, the Windows platform.

Check/indicate

• [X] Relevant for XeTeX
• [ ] Relevant for LuaTeX
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• [ ] Links to <tex.stackexchange.com> discussion if appropriate

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Cambria}[ItalicFont=cmunit.otf]
\begin{document}
hello \textit{hello}
\end{document}

Further details