Cannot use Extension with .ttc files and FontIndex

[adamliter]

Description

I'm not sure if this is a bug or the intended behavior, but it does not seem to be possible to use the Extension keyword with a .ttc file and using the *Features keywords with FontIndex when trying to set the main font.

Check/indicate

• [x] Relevant for XeTeX
• [x] Relevant for LuaTeX
• [x] Issue tracker has been searched for similar issues?
• [x] Links to <tex.stackexchange.com> discussion if appropriate:
  • Issue selecting font from TrueType Collection with LuaTeX

Minimal example demonstrating the issue

The following document does not correctly set the bold, italic, and bold italic fonts for me, even though I am able to directly set a bold font by defining a new font and using the UprightFeatures keyword.

\documentclass{article}

\usepackage{fontspec}
\setmainfont[
  Ligatures=TeX,
  Extension=.ttc,
  UprightFeatures={FontIndex=0},
  BoldFeatures={FontIndex=4},
  ItalicFeatures={FontIndex=2},
  BoldItalicFeatures={FontIndex=5}]{GillSans}

\newfontfamily\SemiBold[
  Ligatures=TeX,
  Extension=.ttc,
  UprightFeatures={FontIndex=4}]{GillSans}

\begin{document}
\textbf{asdf}
\textit{asdf}
asdf
\textbf{\textit{asdf}}

{\SemiBold asdf}
\end{document}