Error: The newtxmath and tensor packages with subscriptcorrection

Question

Compiling the following Minimal Broken Example

\documentclass{article}
\usepackage{newtxtext}
\usepackage[subscriptcorrection]{newtxmath}
%\usepackage[lite,subscriptcorrection]{mtpro2}
\usepackage{tensor}
\begin{document}
Consider the following:
\[
A_f = R\indices{_\mu^{\sigma\nu}}
\]
\end{document}

leads to the following error messages:

! Package tensor Error: Sub/Superscript items out of order on input line 9, 
(tensor)                some index tokens may now have been lost.

See the tensor package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.9 A_f = R\indices{_\mu^{\sigma\nu}}

? 

However, the code compiles perfectly fine if

• one uses \usepackage{newtxmath}, i.e., without subscriptcorrection; OR
• one uses \usepackage[lite]{mtpro2}; OR
• one uses \usepackage[lite,subscriptcorrection]{mtpro2}.

What is going on here? Is there a fix to this problem?

Added: I have noticed that both newtxmath and mtpro2 implement subscriptcorrection by making the underscore _ active (although it is more likely that newtxmath is inspired by mtpro2).

---

P.S. The working output if one uses \usepackage[lite,subscriptcorrection]{mtpro2}:

Answer 1

The tensor package uses a chain of \ifx comparisons to work out what type of index has been given. That works of course if the catcode of _ (and ^) when tensor is loaded is the same as when \indices is used. However, here the catcode of _ in the preamble is different from the document body: when using math-active _, it's catcode is set to 12 rather than 8. The easiest work-around is to load tensor with the correct catcode applied:

\documentclass{article}
\usepackage{newtxtext}
\usepackage[subscriptcorrection]{newtxmath}
\catcode`\_=12 %
\usepackage{tensor}
\catcode`\_=8 %
\begin{document}
Consider the following:
\[
A_f = R\indices{_\mu^{\sigma\nu}}
\]
\end{document}

Presumably the mtpro2 package sets the catcode of _ immediately: the newtxmath package delays until the start of the document.

A slightly fancier version which automates whether to set the catcode:

\documentclass{article}
\usepackage{newtxtext}
\usepackage[subscriptcorrection]{newtxmath}
\makeatletter
\edef\savedcodes{\catcode`\noexpand\_=\the\catcode`\_}
\edef\@tempa{\csname opt@newtxmath.sty\endcsname}
\def\@tempb{{subscriptcorrection}}
\expandafter\expandafter\expandafter
  \in@\expandafter\@tempb\expandafter{\@tempa}
\ifin@
  \catcode`\_=12 %
\fi
\makeatother
\usepackage{tensor}
\savedcodes
\begin{document}
Consider the following:
\[
A_f = R\indices{_\mu^{\sigma\nu}}
\]
\end{document}