LEFTRIGHT<left delimiter><right delimiter>{<contents>}% `mtpro2' exclusive

left<left delimiter> <contents> right<right delimiter>

% From `mtpro2.sty'
DeclareOption{curlybraces}{letmtp@br=c}
DeclareOption{morphedbraces}{letmtp@br=m}
DeclareOption{straightbraces}{letmtp@br=s}

documentclass{article}
usepackage{mathtools}% loads `amsmath'
usepackage{newtxtext}
usepackage[scaled=0.861,lining]{FiraMono}
defbracesshape{curlybraces}% change here to obtain different braces
% curlybraces
% morphedbraces
% straightbraces
usepackage[lite,bracesshape]{mtpro2}

% Patches begin
makeatletter
newsavebox{mtp@casesbox}
% Activate `mathtools' syntax
MHInternalSyntaxOn
% Curly braces are used only if `curlybraces' is set
% From `mtpro2.sty': DeclareOption{curlybraces}{letmtp@br=c}
MH_if_meaning:NN mtp@br c
  defMTP_MT_start_cases:nnn #1#2#3{ % #1=sep,#2=lpreamble,#3=rpreamble
   RIfM@else
     nonmatherr@{begin{@currenvir}}
   fi
   MH_group_align_safe_begin:
   setboxmtp@casesbox=hboxbgroup$% <- put contents in `mtp@casesbox'
   vcenter bgroup
       Let@ chardefdspbrk@context@ne restore@math@cr
       let  math@cr@@AMS@math@cr@@
       spread@equation
       ialignbgroup
         strut@#2 &#1strut@
         #3
         crcr
  }
  defMTP_MH_end_cases:{crcregroup
   restorecolumn@
   egroup
   $egroup% <- close the `hbox'
   MH_group_align_safe_end:
  }
  newcommand*mtp@newcases[6]{% #1=name, #2=sep, #3=lpreamble, #4=rpreamble, #5=left, #6=right
   newenvironment{#1}
     {MTP_MT_start_cases:nnn {#2}{#3}{#4}}
     {MTP_MH_end_cases:LEFTRIGHT#5#6{copymtp@casesbox}}
  }
  newcommand*mtp@renewcases[6]{
   renewenvironment{#1}
     {MTP_MT_start_cases:nnn {#2}{#3}{#4}}
     {MTP_MH_end_cases:LEFTRIGHT#5#6{copymtp@casesbox}}
  }
  mtp@renewcases{dcases}{quad}{%
    $m@thdisplaystyle{##}$hfil}{$m@thdisplaystyle{##}$hfil}{lbrace}{.}
  mtp@renewcases{dcases*}{quad}{%
    $m@thdisplaystyle{##}$hfil}{{##}hfil}{lbrace}{.}
  mtp@renewcases{rcases}{quad}{%
    $m@th{##}$hfil}{$m@th{##}$hfil}{.}{rbrace}
  mtp@renewcases{rcases*}{quad}{%
    $m@th{##}$hfil}{{##}hfil}{.}{rbrace}
  mtp@renewcases{drcases}{quad}{%
    $m@thdisplaystyle{##}$hfil}{$m@thdisplaystyle{##}$hfil}{.}{rbrace}
  mtp@renewcases{drcases*}{quad}{%
    $m@thdisplaystyle{##}$hfil}{{##}hfil}{.}{rbrace}
  mtp@renewcases{cases*}{quad}{%
    $m@th{##}$hfil}{{##}hfil}{lbrace}{.}
MH_fi:
% Deactivate `mathtools' syntax
MHInternalSyntaxOff
makeatother
% Patches end

newcommand*showopendelimitersizes[1]{%
  #1bigl#1Bigl#1biggl#1Biggl#1}

begin{document}

section*{texttt{dcases*} in texttt{align} work?}
verb|dcases*| and friends from verb|mathtools| work,
iff verb|curlybraces| is set.
subsection*{Package texttt{mtpro2} options: texttt{lite,bracesshape}}
begin{align*}
showopendelimitersizes{lbrace}
begin{dcases*}
int_a^b f(x) , mathrm{d}x    & Nothing to see here \
sum_{n=1}^infty frac{1}{n^2} & Otherwise
end{dcases*}
end{align*}
With verb|morphedbraces| or verb|straightbraces|,
errors appear.

end{document}

% Hit <compile>
! Package amsmath Error: begin{document} allowed only in math mode.

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

l.21      nonmatherr@{begin{@currenvir}}

% Hit <return>
! Only one # is allowed per tab.
l.31          #
               3

% Hit <return>
! Extra fi.
l.64 MH_fi:

% Hit <return>
! Undefined control sequence.
dcases* ->MTP_MT_start_cases:nnn
                                   {quad }{$m@th displaystyle {##}$hfil ...
l.85 end{align*}

...