[LyX/master] Fix #11552 in a different way

[Enrico Forestieri]

commit 54147a7140cbb053711dd9f2cf15763b0943659e
Author: Enrico Forestieri <for...@lyx.org>
Date:   Fri Apr 19 17:26:14 2019 +0200

    Fix #11552 in a different way
    
    The reported issue is fixed in a different way, not
    involving the fix for #9348.
    
    This commit supercedes [f2029fb3/lyxgit] and [17abf68b/lyxgit].
---
 src/mathed/MathParser.cpp |   21 +++++++++++++++------
 1 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/src/mathed/MathParser.cpp b/src/mathed/MathParser.cpp
index 0286ba1..c41a6cf 100644
--- a/src/mathed/MathParser.cpp
+++ b/src/mathed/MathParser.cpp
@@ -946,13 +946,22 @@ bool Parser::parse1(InsetMathGrid & grid, unsigned flags,
                        parse(ar, FLAG_BRACE_LAST, mode);
                        // do not create a BraceInset if they were written by 
LyX
                        // this helps to keep the annoyance of  "a choose b"  
to a minimum
-                       // But create a BraceInset if a script follows (#9348)
                        InsetMathBrace const * mb;
-                       if (ar.size() > 0)
-                               mb = ar[0]->asBraceInset();
-                       if (ar.size() == 1 && (ar[0]->extraBraces()
-                                              || (mb && mb->cell(0).size()
-                                                  && 
!mb->cell(0)[0].nucleus()->asScriptInset())))
+                       InsetMathChar const * mc;
+                       for (size_type i = 0; i < ar.size(); ++i) {
+                               mb = ar[i]->asBraceInset();
+                               mc = mb && mb->cell(0).size() > 1 && 
mb->cell(0)[0]->asMacro()
+                                       ? mb->cell(0)[1]->asCharInset(): 0;
+                               if (mc && mc->getChar() == '[') {
+                                       // Remove the BraceInset around a macro
+                                       // with optional arguments. It will be
+                                       // automatically reinserted on write.
+                                       MathData md = mb->cell(0);
+                                       ar.erase(i);
+                                       ar.insert(i,md);
+                               }
+                       }
+                       if (ar.size() == 1 && ar[0]->extraBraces())
                                cell->append(ar);
                        else
                                cell->push_back(MathAtom(new 
InsetMathBrace(ar)));