({

Hello, C++.

I'm having some difficulty (amending Emacs's C++ Mode) reconciling the
two conflicting uses in C++ of ({.

Firstly, it is used as a "statement expression", a GCC enhancement also
found in C, allowing a more relaxed and natural way to write an
expression as the end result of a sequence of statements:

     ({ int y = foo (); int z;
        if (y > 0) z = y;
else z = - y;
z; })

.  I think this usage is very old.

Secondly, there's initialisation expressions like:

  void f4 (int a, int b, int c)
  {
    std::vector<ABC> abcList2(
      {{a+6,
        b+6,
        c+6}
      }
      );
  ....
  }

.  Here the ( on the std::vector line, together with the next {, can be
confused as a statement expression, though it's clearly not meant that
way.  I think this syntax is much newer than the other one, though I may
be wrong here.

In calculating the indentation for source lines in these constructs, the
ambiguity causes mis-indentation for one or the other of them.

Now to my question: how common is GCC's statement expression in the wide
world of C++ source code?  How much would be lost if I simply removed the
statement expression from C++ Mode's parsing functions?

--
Alan Mackenzie (Nuremberg, Germany).