make f(x, y=3) a syntax error

[StefanKarpinski]

This reserves the syntax for keyword arguments in the future (#485).

[o-jasper]

a = :(f(x,=(y,3))).args[3]
b = :(f(x,y=3)).args[3] #This one will produce something else/a syntax error.
((a.head,a.args),(b.head,b.args)) #->((call,{=, y, 3}),(=,{y, 3}))

It isn't clear anymore how to make some arbitrary expression if the parser 'gets in your way'? Can be done though:

c = :(f(x,$Expr(:(=),{:y,3},Any))).args[3] #-> y = 3, that was what we wanted.

[JeffBezanson]

This isn't really a big deal, we're just reserving this syntax for keyword arguments in the future, so it should be an error now to ensure that code doesn't break.

[o-jasper]

In a sense maybe it was better off in eval(or the equivalent) or codegen.cpp somehow, so quote .. end and :(..) aren't affected, but too much goes on there already i guess.