Re: C++ PATCH for c++/70744 (wrong-code with x ?: y extension)

[Jason Merrill <jason at redhat dot com>]

On 04/22/2016 07:50 AM, Marek Polacek wrote:
> This PR shows that we generate wrong code with the x ?: y extension in case the
> first operand contains either predecrement or preincrement.  The problem is
> that we don't emit SAVE_EXPR, thus the operand is evaluated twice, which it
> should not be.
>
> While ++i or --i can be lvalues in C++, i++ or i-- can not.  The code in
> build_conditional_expr_1 has:
>   4635       /* Make sure that lvalues remain lvalues.  See g++.oliva/ext1.C.  */
>   4636       if (real_lvalue_p (arg1))
>   4637         arg2 = arg1 = stabilize_reference (arg1);
>   4638       else
>   4639         arg2 = arg1 = save_expr (arg1);
> so for ++i/--i we call stabilize_reference, but that doesn't know anything
> about PREINCREMENT_EXPR or PREDECREMENT_EXPR and just returns the same
> expression, so SAVE_EXPR isn't created.
>
> I think one fix would be to teach stabilize_reference what to do with those,
> similarly to how we handle COMPOUND_EXPR there.

Your change will turn the expression into an rvalue, so that isn't 
enough.  We need to turn the expression into some sort of _REF before 
passing it to stabilize_reference, perhaps by factoring out the 
cast-to-reference code from force_paren_expr.  This should probably be 
part of a cp_stabilize_reference function that replaces all uses of 
stabilize_reference in the front end.

Jason