Correct parsing of ternary operator.

[Walter Murray]

Mike Lijewski writes:

> Consider the following code:
>    main()
>    {
>        int a = 0, b = 1, c = 2;
>        c ? c = a : c = b;
>        exit(0);
>    }
> According to the precedence rules of C, the conditional expression
> should be evaluated as:
> 	 (c ? c = a : c) = b;
> which should produce and error.  Well, it happens that I have a
> compiler which accepts this and produces results which suggest
> to me that it is evaluating this expression as:
> 	 c ? c = a :(c = b);
> The vendor seems unwilling to accept this as a bug.  

If your vendor claims conformance to the forthcoming ANSI standard,
the answer is easy:  the program is illegal.  An ANSI-conforming
compiler will produce a diagnostic.  If it chooses to make the
diagnostic a warning, it is free to proceed and generate whatever
code it pleases.  The only requirement is that a diagnostic be
produced.

If ANSI conformance is not claimed, the answer is a little harder.
Does the vendor provide a programmer's reference manual that describes
what the behavior should be?

> Am I just being pedantic, or is this a serious bug?  

You gave the compiler an expression that you admit is illegal.
The compiler interpreted the expression in a way that would make
sense to a lot of programmers.  I wouldn't call that a serious
bug.  Some people would call it a feature.

It all boils down to a question of whether the expression is
bad enough to evoke a diagnostic from the compiler, and that's
a matter of opinion.  (ANSI says Yes.)  Does your compiler have
an option to make it more verbose about questionable constructs?
Try lint; it will probably catch the error.

> I haven't been able to come up with an example of correct usage
> of the ternary operator, which produces incorrect code. 

Keep trying.

> If anyone knows of some code which might clearly prove that
> this is a bug, I would sure appreciate receiving it.

For what it's worth, Harbison & Steele (C:  A Reference Manual)
clearly state that the expression is illegal (2nd ed., p. 182).

Walter Murray
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