[Bug preprocessor/12607] preprocessor fails to diagnose an undefined ## result

[Neil Booth]

sebor at roguewave dot com wrote:-

> I see. I think I agree with you: it's the pp-token immediatley preceding the ##
> operator that, after concatenation, must form another valid token, and it is not
> required that the pp-token (or sequence thereof) substituted for the macro
> parameter form a single valid token after concatentation. Looks like HP
> misinterpreted this the same way I did -- their preprocessor complains (see below).

But "sequence thereof" is a useless concept in this context.  The stand
is quite clear - ## pastes just two tokens to create a token.

> Warning (anachronism) 823: "t.cpp", line 6 # Redundant preprocessing
> concatenation operation results in two valid preprocessing tokens. Depending on
> this implementation defined behavior will result in non-portable code.
>     /*   6 */ int j = CAT (s.a, b);   // #2
>                            ^               
>  int j = s.ab ;

This is simply wrong.  I bet you can't find a single compiler that
gets this wrong.  Further, there is nothing redundant about the ##;
if any compiler works in the way you intend with it *missing* then that
compiler is buggy.  I suggest you report this as a bug to EDG.

Neil.