Undocumented #if feature/bug

[Kendall]

I have heard #if recommended over #if(n)def for conditional compilation,
because it is safe from typos; that is, if you misspell the macro name in an
#if(n)def you may silently get the wrong effect, while if you misspell a macro
name in an #if, so the argument goes, you will get an error message.  Unfor-
tunately it is not so, as I recently discovered: in #if, 0 is substituted for
an undefined macro name, so that `#if name' is a synonym of `#if defined(name)
&& (name)'.  Many programs, the cpp itself among them, depend on this.

The cpp should be changed to issue a warning whenever this feature/bug is used,
because it is undocumented, even on System V (where many obscure cpp features
are documented for the first time), and because without the warning the
programmer is denied important error-checking.

				Sam Kendall
				Delft Consulting Corporation
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