On 18 February 2013 13:02, Comeau At9Fans <comeauat9fans@gmail.com> wrote:

seems to be doing is setting up allowing the call to compile and once that is satisfied then the subsequent definition "has" to match it, as perhaps a way to do type punning.

No, the compiler is simply applying scope rules. Without that inner declaration explicitly overriding the outer declaration--whether static or extern is used--
it will not compile (eg, if you put "static void fn(Outer*);" or "extern void fn(Outer*);" and remove static from fn in the file scope).

The behaviour is undefined in ANSI C if two declarations that refer to the same object or function do not have compatible types
(normally, you're protected by another rule that you can't have incompatible declarations *in the same scope*).

ANSI C does, however, forbid the inner static declaration (which surprised me)

"The declaration of an identifier for a function that has block scope shall have no explicit storage-class specifier other than extern." (6.7.1)

We're probably saying the same thing. As you say ANSI C forbids it hence my comment about normally a diagnostic from a so-called mainstream compiler. And as you say without a declaration it would not compile either. The declaration should normally be in global scope (it could have been), which would have also produced a diagnostic since Inner/Outer don't match. That leaves the declaration where Eric showed it, which the Plan 9 compiler obviously allowed. As you note the net effect is it's undefined (if we're using ANSI C as the metric) hence created a kind of type pun (even if the original code did it as a mistake).