[Bug c/67386] missing diagnostic on a use of an undeclared function

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67386

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Fri, 28 Aug 2015, msebor at gcc dot gnu.org wrote:

> My reading was that the implicit declaration is intended to be in effect only
> for the call to the otherwise undeclared function, but GCC and the other
> compilers I've tried let it persist (at least) until the end of the scope and

Yes (remember, two scopes are the same if they end at the same point; 
saying what block the declaration appears in is the same as saying where 
it ends).

> I think Clang and IBM xlc are both wrong since the reference to abs on line 8
> should clearly be diagnosed.  The C90 words aren't completely clear about where
> in the innermost block the extern int identifier(); declaration is supposed to
> appear but it stands to reason that it should appear where all other
> declarations must appear in C90: before any executable code.  So diagnosing the

In general, it's a mistake to interpret "X is equivalent to Y" statements 
in the C standard as referring to a textual substitution; there are plenty 
of other places where applying such a substitution goes wrong.  Cf. cases 
where something is said to be equivalent to a particular sequence of 
declarations and statements and it must be implicitly understood that the 
variable names in those declarations and statements are not special in any 
way.