Implicit built-in function declarations

["Joseph S. Myers" <jsm at polyomino dot org dot uk>]

Part of a patch I'm testing for bug 13801 leads to mandatory warnings for
implicit declarations of built-in functions (other than those in the
implementation namespace ^_[_A-Z]) if the type of the implicit declaration
is not compatible with the type of the built-in function.  Are there any
comments on this (that the warnings shouldn't be on by default, that they
should be mandatory errors, ...)?

Note that all implicit declarations of built-in functions (not just those
with incompatible types) have had warnings for some time at -Wall and by
default in C99 mode.

The most common instances in gcc.dg are for implicit declarations of abort
and exit, but other functions involved (outside the tests for built-in
functions) include printf, memcpy, strlen, memset, fabs and alloca; any 
built-in function with a return type that is not int, or which is 
variadic, or which has an argument changed by the default argument 
promotions, will receive the warning if implicitly declared.

-- 
Joseph S. Myers               http://www.srcf.ucam.org/~jsm28/gcc/
    jsm@polyomino.org.uk (personal mail)
    jsm28@gcc.gnu.org (Bugzilla assignments and CCs)