GNU C: Implicit int and implicit function definitions

[Florian Weimer]

I just spend an hour hunting down someone else's GCC code generation 
bug, when it turned out it was caused by an implicit function 
definition, where the real return type was incompatible with int.

C99 got rid of implicit function definitions and implicit ints.  Would 
it be possible to remove them retroactively from the -std=gnu99 and 
-std=gnu11 language variants (as well as -std=c99 and -std=c11), so that 
they are rejected by default?

Thanks,
Florian