[Bug c/85676] New: Obsolete function declarations should have warnings by default

[david at westcontrol dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85676

            Bug ID: 85676
           Summary: Obsolete function declarations should have warnings by
                    default
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david at westcontrol dot com
  Target Milestone: ---

There are a number of features of C that are still legal code even in C11, but
have been obsolete for many generations of C because they are unnecessary and
have a high risk of errors.  gcc has to be able to support old code, but it is
perfectly valid to give warnings on such old code.

A prime example is non-prototype function declarations, such as "void foo()". 
This has been obsolete since C89, almost 30 years ago, yet if you have:

// a.c
void foo();

gcc -std=c11 -Wall -Wextra -pedantic -c a.c

it compiles without warning.  You need "-Wstrict-prototypes" to get a warning. 
Surely this warning ought to be part of -Wall, or enabled by default when a C11
or C99 standard is chosen?

The same applies to K&R style function definitions.