Will autoconf work with -Werror=implicit-int and -Werror=implicit-function-declaration ?

[David A. Wheeler]

All:

Will the latest version of autoconf work by default when the compiler has these 
options enabled?:
-Werror=implicit-int
-Werror=implicit-function-declaration

If not, can that be fixed at least in the most common cases (e.g., by declaring 
exit() so it's no longer an implicit function nor using an implicit int)?

Below are details.

Sorry for the late notice/question. I realize an autoconf update is about to be 
released. However, I *just* became aware of this issue. This particularly hints 
clang, because a late change in Clang appears to cause implicit function 
declarations to be errors by default. Details/context below.

--- David A. Wheeler


=== Details ===

The Open Source Security Foundation (OpenSSF) has developed
"Compiler Options Hardening Guide for C and C++"
<https://best.openssf.org/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++>
which gives recommended compiler flags to detect/counter bugs (especially 
vulnerabilities).
The guide is in active development. We want to strongly encourage using various 
options to
detect bugs as soon as possible (ideally at compile time when that's practical).

A proposed change here: 
<https://github.com/ossf/wg-best-practices-os-developers/issues/339>
suggests implementing the Fedora project's "Porting to Modern C"
<https://fedoraproject.org/wiki/Changes/PortingToModernC>
where "modern" is C99 (~24 years old).

That Fedora document recommends turning on "-Werror=implicit-int", however, it 
notes that:
>... introducing errors for these constructs (as required by C99) alters the 
>result of autoconf configure checks. Quite a few such checks use an implicitly 
>declared exit function, for instance. These failures are not really related to 
>the feature under test. If the build system is well written, the build still 
>succeeds, the relevant features are automatically disabled in the test suite 
>and removed from reference ABI lists, and it's not immediately apparent that 
>feature is gone. Therefore, some care is needed that no such alterations 
>happen, and packages need to be ported to C99. Various tools for this porting 
>activity are being developed to support this proposal. Cross-distribution 
>collaboration will help as well, sharing patches and insights.

*Another* proposed change to the OpenSSF guide
<https://github.com/ossf/wg-best-practices-os-developers/issues/342>
*also* recommends adding "-Werror=implicit-function-declaration", in this case 
because it's listed in
"Florian Weimer's 2018 article "Recommended compiler and linker flags for GCC"
<https://developers.redhat.com/blog/2018/03/21/compiler-and-linker-flags-gcc>.

However, there are discussions that suggest that autoconf struggles when 
-Werror=implicit-function-declaration is enabled (e.g., by clang default): 
https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213/5
> "We’ve noticed some worrying problems resulting from the late change in Clang 
> that causes implicit function declarations to be errors by default 105. It 
> seems that many configure (e.g. autoconf) scripts were silently relying on 
> the old behavior, and the change is causing them to misfire."

It'd be great if autoconf "just worked" when enabling "modern C" options, 
especially
"-Werror=implicit-int" and "-Werror=implicit-function-declaration".