https://git.ruby-lang.org/ruby.git/commit/?id=329c89bb42

From 329c89bb42bb44467588afc1d41d9f99172dfeb5 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Thu, 17 Mar 2022 14:29:35 +0900
Subject: Make implicit function declaration error [Feature #18615]

Enable `-Werror=implicit-function-declaration` by default for
building C extensions for early failures.
---
 lib/mkmf.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 276e7302f4..ef14f3042e 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -2572,7 +2572,7 @@ site-install-rb: install-rb https://github.com/ruby/ruby/blob/trunk/lib/mkmf.rb#L2572
 
     if $warnflags = CONFIG['warnflags'] and CONFIG['GCC'] == 'yes'
       # turn warnings into errors only for bundled extensions.
-      config['warnflags'] = $warnflags.gsub(/(\A|\s)-Werror[-=]/, '\1-W')
+      config['warnflags'] = $warnflags.gsub(/(?:\A|\s)-W\Kerror[-=](?!implicit-function-declaration)/, '')
       if /icc\z/ =~ config['CC']
         config['warnflags'].gsub!(/(\A|\s)-W(?:division-by-zero|deprecated-declarations)/, '\1')
       end
-- 
cgit v1.2.1


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