From 6a4062b3a77f93a75b4be363c0958ea708d30b23 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Tue, 20 Jan 2026 11:35:16 +0100
Subject: [PATCH 3/5] Enable -Wimplicit-fallthrough option for clang

On clang, -Wimplicit-fallthrough requires annotations with attributes,
but on gcc, -Wimplicit-fallthrough is the same as
-Wimplicit-fallthrough=3, which allows annotations with comments.  In
order to enforce consistent annotations with attributes on both
compilers, we test first for -Wimplicit-fallthrough=5, which will
succeed on gcc, and if that is not found we test for
-Wimplicit-fallthrough.
---
 configure    | 103 +++++++++++++++++++++++++++++++++++++++++++++++++++
 configure.ac |  14 +++++++
 meson.build  |  18 ++++++++-
 3 files changed, 134 insertions(+), 1 deletion(-)

diff --git a/configure b/configure
index 4f41b1ba007..5ec4048751f 100755
--- a/configure
+++ b/configure
@@ -5524,6 +5524,12 @@ fi
 
 
 
+  # To require fallthrough attribute annotations, use
+  # -Wimplicit-fallthrough=5 with gcc and -Wimplicit-fallthrough with
+  # clang.  The latter is also accepted on gcc but does not enforce
+  # attribute annotations, so test the former first.
+  save_CFLAGS=$CFLAGS
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wimplicit-fallthrough=5, for CFLAGS" >&5
 $as_echo_n "checking whether ${CC} supports -Wimplicit-fallthrough=5, for CFLAGS... " >&6; }
 if ${pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_5+:} false; then :
@@ -5563,6 +5569,49 @@ if test x"$pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_5" = x"yes"; then
 fi
 
 
+  if test x"$save_CFLAGS" = x"$CFLAGS"; then
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wimplicit-fallthrough, for CFLAGS" >&5
+$as_echo_n "checking whether ${CC} supports -Wimplicit-fallthrough, for CFLAGS... " >&6; }
+if ${pgac_cv_prog_CC_cflags__Wimplicit_fallthrough+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+pgac_save_CC=$CC
+CC=${CC}
+CFLAGS="${CFLAGS} -Wimplicit-fallthrough"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_prog_CC_cflags__Wimplicit_fallthrough=yes
+else
+  pgac_cv_prog_CC_cflags__Wimplicit_fallthrough=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$pgac_save_CFLAGS"
+CC="$pgac_save_CC"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CC_cflags__Wimplicit_fallthrough" >&5
+$as_echo "$pgac_cv_prog_CC_cflags__Wimplicit_fallthrough" >&6; }
+if test x"$pgac_cv_prog_CC_cflags__Wimplicit_fallthrough" = x"yes"; then
+  CFLAGS="${CFLAGS} -Wimplicit-fallthrough"
+fi
+
+
+  fi
+  save_CXXFLAGS=$CXXFLAGS
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CXX} supports -Wimplicit-fallthrough=5, for CXXFLAGS" >&5
 $as_echo_n "checking whether ${CXX} supports -Wimplicit-fallthrough=5, for CXXFLAGS... " >&6; }
 if ${pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_5+:} false; then :
@@ -5614,6 +5663,60 @@ if test x"$pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_5" = x"yes"; then
 fi
 
 
+  if test x"$save_CXXFLAGS" = x"$CXXFLAGS"; then
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CXX} supports -Wimplicit-fallthrough, for CXXFLAGS" >&5
+$as_echo_n "checking whether ${CXX} supports -Wimplicit-fallthrough, for CXXFLAGS... " >&6; }
+if ${pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CXXFLAGS=$CXXFLAGS
+pgac_save_CXX=$CXX
+CXX=${CXX}
+CXXFLAGS="${CXXFLAGS} -Wimplicit-fallthrough"
+ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
+ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough=yes
+else
+  pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
+CXXFLAGS="$pgac_save_CXXFLAGS"
+CXX="$pgac_save_CXX"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough" >&5
+$as_echo "$pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough" >&6; }
+if test x"$pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough" = x"yes"; then
+  CXXFLAGS="${CXXFLAGS} -Wimplicit-fallthrough"
+fi
+
+
+  fi
+
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wcast-function-type, for CFLAGS" >&5
 $as_echo_n "checking whether ${CC} supports -Wcast-function-type, for CFLAGS... " >&6; }
diff --git a/configure.ac b/configure.ac
index 93503753971..1d20b526b84 100644
--- a/configure.ac
+++ b/configure.ac
@@ -516,8 +516,22 @@ if test "$GCC" = yes -a "$ICC" = no; then
   PGAC_PROG_CXX_CFLAGS_OPT([-Wendif-labels])
   PGAC_PROG_CC_CFLAGS_OPT([-Wmissing-format-attribute])
   PGAC_PROG_CXX_CFLAGS_OPT([-Wmissing-format-attribute])
+
+  # To require fallthrough attribute annotations, use
+  # -Wimplicit-fallthrough=5 with gcc and -Wimplicit-fallthrough with
+  # clang.  The latter is also accepted on gcc but does not enforce
+  # attribute annotations, so test the former first.
+  save_CFLAGS=$CFLAGS
   PGAC_PROG_CC_CFLAGS_OPT([-Wimplicit-fallthrough=5])
+  if test x"$save_CFLAGS" = x"$CFLAGS"; then
+    PGAC_PROG_CC_CFLAGS_OPT([-Wimplicit-fallthrough])
+  fi
+  save_CXXFLAGS=$CXXFLAGS
   PGAC_PROG_CXX_CFLAGS_OPT([-Wimplicit-fallthrough=5])
+  if test x"$save_CXXFLAGS" = x"$CXXFLAGS"; then
+    PGAC_PROG_CXX_CFLAGS_OPT([-Wimplicit-fallthrough])
+  fi
+
   PGAC_PROG_CC_CFLAGS_OPT([-Wcast-function-type])
   PGAC_PROG_CXX_CFLAGS_OPT([-Wcast-function-type])
   PGAC_PROG_CC_CFLAGS_OPT([-Wshadow=compatible-local])
diff --git a/meson.build b/meson.build
index f428371a4c4..9c4df1f1031 100644
--- a/meson.build
+++ b/meson.build
@@ -2125,7 +2125,6 @@ common_warning_flags = [
   '-Werror=unguarded-availability-new',
   '-Wendif-labels',
   '-Wmissing-format-attribute',
-  '-Wimplicit-fallthrough=5',
   '-Wcast-function-type',
   '-Wshadow=compatible-local',
   # This was included in -Wall/-Wformat in older GCC versions
@@ -2137,6 +2136,23 @@ if have_cxx
   cxxflags_warn += cxx.get_supported_arguments(common_warning_flags)
 endif
 
+# To require fallthrough attribute annotations, use
+# -Wimplicit-fallthrough=5 with gcc and -Wimplicit-fallthrough with
+# clang.  The latter is also accepted on gcc but does not enforce
+# attribute annotations, so test the former first.
+fallthrough_warning_flags = ['-Wimplicit-fallthrough=5', '-Wimplicit-fallthrough']
+foreach w : fallthrough_warning_flags
+  if cc.has_argument(w)
+    cflags_warn += w
+    break
+  endif
+endforeach
+foreach w : fallthrough_warning_flags
+  if have_cxx and cxx.has_argument(w)
+    cxxflags_warn += w
+  endif
+endforeach
+
 # A few places with imported code get a pass on -Wdeclaration-after-statement, remember
 # the result for them
 cflags_no_decl_after_statement = []
-- 
2.52.0