Build Ruby < 3.2 with GCC 15 everywhere, not only on Fedora

bin/ruby-build

@@ -208,16 +208,24 @@ is_freebsd() {
   [ "$(uname -s)" = "FreeBSD" ]
 }
 
-is_fedora() {
-  [ -r /etc/os-release ] || return 1
-  # shellcheck disable=SC1091
-  source /etc/os-release
-  # Treat Aurora, Bazzite, BlueFin, uCore, and other Fedora variants as "fedora".
-  if [[ "${ID_LIKE:-$ID}" != "fedora" || ( $# -gt 0 && "${VERSION_ID%%.*}" -lt "$1") ]]; then
-    return 1
+# The C compiler that Ruby's configure picks: $CC, otherwise gcc if available, otherwise cc.
+c_compiler() {
+  if [ -n "$CC" ]; then
+    echo "$CC"
+  elif command -v gcc >/dev/null; then
+    echo gcc
+  else
+    echo cc
   fi
 }
 
+# Prints the major version of GCC, or nothing if the given C compiler is not GCC (e.g. Clang).
+gcc_major_version() {
+  # shellcheck disable=SC2086
+  $1 -E -dM - </dev/null 2>/dev/null |
+    awk '/^#define __clang__ /{clang=1} /^#define __GNUC__ /{gnuc=$3} END{if (!clang) print gnuc}'
+}
+
 freebsd_package_prefix() {
   local package="$1"
   pkg info --prefix "$package" 2>/dev/null | cut -wf2
@@ -735,24 +743,36 @@ build_package_standard() {
     fi
   fi
 
+  local cc_override extra_cflags
+  if [ -z "$CC" ] && is_mac 1010; then
+    cc_override=clang
+  elif [[ "$package_var_name" = "RUBY" && "$ruby_semver" -lt 30200 ]]; then
+    local cc gcc_version
+    cc="$(c_compiler)"
+    gcc_version="$(gcc_major_version "$cc")"
+    if [ "${gcc_version:-0}" -ge 15 ]; then
+      # GCC 15 changed the default C mode from gnu17 to gnu23: https://gcc.gnu.org/gcc-15/changes.html#c
+      # Ruby < 3.2 can't compile as C23, where `()` in a declaration means `(void)`.
+      extra_cflags="-std=gnu17"
+    fi
+    if [[ -n "$extra_cflags" && "$ruby_semver" -ge 20600 && "$ruby_semver" -lt 20700 ]]; then
+      # Ruby 2.6 checks its MJIT header with a bare $(CC), so the flags must be part of CC there.
+      cc_override="$cc $extra_cflags"
+      extra_cflags=
+    fi
+  fi
+
   ( if [ "${CFLAGS+defined}" ] || [ "${!PACKAGE_CFLAGS+defined}" ]; then
       export CFLAGS="$CFLAGS ${!PACKAGE_CFLAGS}"
     fi
-    if [ -z "$CC" ] && is_mac 1010; then
-      export CC=clang
-    elif [[ "$package_var_name" = "RUBY" && "$ruby_semver" -lt 30200 ]] && is_fedora 42; then
-      # Fedora 42+ has updated to GCC v15. GCC v15 changed the default
-      # mode from gnu17 to gnu23: https://gcc.gnu.org/gcc-15/changes.html#c
-      #
-      # Ruby < 3.2 can't compile on GCC v15 in the default mode of gnu23.
-      #
-      # TODO: Consider changing this to check for GCC v15 specifically
-      # rather than inspecting the OS itself.
-
+    if [ -n "$cc_override" ]; then
+      export CC="$cc_override"
+    fi
+    if [ -n "$extra_cflags" ]; then
       # Use the cflags configure variable, which Ruby's configure appends to
       # its default optflags/debugflags/warnflags, unlike CFLAGS which would
       # override them and notably lose the -O3, producing a -O0 Ruby.
-      export cflags="-std=gnu17 $cflags"
+      export cflags="$extra_cflags${cflags:+ $cflags}"
     fi
     if [ "$package_var_name" = "RUBY" ] && is_mac && ! is_mac 2700; then
       # The macOS 27 SDK declares dup3() and pipe2() as available since macOS 27.0.

test/compiler.bats

@@ -117,3 +117,179 @@ OUT
   unstub brew
   unstub make
 }
+
+# Stubs a Linux host with a ./configure that logs CC
+stub_linux_build() {
+  mkdir -p "$INSTALL_ROOT"
+  cd "$INSTALL_ROOT" || return 1
+
+  stub_repeated uname '-s : echo Linux'
+  stub_repeated brew 'false'
+  # shellcheck disable=SC2016
+  stub_repeated make 'echo "make $(inspect_args "$@")" >> build.log'
+
+  cat > ./configure <<CON
+#!${BASH}
+echo ./configure "\$@" > build.log
+echo CC=\${CC-unset} >> build.log
+echo cflags=\${cflags-unset} >> build.log
+CON
+  chmod +x ./configure
+}
+
+unstub_linux_build() {
+  unstub uname
+  unstub brew
+  unstub make
+}
+
+# Stubs the given C compiler (default: gcc) to report the given preprocessor defines
+stub_cc() {
+  stub_repeated "${2:-gcc}" "-E -dM - : printf '%s\n' $1"
+}
+
+@test "GCC 15 compiles Ruby < 3.2 as gnu17" {
+  stub_linux_build
+  stub_cc '"#define __GNUC__ 15"'
+
+  run_inline_definition <<DEF
+build_package_standard ruby-3.1.7
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=unset
+cflags=-std=gnu17
+make -j 2
+make install
+OUT
+
+  unstub gcc
+  unstub_linux_build
+}
+
+@test "GCC 15 compiles Ruby 2.6 as gnu17 via CC" {
+  stub_linux_build
+  stub_cc '"#define __GNUC__ 15"'
+
+  run_inline_definition <<DEF
+build_package_standard ruby-2.6.10
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=gcc -std=gnu17
+cflags=unset
+make -j 2
+make install
+OUT
+
+  unstub gcc
+  unstub_linux_build
+}
+
+@test "GCC 15 compiles Ruby <= 2.2 as gnu17" {
+  stub_linux_build
+  stub_cc '"#define __GNUC__ 15"'
+
+  run_inline_definition <<DEF
+build_package_standard ruby-2.2.10
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT
+CC=unset
+cflags=-std=gnu17
+make -j 2
+make install
+OUT
+
+  unstub gcc
+  unstub_linux_build
+}
+
+@test "GCC 15 adds -std=gnu17 to a custom CC for Ruby 2.6" {
+  stub_linux_build
+  stub_cc '"#define __GNUC__ 15"' gcc-15
+  export CC=gcc-15
+
+  run_inline_definition <<DEF
+build_package_standard ruby-2.6.10
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=gcc-15 -std=gnu17
+cflags=unset
+make -j 2
+make install
+OUT
+
+  unstub gcc-15
+  unstub_linux_build
+}
+
+@test "the C compiler is not probed for Ruby 3.2+" {
+  stub_linux_build
+
+  run_inline_definition <<DEF
+build_package_standard ruby-3.2.0
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=unset
+cflags=unset
+make -j 2
+make install
+OUT
+
+  unstub_linux_build
+}
+
+@test "GCC 14 adds no flags for Ruby >= 2.3" {
+  stub_linux_build
+  stub_cc '"#define __GNUC__ 14"'
+
+  run_inline_definition <<DEF
+build_package_standard ruby-3.1.7
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=unset
+cflags=unset
+make -j 2
+make install
+OUT
+
+  unstub gcc
+  unstub_linux_build
+}
+
+@test "Clang masquerading as gcc gets no flags" {
+  stub_linux_build
+  stub_cc '"#define __clang__ 1" "#define __GNUC__ 15"'
+
+  run_inline_definition <<DEF
+build_package_standard ruby-3.1.7
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=unset
+cflags=unset
+make -j 2
+make install
+OUT
+
+  unstub gcc
+  unstub_linux_build
+}