feat: select the GCC version through a build flag

.github/workflows/default.yaml

@@ -60,6 +60,30 @@ jobs:
         run: |
           ln -s .github/workflows/.bazelrc.ci .bazelrc.ci
           bazel test --compilation_mode ${{ matrix.compilation_mode }} //...
+  gcc_versions:
+    strategy:
+      matrix:
+        gcc_version:
+          - 12.5.0
+          - 13.4.0
+          - 14.3.0
+          - 15.2.0
+          - 16.1.0
+          - 16.2.0
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          lfs: true
+      - uses: bazel-contrib/setup-bazel@0.15.0
+        with:
+          bazelisk-cache: true
+          disk-cache: ${{ github.workflow }}
+          repository-cache: true
+      - name: Test GCC ${{ matrix.gcc_version }}
+        run: |
+          ln -s .github/workflows/.bazelrc.ci .bazelrc.ci
+          bazel test --//toolchain:gcc_version=${{ matrix.gcc_version }} //...
   sanitizers:
     strategy:
       matrix:

MODULE.bazel

@@ -19,8 +19,7 @@ gcc_toolchains = use_extension("//toolchain:module_extensions.bzl", "gcc_toolcha
         ),
         use_repo(gcc_toolchains, "gcc_toolchain_{}".format(arch)),
         register_toolchains(
-            "@gcc_toolchain_{}//:cc_toolchain".format(arch),
-            "@gcc_toolchain_{}//:fortran_toolchain".format(arch),
+            "@gcc_toolchain_{}//:all".format(arch),
             # Register toolchains as dev dependencies so that we don't pollute the toolchain resolution of consumers.
             dev_dependency = True,
         ),

tests/gcc_version/BUILD.bazel

@@ -0,0 +1,39 @@
+# Copyright (c) Thulio Ferraz Assis 2026
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("@bazel_skylib//lib:dicts.bzl", "dicts")
+load("//rules_cc:defs.bzl", "cc_test")
+load("//toolchain:defs.bzl", "AVAILABLE_GCC_VERSIONS", "DEFAULT_GCC_VERSION")
+
+# Asserts that the compiler picked by toolchain resolution is the one the gcc_version flag asked
+# for. The build fails to compile when the selected toolchain reports a different version.
+cc_test(
+    name = "gcc_version_test",
+    srcs = ["main.c"],
+    copts = select(dicts.add(
+        {
+            "//toolchain:gcc_version_{}".format(gcc_version.replace(".", "_")): [
+                "-DEXPECTED_GCC_MAJOR={}".format(gcc_version.split(".")[0]),
+                "-DEXPECTED_GCC_MINOR={}".format(gcc_version.split(".")[1]),
+            ]
+            for gcc_version in AVAILABLE_GCC_VERSIONS
+        },
+        {
+            "//conditions:default": [
+                "-DEXPECTED_GCC_MAJOR={}".format(DEFAULT_GCC_VERSION.split(".")[0]),
+                "-DEXPECTED_GCC_MINOR={}".format(DEFAULT_GCC_VERSION.split(".")[1]),
+            ],
+        },
+    )),
+)

tests/gcc_version/main.c

@@ -0,0 +1,28 @@
+// Copyright (c) Thulio Ferraz Assis 2026
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdio.h>
+
+#if __GNUC__ != EXPECTED_GCC_MAJOR
+#error "The compiling GCC major version does not match the gcc_version flag."
+#endif
+
+#if __GNUC_MINOR__ != EXPECTED_GCC_MINOR
+#error "The compiling GCC minor version does not match the gcc_version flag."
+#endif
+
+int main(void) {
+  printf("%d.%d.%d\n", __GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__);
+  return 0;
+}

tests/lld/BUILD.bazel

@@ -12,15 +12,22 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-load("//rules_cc:defs.bzl", "cc_binary")
 load("@rules_shell//shell:sh_test.bzl", "sh_test")
+load("//rules_cc:defs.bzl", "cc_binary")
 
 # This binary is always linked with lld via the target-level `linker-lld`
-# feature, which requires the toolchain archive to ship `ld.lld`.
+# feature, which requires the toolchain archive to ship `ld.lld`. GCC versions
+# older than 15.2.0 are built without lld, so the test is skipped for them.
 cc_binary(
     name = "hello_world_lld",
     srcs = ["main.c"],
     features = ["linker-lld"],
+    target_compatible_with = select({
+        "//toolchain:gcc_version_12_5_0": ["@platforms//:incompatible"],
+        "//toolchain:gcc_version_13_4_0": ["@platforms//:incompatible"],
+        "//toolchain:gcc_version_14_3_0": ["@platforms//:incompatible"],
+        "//conditions:default": [],
+    }),
 )
 
 sh_test(

toolchain/BUILD.bazel

@@ -16,6 +16,27 @@
 # limitations under the License.
 
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+load(":defs.bzl", "AVAILABLE_GCC_VERSIONS")
+
+# Selects which GCC version the registered toolchains resolve to. The empty default defers to the
+# gcc_version each toolchain was declared with.
+string_flag(
+    name = "gcc_version",
+    build_setting_default = "",
+    visibility = ["//visibility:public"],
+)
+
+# Matches when gcc_version is set to that exact version. None of these match while the flag is left
+# at its empty default, so a select over them needs a //conditions:default branch.
+[
+    config_setting(
+        name = "gcc_version_{}".format(gcc_version.replace(".", "_")),
+        flag_values = {":gcc_version": gcc_version},
+        visibility = ["//visibility:public"],
+    )
+    for gcc_version in AVAILABLE_GCC_VERSIONS
+]
 
 exports_files(
     ["config.bzl"],