add test for incompatible flag

proto/proto_common.bzl

diff --git a/proto/proto_common.bzl b/proto/proto_common.bzl
index 26347a8..acc0c86 100644
--- a/proto/proto_common.bzl
+++ b/proto/proto_common.bzl
@@ -24,7 +24,7 @@
 ProtoLangToolchainInfo = proto_common.ProtoLangToolchainInfo
 
 def _incompatible_toolchains_enabled():
-    getattr(proto_common, "INCOMPATIBLE_ENABLE_PROTO_TOOLCHAIN_RESOLUTION", False)
+    return getattr(proto_common, "INCOMPATIBLE_ENABLE_PROTO_TOOLCHAIN_RESOLUTION", False)
 
 def _find_toolchain(ctx, legacy_attr, toolchain_type):
     if _incompatible_toolchains_enabled():

tests/BUILD

diff --git a/tests/BUILD b/tests/BUILD
index 2381713..57bb308 100644
--- a/tests/BUILD
+++ b/tests/BUILD
@@ -1,4 +1,18 @@
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load("//proto:defs.bzl", "proto_library")
+load("//proto:proto_common.bzl", "proto_common")
+
+config_setting(
+    name = "incompatible_enable_proto_toolchain_resolution",
+    flag_values = {
+        ":incompatible_enable_proto_toolchain_resolution_flag": "true",
+    },
+)
+
+bool_flag(
+    name = "incompatible_enable_proto_toolchain_resolution_flag",
+    build_setting_default = getattr(proto_common, "INCOMPATIBLE_ENABLE_PROTO_TOOLCHAIN_RESOLUTION", False),
+)
 
 proto_library(
     name = "empty_proto",

tests/proto_common/toolchains.bzl

diff --git a/tests/proto_common/toolchains.bzl b/tests/proto_common/toolchains.bzl
index 3a639af..ad0ae62 100644
--- a/tests/proto_common/toolchains.bzl
+++ b/tests/proto_common/toolchains.bzl
@@ -1,21 +1,49 @@
+load("@bazel_skylib//lib:partial.bzl", "partial")
 load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
 load("//proto:proto_common.bzl", "toolchains")
 
-def _test_toolchains(ctx):
+def _test_toolchains_without_incompatible_flag(ctx):
     env = unittest.begin(ctx)
 
     asserts.equals(env, {}, toolchains.if_legacy_toolchain({}))
     asserts.equals(env, None, toolchains.if_legacy_toolchain(None))
     asserts.equals(env, False, toolchains.if_legacy_toolchain(False))
-    asserts.equals(env, False, toolchains.if_legacy_toolchain(False))
 
     return unittest.end(env)
 
-toolchains_test = unittest.make(_test_toolchains)
+toolchains_without_incompatible_flags_test = unittest.make(_test_toolchains_without_incompatible_flag)
+
+def _test_toolchains_with_incompatible_flag(ctx):
+    env = unittest.begin(ctx)
+
+    asserts.equals(env, {}, toolchains.if_legacy_toolchain({}))
+    asserts.equals(env, {}, toolchains.if_legacy_toolchain(None))
+    asserts.equals(env, {}, toolchains.if_legacy_toolchain(False))
+    toolchain = toolchains.use_toolchain("//nonexistent:toolchain_type")
+    asserts.equals(env, 1, len(toolchain))
+    asserts.equals(env, False, toolchain[0].mandatory)
+    asserts.equals(env, str(Label("//nonexistent:toolchain_type")), str(toolchain[0].toolchain_type))
+
+    return unittest.end(env)
+
+toolchains_with_incompatible_flag_test = unittest.make(_test_toolchains_with_incompatible_flag)
 
 # buildifier: disable=unnamed-macro
 def unittest_toolchains():
     unittest.suite(
         "test_toolchains",
-        toolchains_test
+        partial.make(
+            toolchains_without_incompatible_flags_test,
+            target_compatible_with = select({
+                "//tests:incompatible_enable_proto_toolchain_resolution": ["@platforms//:incompatible"],
+                "//conditions:default": [],
+            }),
+        ),
+        partial.make(
+            toolchains_with_incompatible_flag_test,
+            target_compatible_with = select({
+                "//tests:incompatible_enable_proto_toolchain_resolution": [],
+                "//conditions:default": ["@platforms//:incompatible"],
+            }),
+        )
     )