GN: Don't define argument overrides globally

tools/gn/args_unittest.cc

Index: tools/gn/args_unittest.cc
diff --git a/tools/gn/args_unittest.cc b/tools/gn/args_unittest.cc
index 098f3dec8504975a55a136204f196cc12b74bd4c..cf51036fb66ea4af26f27bacda6078ffbfb965e7 100644
--- a/tools/gn/args_unittest.cc
+++ b/tools/gn/args_unittest.cc
@@ -39,3 +39,43 @@ TEST(ArgsTest, VerifyAllOverridesUsed) {
   args.AddArgOverride("c", Value(nullptr, true));
   EXPECT_FALSE(args.VerifyAllOverridesUsed(&err));
 }
+
+// Ensure that arg overrides get only set after the they were declared.
+TEST(ArgsTest, VerifyOverrideScope) {
+  TestWithScope setup;
+  Args args;
+  Err err;
+
+  args.AddArgOverride("a", Value(nullptr, "avalue"));
+  args.AddArgOverride("current_os", Value(nullptr, "theiros"));
+
+  Scope::KeyValueMap toolchain_overrides;
+  toolchain_overrides["b"] = Value(nullptr, "bvalue");
+  toolchain_overrides["current_os"] = Value(nullptr, "myos");
+  args.SetupRootScope(setup.scope(), toolchain_overrides);
+
+  // Overrides of arguments not yet declared aren't applied yet.
+  EXPECT_EQ(nullptr, setup.scope()->GetValue("a"));
+  EXPECT_EQ(nullptr, setup.scope()->GetValue("b"));
+
+  // |current_os| is a system var. and already declared.
+  // Thus it should have our override value.
+  ASSERT_NE(nullptr, setup.scope()->GetValue("current_os"));
+  EXPECT_EQ(Value(nullptr, "myos"), *setup.scope()->GetValue("current_os"));
+
+  Scope::KeyValueMap key_value_map1;
+  key_value_map1["a"] = Value(nullptr, "avalue2");
+  key_value_map1["b"] = Value(nullptr, "bvalue2");
+  key_value_map1["c"] = Value(nullptr, "cvalue2");
+  EXPECT_TRUE(args.DeclareArgs(key_value_map1, setup.scope(), &err));
+
+  ASSERT_NE(nullptr, setup.scope()->GetValue("a"));
+  EXPECT_EQ(Value(nullptr, "avalue"), *setup.scope()->GetValue("a"));
+
+  ASSERT_NE(nullptr, setup.scope()->GetValue("b"));
+  EXPECT_EQ(Value(nullptr, "bvalue"), *setup.scope()->GetValue("b"));
+
+  // This wasn't overwritten, so it should have the default value.
+  ASSERT_NE(nullptr, setup.scope()->GetValue("c"));
+  EXPECT_EQ(Value(nullptr, "cvalue2"), *setup.scope()->GetValue("c"));
+}