Change GN to disallow reading args defined in the same declare_args() call.

tools/gn/functions_unittest.cc

Index: tools/gn/functions_unittest.cc
diff --git a/tools/gn/functions_unittest.cc b/tools/gn/functions_unittest.cc
index 7156747471225952ae48d78b3b5f4ca8e509449b..949b844b34bc3a9b2ad5eafdd6653ad3802ccb35 100644
--- a/tools/gn/functions_unittest.cc
+++ b/tools/gn/functions_unittest.cc
@@ -125,3 +125,48 @@ TEST(Functions, SplitList) {
       "rounding = [[1, 2], [3, 4], [5], [6]]\n",
       setup.print_output());
 }
+
+TEST(Functions, DeclareArgs) {
+  TestWithScope setup;
+  Err err;
+
+  // It is not legal to read the value of an argument declared in a
+  // declare_args() from inside the call, but outside the call and in
+  // a separate call should work.
+
+  TestParseInput reading_from_same_call(R"(
+      declare_args() {
+        foo = true
+        bar = foo
+      })");
+  reading_from_same_call.parsed()->Execute(setup.scope(), &err);
+  ASSERT_TRUE(err.has_error());
+
+  TestParseInput reading_from_outside_call(R"(
+      declare_args() {
+        foo = true
+      }
+
+      bar = foo
+      assert(bar)
+      )");
+  err = Err();
+  reading_from_outside_call.parsed()->Execute(setup.scope(), &err);
+  ASSERT_FALSE(err.has_error());
+
+  TestParseInput reading_from_different_call(R"(
+      declare_args() {
+        foo = true
+      }
+
+      declare_args() {
+        bar = foo
+      }
+
+      assert(bar)
+      )");
+  err = Err();
+  TestWithScope setup2;
+  reading_from_different_call.parsed()->Execute(setup2.scope(), &err);
+  ASSERT_FALSE(err.has_error());
+}