GN: Throw an error for duplicate object files.

tools/gn/ninja_binary_target_writer_unittest.cc

Index: tools/gn/ninja_binary_target_writer_unittest.cc
diff --git a/tools/gn/ninja_binary_target_writer_unittest.cc b/tools/gn/ninja_binary_target_writer_unittest.cc
index 2ad816ac14b2e893da9450d30442809885c3ae2c..5438c4dc6bd463db230cf98c94ad79bfb30fa81c 100644
--- a/tools/gn/ninja_binary_target_writer_unittest.cc
+++ b/tools/gn/ninja_binary_target_writer_unittest.cc
@@ -6,6 +6,7 @@
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/ninja_binary_target_writer.h"
+#include "tools/gn/scheduler.h"
 #include "tools/gn/target.h"
 #include "tools/gn/test_with_scope.h"
 
@@ -459,3 +460,23 @@ TEST(NinjaBinaryTargetWriter, WinPrecompiledHeaders) {
     EXPECT_EQ(pch_win_expected, out.str());
   }
 }
+
+// Should throw an error with the scheduler if a duplicate object file exists.
+// This is dependent on the toolchain's object file mapping.
+TEST(NinjaBinaryTargetWriter, DupeObjFileError) {
+  Scheduler scheduler;
+
+  TestWithScope setup;
+  TestTarget target(setup, "//foo:bar", Target::EXECUTABLE);
+  target.sources().push_back(SourceFile("//a.cc"));
+  target.sources().push_back(SourceFile("//a.cc"));
+
+  EXPECT_FALSE(scheduler.is_failed());
+
+  std::ostringstream out;
+  NinjaBinaryTargetWriter writer(&target, out);
+  writer.Run();
+
+  // Should have issued an error.
+  EXPECT_TRUE(scheduler.is_failed());
+}