GN: forward_variables_from shouldn't clobber vars.

tools/gn/function_forward_variables_from.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 
 namespace functions {
@@ skipping 36 matching lines @@
       // expect a persistent StringPiece (it won't copy). Not doing this will
       // lead the scope's key to point to invalid memory after this returns.
       base::StringPiece storage_key = source->GetStorageKey(cur.string_value());
       if (storage_key.empty()) {
         // Programmatic value, don't allow copying.
         *err = Err(cur, "This value can't be forwarded.",
             "The variable \"" + cur.string_value() + "\" is a built-in.");
         return;
       }
 
+      // Don't allow clobbering existing values.
+      const Value* existing_value = dest->GetValue(storage_key);
+      if (existing_value) {
+        *err = Err(cur, "Clobbering existing value.",
+            "The current scope already defines a value \"" +
+             cur.string_value() + "\".\nforward_variables_from() won't clobber "
+             "existing values. If you want to\nmerge lists, you'll need to "
+             "do this explicitly.");
+        err->AppendSubErr(Err(*existing_value, "value being clobbered."));
+        return;
+      }
+
       // Keep the origin information from the original value. The normal
       // usage is for this to be used in a template, and if there's an error,
       // the user expects to see the line where they set the variable
       // blamed, rather than a template call to forward_variables_from().
       dest->SetValue(storage_key, *value, value->origin());
     }
   }
 }
 
 }  // namespace
@@ skipping 151 matching lines @@
     }
   }
 
   // Not the right type of argument.
   *err = Err(what_value, "Not a valid list of variables to copy.",
              "Expecting either the string \"*\" or a list of strings.");
   return Value();
 }
 
 }  // namespace functions