GN: Throw an error overwriting a nonempty scope.

tools/gn/scope.cc

 // Copyright (c) 2013 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/scope.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/template.h"
@@ skipping 53 matching lines @@
 }
 
 Scope::~Scope() {
 }
 
 void Scope::DetachFromContaining() {
   const_containing_ = nullptr;
   mutable_containing_ = nullptr;
 }
 
+bool Scope::HasValues(SearchNested search_nested) const {
+  DCHECK(search_nested == SEARCH_CURRENT);
+  return !values_.empty();
+}
+
 const Value* Scope::GetValue(const base::StringPiece& ident,
                              bool counts_as_used) {
   // First check for programmatically-provided values.
   for (auto* provider : programmatic_providers_) {
     const Value* v = provider->GetProgrammaticValue(ident);
     if (v)
       return v;
   }
 
   RecordMap::iterator found = values_.find(ident);
@@ skipping 430 matching lines @@
     return false;
   for (const auto& pair : a) {
     const auto& found_b = b.find(pair.first);
     if (found_b == b.end())
       return false;  // Item in 'a' but not 'b'.
     if (pair.second.value != found_b->second.value)
       return false;  // Values for variable in 'a' and 'b' are different.
   }
   return true;
 }