From f1dd0eeaaf264ebb35b048f423c66296e6967d16 Mon Sep 17 00:00:00 2001
From: Brad King <brad.king@kitware.com>
Date: Mon, 24 Sep 2018 10:54:18 -0400
Subject: cmListFileCache: Add wrapper template for values with a backtrace

---
 Source/cmListFileCache.cxx |  7 ++++++-
 Source/cmListFileCache.h   | 29 +++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/Source/cmListFileCache.cxx b/Source/cmListFileCache.cxx
index 4d7e1e2..d79afc6 100644
--- a/Source/cmListFileCache.cxx
+++ b/Source/cmListFileCache.cxx
@@ -9,10 +9,10 @@
 #include "cmSystemTools.h"
 #include "cmake.h"
 
-#include <algorithm>
 #include <assert.h>
 #include <memory>
 #include <sstream>
+#include <utility>
 
 cmCommandContext::cmCommandName& cmCommandContext::cmCommandName::operator=(
   std::string const& name)
@@ -474,3 +474,8 @@ bool operator!=(const cmListFileContext& lhs, const cmListFileContext& rhs)
 {
   return !(lhs == rhs);
 }
+
+std::ostream& operator<<(std::ostream& os, BT<std::string> const& s)
+{
+  return os << s.Value;
+}
diff --git a/Source/cmListFileCache.h b/Source/cmListFileCache.h
index 3d3afdf..9f16b16 100644
--- a/Source/cmListFileCache.h
+++ b/Source/cmListFileCache.h
@@ -9,6 +9,7 @@
 #include <memory> // IWYU pragma: keep
 #include <stddef.h>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "cmStateSnapshot.h"
@@ -169,6 +170,34 @@ private:
   cmListFileBacktrace(std::shared_ptr<Entry const> top);
 };
 
+// Wrap type T as a value with a backtrace.  For purposes of
+// ordering and equality comparison, only the original value is
+// used.  The backtrace is considered incidental.
+template <typename T>
+class BT
+{
+public:
+  BT(T v = T(), cmListFileBacktrace bt = cmListFileBacktrace())
+    : Value(std::move(v))
+    , Backtrace(std::move(bt))
+  {
+  }
+  T Value;
+  cmListFileBacktrace Backtrace;
+  friend bool operator==(BT<T> const& l, BT<T> const& r)
+  {
+    return l.Value == r.Value;
+  }
+  friend bool operator<(BT<T> const& l, BT<T> const& r)
+  {
+    return l.Value < r.Value;
+  }
+  friend bool operator==(BT<T> const& l, T const& r) { return l.Value == r; }
+  friend bool operator==(T const& l, BT<T> const& r) { return l == r.Value; }
+};
+
+std::ostream& operator<<(std::ostream& os, BT<std::string> const& s);
+
 struct cmListFile
 {
   bool ParseFile(const char* path, cmMessenger* messenger,
-- 
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