diff --git a/clang/Lex/PPExpressions.cpp b/clang/Lex/PPExpressions.cpp
index b9a73cf8e313..c46dcb1f1070 100644
--- a/clang/Lex/PPExpressions.cpp
+++ b/clang/Lex/PPExpressions.cpp
@@ -430,9 +430,19 @@ static bool EvaluateDirectiveSubExpr(APSInt &LHS, unsigned MinPrec,
     // either operand is unsigned.  Don't do this for x and y in "x ? y : z".
     APSInt Res(LHS.getBitWidth());
     if (Operator != tok::question) {
-      if (RHS.isUnsigned()) LHS.setIsUnsigned(true);
-      RHS.setIsUnsigned(LHS.isUnsigned());
-      Res.setIsUnsigned(LHS.isUnsigned());
+      Res.setIsUnsigned(LHS.isUnsigned()|RHS.isUnsigned());
+      // If this just promoted something from signed to unsigned, and if the
+      // value was negative, warn about it.
+      if (ValueLive && Res.isUnsigned()) {
+        if (!LHS.isUnsigned() && LHS.isNegative())
+          PP.Diag(OpToken, diag::warn_pp_convert_lhs_to_positive,
+                  LHS.toString(10, true) + " to " + LHS.toString(10, false));
+        if (!RHS.isUnsigned() && RHS.isNegative())
+          PP.Diag(OpToken, diag::warn_pp_convert_rhs_to_positive,
+                  RHS.toString(10, true) + " to " + RHS.toString(10, false));
+      }
+      LHS.setIsUnsigned(Res.isUnsigned());
+      RHS.setIsUnsigned(Res.isUnsigned());
     }
     
     // FIXME: All of these should detect and report overflow??
diff --git a/clang/include/clang/Basic/DiagnosticKinds.def b/clang/include/clang/Basic/DiagnosticKinds.def
index abfeaa1f9505..9dd47263d54c 100644
--- a/clang/include/clang/Basic/DiagnosticKinds.def
+++ b/clang/include/clang/Basic/DiagnosticKinds.def
@@ -116,6 +116,10 @@ DIAG(pp_invalid_string_literal, WARNING,
      "invalid string literal, ignoring final '\\'")
 DIAG(warn_pp_expr_overflow, WARNING,
      "integer overflow in preprocessor expression")
+DIAG(warn_pp_convert_lhs_to_positive, WARNING,
+     "left side of operator converted from negative value to unsigned: %s")
+DIAG(warn_pp_convert_rhs_to_positive, WARNING,
+     "right side of operator converted from negative value to unsigned: %s")
 
 DIAG(ext_pp_import_directive, EXTENSION,
      "#import is a language extension")
diff --git a/clang/test/Preprocessor/expr_usual_conversions.c b/clang/test/Preprocessor/expr_usual_conversions.c
new file mode 100644
index 000000000000..b2ccc4090075
--- /dev/null
+++ b/clang/test/Preprocessor/expr_usual_conversions.c
@@ -0,0 +1,8 @@
+// RUN: clang %s -E  2>&1 | grep warning | wc -l | grep 2
+
+#define INTMAX_MIN (-9223372036854775807LL -1)
+
+#if (-42 + 0U) / -2
+foo
+#endif
+