Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -721,7 +721,7 @@
     /// StepsLeft - The remaining number of evaluation steps we're permitted
     /// to perform. This is essentially a limit for the number of statements
     /// we will evaluate.
-    unsigned StepsLeft;
+    int StepsLeft;
 
     /// BottomFrame - The frame in which evaluation started. This must be
     /// initialized after CurrentCall and CallStackDepth.
@@ -898,7 +898,8 @@
         FFDiag(S->getBeginLoc(), diag::note_constexpr_step_limit_exceeded);
         return false;
       }
-      --StepsLeft;
+      if (StepsLeft > 0)
+        --StepsLeft;
       return true;
     }