Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -112,6 +112,24 @@
   /// Note that NUW and NSW are also valid properties of a recurrence, and
   /// either implies NW. For convenience, NW will be set for a recurrence
   /// whenever either NUW or NSW are set.
+  ///
+  /// We require that the flag on a SCEV apply to the entire scope in which
+  /// that SCEV is defined.  A SCEV's scope is set of locations dominated by
+  /// a defining location, which is in turn describes by the following rules:
+  /// * A SCEVUnknown is at the point of definition of the Value.
+  /// * A SCEVConstant is defined at all points.
+  /// * A SCEVAddRec is defined starting with the header of the associated
+  ///   loop.
+  /// * All other SCEVs are defined at the earlest point all operands are
+  ///   defined.
+  ///
+  /// The above rules describe a maximally hoisted form (without regards to
+  /// potential control dependence).  A SCEV is defined anywhere a
+  /// corresponding instruction could be defined in said maximally hoisted
+  /// form.  Note that SCEVUDivExpr (currently the only expression type which
+  /// can trap) can be defined per these rules in regions where it would trap
+  /// at runtime.  A SCEV being defined does not require the existence of any
+  /// instruction within the defined scope.
   enum NoWrapFlags {
     FlagAnyWrap = 0,    // No guarantee.
     FlagNW = (1 << 0),  // No self-wrap.
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13495,6 +13495,11 @@
 const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
   auto CollectCondition = [&](ICmpInst::Predicate Predicate, const SCEV *LHS,
                               const SCEV *RHS, ValueToSCEVMapTy &RewriteMap) {
+    // WARNING: It is generally unsound to apply any wrap flags to the proposed
+    // replacement SCEV which isn't directly implied by the replacee SCEV.  In
+    // particular, using contextual facts to imply flags is *NOT* legal.  See
+    // the scoping rules for flags in the header to understand why.
+
     // If we have LHS == 0, check if LHS is computing a property of some unknown
     // SCEV %v which we can rewrite %v to express explicitly.
     const SCEVConstant *RHSC = dyn_cast<SCEVConstant>(RHS);
@@ -13509,7 +13514,7 @@
           Value *V = LHSUnknown->getValue();
           auto Multiple =
               getMulExpr(getUDivExpr(URemLHS, URemRHS), URemRHS,
-                         (SCEV::NoWrapFlags)(SCEV::FlagNUW | SCEV::FlagNSW));
+                         SCEV::FlagAnyWrap);
           RewriteMap[V] = Multiple;
           return;
         }