Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4135,9 +4135,11 @@
       // one element is undef, choose the defined element as the safe result.
       if (TEltC == FEltC)
         NewC.push_back(TEltC);
-      else if (isa<UndefValue>(TEltC))
+      else if (isa<UndefValue>(TEltC) &&
+               isGuaranteedNotToBeUndefOrPoison(FEltC))
         NewC.push_back(FEltC);
-      else if (isa<UndefValue>(FEltC))
+      else if (isa<UndefValue>(FEltC) &&
+               isGuaranteedNotToBeUndefOrPoison(TEltC))
         NewC.push_back(TEltC);
       else
         break;
Index: llvm/test/Transforms/InstSimplify/select.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/select.ll
+++ llvm/test/Transforms/InstSimplify/select.ll
@@ -848,3 +848,17 @@
   %s = select i1 %cond, i32 undef, i32 %xf
   ret i32 %s
 }
+
+@g = external global i32, align 1
+
+; Make sure we don't fold partial undef vectors when constexprs are involved.
+; We would need to prove the constexpr doesn't result in poison which we aren't
+; equiped to do yet.
+define <2 x i32> @false_undef_true_constextpr_vec(i1 %cond) {
+; CHECK-LABEL: @false_undef_true_constextpr_vec(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> <i32 undef, i32 ptrtoint (i32* @g to i32)>, <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 undef>
+; CHECK-NEXT:    ret <2 x i32> [[S]]
+;
+  %s = select i1 %cond, <2 x i32> <i32 undef, i32 ptrtoint (i32* @g to i32)>, <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 undef>
+  ret <2 x i32> %s
+}