Index: lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- lib/Transforms/Scalar/SCCP.cpp
+++ lib/Transforms/Scalar/SCCP.cpp
@@ -223,6 +223,10 @@
   /// represented here for efficient lookup.
   SmallPtrSet<Function *, 16> MRVFunctionsTracked;
 
+  /// MustTailFunctions - Each function here is a callee of non-removable
+  /// musttail call site.
+  SmallPtrSet<Function *, 16> MustTailCallees;
+
   /// TrackingIncomingArguments - This is the set of functions for whose
   /// arguments we make optimistic assumptions about and try to prove as
   /// constants.
@@ -289,6 +293,18 @@
       TrackedRetVals.insert(std::make_pair(F, LatticeVal()));
   }
 
+  /// AddMustTailCallee - If the SCCP solver finds that this function is called
+  /// from non-removable musttail call site.
+  void AddMustTailCallee(Function *F) {
+    MustTailCallees.insert(F);
+  }
+
+  /// Returns true if the given function is called from non-removable musttail
+  /// call site.
+  bool isMustTailCallee(Function *F) {
+    return MustTailCallees.count(F);
+  }
+
   void AddArgumentTrackedFunction(Function *F) {
     TrackingIncomingArguments.insert(F);
   }
@@ -358,6 +374,12 @@
     return MRVFunctionsTracked;
   }
 
+  /// getMustTailCallees - Get the set of functions which are called
+  /// from non-removable musttail call sites.
+  const SmallPtrSet<Function *, 16> getMustTailCallees() {
+    return MustTailCallees;
+  }
+
   /// markOverdefined - Mark the specified value overdefined.  This
   /// works with both scalars and structs.
   void markOverdefined(Value *V) {
@@ -1672,6 +1694,23 @@
           IV.isConstant() ? IV.getConstant() : UndefValue::get(V->getType());
   }
   assert(Const && "Constant is nullptr here!");
+
+  // Replacing `musttail` instructions with constant breaks `musttail` invariant
+  // unless the call itself can be removed
+  CallInst *CI = dyn_cast<CallInst>(V);
+  if (CI && CI->isMustTailCall() && !CI->isSafeToRemove()) {
+    CallSite CS(CI);
+    Function *F = CS.getCalledFunction();
+
+    // Don't zap returns of the callee
+    if (F)
+      Solver.AddMustTailCallee(F);
+
+    DEBUG(dbgs() << "  Can\'t treat the result of musttail call : " << *CI
+                 << " as a constant\n");
+    return false;
+  }
+
   DEBUG(dbgs() << "  Constant: " << *Const << " = " << *V << '\n');
 
   // Replaces all of the uses of a variable with uses of the constant.
@@ -1802,10 +1841,25 @@
   if (!Solver.isArgumentTrackedFunction(&F))
     return;
 
-  for (BasicBlock &BB : F)
+  // There is a non-removable musttail call site of this function. Zapping
+  // returns is not allowed.
+  if (Solver.isMustTailCallee(&F)) {
+    DEBUG(dbgs() << "Can't zap returns of the function : " << F.getName()
+                 << " due to present musttail call of it\n");
+    return;
+  }
+
+  for (BasicBlock &BB : F) {
+    if (CallInst *CI = BB.getTerminatingMustTailCall()) {
+      DEBUG(dbgs() << "Can't zap return of the block due to present "
+                   << "musttail call : " << *CI << "\n");
+      return;
+    }
+
     if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator()))
       if (!isa<UndefValue>(RI->getOperand(0)))
         ReturnsToZap.push_back(RI);
+  }
 }
 
 static bool runIPSCCP(Module &M, const DataLayout &DL,
Index: test/Transforms/IPConstantProp/musttail-call.ll
===================================================================
--- /dev/null
+++ test/Transforms/IPConstantProp/musttail-call.ll
@@ -0,0 +1,58 @@
+; RUN: opt < %s -ipsccp -S | FileCheck %s
+; PR36485
+; musttail call result can\'t be replaced with a constant, unless the call
+; can be removed
+
+declare i32 @external()
+
+define i8* @start(i8 %v) {
+  %c1 = icmp eq i8 %v, 0
+  br i1 %c1, label %true, label %false
+true:
+  ; CHECK: %ca = musttail call i8* @side_effects(i8 %v)
+  ; CHECK: ret i8* %ca
+  %ca = musttail call i8* @side_effects(i8 %v)
+  ret i8* %ca
+false:
+  %c2 = icmp eq i8 %v, 1
+  br i1 %c2, label %c2_true, label %c2_false
+c2_true:
+  %ca1 = musttail call i8* @no_side_effects(i8 %v)
+  ; CHECK: ret i8* null
+  ret i8* %ca1
+c2_false:
+  ; CHECK: %ca2 = musttail call i8* @dont_zap_me(i8 %v)
+  ; CHECK: ret i8* %ca2
+  %ca2 = musttail call i8* @dont_zap_me(i8 %v)
+  ret i8* %ca2
+}
+
+define internal i8* @side_effects(i8 %v) {
+  %i1 = call i32 @external()
+
+  ; since this goes back to `start` the SCPP should be see that the return value
+  ; is always `null`.
+  ; The call can't be removed due to `external` call above, though.
+
+  ; CHECK: %ca = musttail call i8* @start(i8 %v)
+  %ca = musttail call i8* @start(i8 %v)
+
+  ; Thus the result must be returned anyway
+  ; CHECK: ret i8* %ca
+  ret i8* %ca
+}
+
+define internal i8* @no_side_effects(i8 %v) readonly nounwind {
+  ; The call to this function is removed, so the return value must be zapped
+  ; CHECK: ret i8* undef
+  ret i8* null
+}
+
+define internal i8* @dont_zap_me(i8 %v) {
+  %i1 = call i32 @external()
+
+  ; The call to this function cannot be removed due to side effects. Thus the
+  ; return value should stay as it is, and should not be zapped.
+  ; CHECK: ret i8* null
+  ret i8* null
+}