diff --git a/lib/SILPasses/Devirtualizer.cpp b/lib/SILPasses/Devirtualizer.cpp
index 5907e7dcacc..d13cb96c452 100644
--- a/lib/SILPasses/Devirtualizer.cpp
+++ b/lib/SILPasses/Devirtualizer.cpp
@@ -1216,10 +1216,11 @@ void SILDevirtualizer::optimizeApplyInst(ApplyInst *AI) {
   //   %9 = apply %8<Self = CodeUnit?>(%6#1) : ...
   //
   WitnessMethodInst *AMI = dyn_cast<WitnessMethodInst>(AI->getCallee());
-  if (AMI && AMI->getConformance() ) {
+  if (AMI && AMI->getConformance()) {
+    ProtocolConformance *C = AMI->getConformance();
     // Lookup the function reference in the witness tables.
     std::pair<SILWitnessTable *, ArrayRef<Substitution>> Ret =
-      AI->getModule().lookUpWitnessTable(AMI->getConformance());
+      AI->getModule().lookUpWitnessTable(C);
 
     if (!Ret.first)
       return;
@@ -1229,19 +1230,37 @@ void SILDevirtualizer::optimizeApplyInst(ApplyInst *AI) {
       if (Entry.getKind() != SILWitnessTable::WitnessKind::Method)
         continue;
 
-      SILWitnessTable::MethodWitness MethodEntry = Entry.getMethodWitness();
       // Check if this is the member we were looking for.
+      SILWitnessTable::MethodWitness MethodEntry = Entry.getMethodWitness();
       if (MethodEntry.Requirement != AMI->getMember())
         continue;
 
+      // Ok, we found the member we are looking for. Devirtualize away!
       SILBuilder Builder(AI);
       SILLocation Loc = AI->getLoc();
       SILFunction *F = MethodEntry.Witness;
       FunctionRefInst *FRI = Builder.createFunctionRef(Loc, F);
 
-      // Collect the function arguments.
+      // Collect args from the apply inst.
+      ArrayRef<Operand> ApplyArgs = AI->getArgumentOperands();
       SmallVector<SILValue, 4> Args;
-      for (auto &A : AI->getArgumentOperands())
+
+      // Begin by upcasting self to the appropriate type if we have an inherited
+      // conformance...
+      SILValue Self = ApplyArgs[0].get();
+      if (C->getKind() == ProtocolConformanceKind::Inherited) {
+        CanType Ty = Ret.first->getConformance()->getType()->getCanonicalType();
+        SILType SILTy = SILType::getPrimitiveType(Ty,
+                                                  Self.getType().getCategory());
+        assert(SILTy.isSuperclassOf(Self.getType()) &&
+               "Should only create upcasts for sub class devirtualization.");
+        Self = Builder.createUpcast(Loc, Self, SILTy);
+      }
+      // and then adding in either the original or newly upcasted value.
+      Args.push_back(Self);
+
+      // Then add the rest of the arguments.
+      for (auto &A : ApplyArgs.slice(1))
         Args.push_back(A.get());
 
       SmallVector<Substitution, 16> NewSubstList(Ret.second.begin(),
diff --git a/test/SILPasses/devirt_inherited_conformance.swift b/test/SILPasses/devirt_inherited_conformance.swift
new file mode 100644
index 00000000000..3826df8b391
--- /dev/null
+++ b/test/SILPasses/devirt_inherited_conformance.swift
@@ -0,0 +1,37 @@
+// RUN: %swift -O3 %s -emit-sil -sil-inline-threshold 0 | FileCheck %s
+
+// CHECK-LABEL: sil shared @_TTSC28devirt_inherited_conformance2B2S0_S_1P___TF28devirt_inherited_conformance11doSomethingUS_1P__FT1tQ__T_ : $@thin (@in B2) -> () {
+// CHECK: bb0([[INPUT_PTR:%[0-9]+]] : $*B2):
+// CHECK-NEXT: function_ref protocol witness for devirt_inherited_conformance.P.doSomething <A : devirt_inherited_conformance.P>(@inout devirt_inherited_conformance.P.Self)() -> () in conformance devirt_inherited_conformance.B : devirt_inherited_conformance.P
+// CHECK-NEXT: [[FUNCTION_REF:%[0-9]+]] = function_ref @_TTWC28devirt_inherited_conformance1BS_1PFS1_11doSomethingUS1___fRQPS1_FT_T_ : $@cc(witness_method) @thin (@inout B) -> ()
+// CHECK-NEXT: [[CAST_PTR:%[0-9]+]] = upcast [[INPUT_PTR]] : $*B2 to $*B
+// CHECK-NEXT: apply [[FUNCTION_REF]]([[CAST_PTR]]) : $@cc(witness_method) @thin (@inout B) -> ()
+// CHECK-NEXT: load
+// CHECK-NEXT: strong_release
+// CHECK-NEXT: tuple
+// CHECK-NEXT: return
+
+@asmname("unknown1")
+func unknown1() -> ()
+
+protocol P {
+  func doSomething()
+}
+
+class B : P {
+  func doSomething() {
+    unknown1()
+  }
+}
+
+class B2 : B {
+
+}
+
+func doSomething<T : P>(t : T) {
+  t.doSomething()
+}
+
+var b2 = B2()
+
+doSomething(b2)