diff --git a/lib/SIL/IR/SILFunctionType.cpp b/lib/SIL/IR/SILFunctionType.cpp
index a32423aeb18a9..0fc26a1c02f70 100644
--- a/lib/SIL/IR/SILFunctionType.cpp
+++ b/lib/SIL/IR/SILFunctionType.cpp
@@ -2955,8 +2955,10 @@ static CanSILFunctionType getSILFunctionTypeForInitAccessor(
 
   // Make a new 'self' parameter.
   if (!declContext->isLocalContext()) {
-    auto selfInterfaceType =
-        MetatypeType::get(declContext->getSelfInterfaceType());
+    auto selfInterfaceType = MetatypeType::get(
+        (genericSig)
+            ? declContext->getSelfInterfaceType()->getReducedType(genericSig)
+            : declContext->getSelfTypeInContext());
     AbstractionPattern origSelfType(genericSig,
                                     selfInterfaceType->getCanonicalType());
     auto loweredSelfType = TC.getLoweredType(
diff --git a/lib/SILGen/SILGenFunction.cpp b/lib/SILGen/SILGenFunction.cpp
index 37b5205224f8b..e39ebbfc0b8e5 100644
--- a/lib/SILGen/SILGenFunction.cpp
+++ b/lib/SILGen/SILGenFunction.cpp
@@ -1931,11 +1931,6 @@ void SILGenFunction::emitAssignOrInit(SILLocation loc, ManagedValue selfValue,
 
   auto initTy = initFRef->getType().castTo<SILFunctionType>();
 
-  // If there are substitutions we need to emit partial apply to
-  // apply substitutions to the init accessor reference type.
-  initTy = initTy->substGenericArgs(SGM.M, substitutions,
-                                    getTypeExpansionContext());
-
   // Emit partial apply with self metatype argument to produce a substituted
   // init accessor reference.
   auto selfTy = selfValue.getType().getASTType();
@@ -1949,8 +1944,9 @@ void SILGenFunction::emitAssignOrInit(SILLocation loc, ManagedValue selfValue,
     selfMetatype = B.createMetatype(loc, getLoweredType(metatypeTy));
   }
 
-  auto expectedSelfTy = initAccessor->getDeclContext()->getSelfInterfaceType()
-      .subst(substitutions);
+  auto expectedSelfTy =
+      initAccessor->getDeclContext()->getSelfInterfaceType().subst(
+          substitutions);
 
   // This should only happen in the invalid case where we attempt to initialize
   // superclass storage from a subclass initializer. However, we shouldn't
@@ -1960,6 +1956,14 @@ void SILGenFunction::emitAssignOrInit(SILLocation loc, ManagedValue selfValue,
     selfMetatype = B.createUpcast(loc, selfMetatype,
                              getLoweredType(MetatypeType::get(expectedSelfTy)));
   }
+
+  if (auto invocationSig = initTy->getInvocationGenericSignature()) {
+    if (invocationSig->areAllParamsConcrete())
+      substitutions = SubstitutionMap();
+  } else {
+    substitutions = SubstitutionMap();
+  }
+
   PartialApplyInst *initPAI =
       B.createPartialApply(loc, initFRef, substitutions, selfMetatype,
                            ParameterConvention::Direct_Guaranteed,
diff --git a/test/SILOptimizer/init_accessor_in_constrained_extensions.swift b/test/SILOptimizer/init_accessor_in_constrained_extensions.swift
new file mode 100644
index 0000000000000..a8337ee955b4b
--- /dev/null
+++ b/test/SILOptimizer/init_accessor_in_constrained_extensions.swift
@@ -0,0 +1,73 @@
+// RUN: %target-swift-frontend -Xllvm -sil-print-types -Xllvm -sil-print-after=definite-init -emit-sil -module-name assign_or_init_lowering %s -o /dev/null 2>&1 | %FileCheck %s
+
+struct S1<T> {
+}
+
+extension S1 where T == Int {
+  class Test {
+    var test1: Int {
+      init(initialValue) { }
+      set {}
+      get { 0 }
+    }
+
+    var test2: T {
+      init(initialValue) { }
+      set {}
+      get { 0 }
+    }
+
+    // CHECK-LABEL: sil hidden [ossa] @$s23assign_or_init_lowering2S1VAASiRszlE4TestCAEySi_Gycfc : $@convention(method) (@owned S1<Int>.Test) -> @owned S1<Int>.Test
+    //
+    // CHECK: [[TEST1_INIT_REF:%.*]] = function_ref @$s23assign_or_init_lowering2S1VAASiRszlE4TestC5test1Sivi : $@convention(thin) (Int, @thick S1<Int>.Test.Type) -> ()
+    // CHECK-NEXT: [[SELF:%.*]] = value_metatype $@thick S1<Int>.Test.Type, {{.*}} : $S1<Int>.Test
+    // CHECK-NEXT: [[TEST1_INIT_REF_WITH_SELF_APPLIED:%.*]] = partial_apply [callee_guaranteed] [on_stack] [[TEST1_INIT_REF]]([[SELF]]) : $@convention(thin) (Int, @thick S1<Int>.Test.Type) -> ()
+    // CHECK: assign_or_init [init] #S1.Test.test1, self {{.*}} : $S1<Int>.Test, value {{.*}} : $Int, init [[TEST1_INIT_REF_WITH_SELF_APPLIED]] : $@noescape @callee_guaranteed (Int) -> (), set {{.*}} : $@noescape @callee_guaranteed (Int) -> ()
+    //
+    // CHECK: [[TEST2_INIT_REF:%.*]] = function_ref @$s23assign_or_init_lowering2S1VAASiRszlE4TestC5test2Sivi : $@convention(thin) (Int, @thick S1<Int>.Test.Type) -> ()
+    // CHECK-NEXT: [[SELF:%.*]] = value_metatype $@thick S1<Int>.Test.Type, {{.*}} : $S1<Int>.Test
+    // CHECK-NEXT: [[TEST2_INIT_REF_WITH_SELF_APPLIED:%.*]] = partial_apply [callee_guaranteed] [on_stack] [[TEST2_INIT_REF]]([[SELF]]) : $@convention(thin) (Int, @thick S1<Int>.Test.Type) -> ()
+    // CHECK: assign_or_init [init] #S1.Test.test2, self {{.*}} : $S1<Int>.Test, value {{.*}} : $Int, init [[TEST2_INIT_REF_WITH_SELF_APPLIED]] : $@noescape @callee_guaranteed (Int) -> (), set {{.*}} : $@noescape @callee_guaranteed (Int) -> ()
+    // CHECK: } // end sil function '$s23assign_or_init_lowering2S1VAASiRszlE4TestCAEySi_Gycfc'
+    init() {
+      test1 = 0
+      test2 = 1
+    }
+  }
+}
+
+struct S2<T, U> {
+}
+
+extension S2 where T == Int {
+  class Test {
+    var test1: T {
+      init(initialValue) { }
+      set {}
+      get { 0 }
+    }
+
+    var test2: U {
+      init(initialValue) { }
+      set {}
+      get { fatalError() }
+    }
+
+    // CHECK-LABEL: sil hidden [ossa] @$s23assign_or_init_lowering2S2VAASiRszrlE4TestC1uAEySiq__Gq__tcfc : $@convention(method) <T, U where T == Int> (@in U, @owned S2<Int, U>.Test) -> @owned S2<Int, U>.Test {
+    //
+    // CHECK: [[TEST1_INIT_REF:%.*]] = function_ref @$s23assign_or_init_lowering2S2VAASiRszrlE4TestC5test1Sivi : $@convention(thin) <τ_0_0, τ_0_1 where τ_0_0 == Int> (Int, @thick S2<Int, τ_0_1>.Test.Type) -> ()
+    // CHECK-NEXT: [[SELF:%.*]] = value_metatype $@thick S2<Int, U>.Test.Type, {{.*}} : $S2<Int, U>.Test
+    // CHECK-NEXT: [[TEST1_INIT_REF_WITH_SELF_APPLIED:%.*]] = partial_apply [callee_guaranteed] [on_stack] [[TEST1_INIT_REF]]<Int, U>([[SELF]]) : $@convention(thin) <τ_0_0, τ_0_1 where τ_0_0 == Int> (Int, @thick S2<Int, τ_0_1>.Test.Type) -> ()
+    // CHECK: assign_or_init [init] #S2.Test.test1, self {{.*}} : $S2<Int, U>.Test, value {{.*}} : $Int, init [[TEST1_INIT_REF_WITH_SELF_APPLIED]] : $@noescape @callee_guaranteed (Int) -> (), set {{.*}} : $@noescape @callee_guaranteed (Int) -> ()
+    //
+    // CHECK: [[TEST2_INIT_REF:%.*]] = function_ref @$s23assign_or_init_lowering2S2VAASiRszrlE4TestC5test2q_vi : $@convention(thin) <τ_0_0, τ_0_1 where τ_0_0 == Int> (@in τ_0_1, @thick S2<Int, τ_0_1>.Test.Type) -> ()
+    // CHECK-NEXT: [[SELF:%.*]] = value_metatype $@thick S2<Int, U>.Test.Type, {{.*}} : $S2<Int, U>.Test
+    // CHECK-NEXT: [[TEST2_INIT_REF_WITH_SELF_APPLIED:%.*]] = partial_apply [callee_guaranteed] [on_stack] [[TEST2_INIT_REF]]<Int, U>([[SELF]]) : $@convention(thin) <τ_0_0, τ_0_1 where τ_0_0 == Int> (@in τ_0_1, @thick S2<Int, τ_0_1>.Test.Type) -> ()
+    // CHECK: assign_or_init [init] #S2.Test.test2, self {{.*}} : $S2<Int, U>.Test, value {{.*}} : $*U, init [[TEST2_INIT_REF_WITH_SELF_APPLIED]] : $@noescape @callee_guaranteed (@in U) -> (), set {{.*}} : $@noescape @callee_guaranteed (@in U) -> ()
+    // CHECK: } // end sil function '$s23assign_or_init_lowering2S2VAASiRszrlE4TestC1uAEySiq__Gq__tcfc'
+    init(u: U) {
+      test1 = 0
+      test2 = u
+    }
+  }
+}