diff --git a/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyAllocStack.swift b/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyAllocStack.swift
index 5fe28a090ea..e7fbdad4933 100644
--- a/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyAllocStack.swift
+++ b/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyAllocStack.swift
@@ -179,7 +179,8 @@ private extension AllocStackInst {
   ///   use %3
   /// ```
   func optimizeExistential(_ context: SimplifyContext) -> Bool {
-    guard type.isExistential || type.isExistentialArchetype,
+    // TODO: support non-root existential archetypes
+    guard type.isExistential || type.isRootExistentialArchetype,
           let concreteFormalType = getConcreteTypeOfExistential()
     else {
       return false
diff --git a/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift b/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift
index 7def68d6389..046ed4a08f4 100644
--- a/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift
+++ b/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift
@@ -170,8 +170,9 @@ private extension FullApplySite {
     // Make sure that existential archetype _is_ a replacement type and not e.g. _contained_ in a
     // replacement type, like
     //    apply %1<Array<@opened("...")>()
-    guard substitutionMap.replacementTypes.contains(where: { $0.isExistentialArchetype }),
-          substitutionMap.replacementTypes.allSatisfy({ $0.isExistentialArchetype || !$0.hasLocalArchetype })
+    // TODO: support non-root existential archetypes
+    guard substitutionMap.replacementTypes.contains(where: { $0.isRootExistentialArchetype }),
+          substitutionMap.replacementTypes.allSatisfy({ $0.isRootExistentialArchetype || !$0.hasLocalArchetype })
     else {
       return false
     }
@@ -191,9 +192,9 @@ private extension FullApplySite {
       let type = value.type
       // Allow three cases:
              // case 1. the argument _is_ the existential archetype
-      return type.isExistentialArchetype ||
+      return type.isRootExistentialArchetype ||
              // case 2. the argument _is_ a metatype of the existential archetype
-             (type.isMetatype && type.canonicalType.instanceTypeOfMetatype.isExistentialArchetype) ||
+             (type.isMetatype && type.canonicalType.instanceTypeOfMetatype.isRootExistentialArchetype) ||
              // case 3. the argument has nothing to do with the existential archetype (or any other local archetype)
              !type.hasLocalArchetype
     }
diff --git a/test/SILOptimizer/simplify_alloc_stack.sil b/test/SILOptimizer/simplify_alloc_stack.sil
index 9a5e4b5e143..d1623d23cef 100644
--- a/test/SILOptimizer/simplify_alloc_stack.sil
+++ b/test/SILOptimizer/simplify_alloc_stack.sil
@@ -10,11 +10,27 @@ public struct S {}
 protocol P {
 }
 
+protocol Q {
+  associatedtype A
+}
+
 public class C2: P {}
 
 public struct T: P {
   let c: C
   let s: S
+
+  struct A {
+    var y: String
+  }
+}
+
+public struct U: Q {
+  let s: String
+
+  struct A {
+    var y: String
+  }
 }
 
 public struct T1: P {
@@ -235,6 +251,24 @@ bb0(%0 : @owned $T):
   return %r
 }
 
+// TODO: we don't support dependent existential archetypes, yet. Once we do, change this test case.
+// CHECK-LABEL: sil [ossa] @dont_replace_dependent_archetype :
+// CHECK:         alloc_stack {{.*}}@opened
+// CHECK:       } // end sil function 'dont_replace_dependent_archetype'
+sil [ossa] @dont_replace_dependent_archetype : $@convention(thin) (@owned U.A) -> () {
+bb0(%0 : @owned $U.A):
+  %1 = metatype $@thick U.Type
+  %2 = init_existential_metatype %1, $@thick Q.Type
+  %3 = open_existential_metatype %2 to $@thick (@opened("82105EE0-DCB0-11E5-865D-C8E0EB309914", Q) Self).Type
+  %4 = alloc_stack $(@opened("82105EE0-DCB0-11E5-865D-C8E0EB309914", Q) Self).A
+  %5 = unchecked_addr_cast %4 to $*U.A
+  store %0 to [init] %5
+  destroy_addr %4
+  dealloc_stack %4
+  %r = tuple ()
+  return %r
+}
+
 // CHECK-LABEL: sil [ossa] @replace_existential_with_concrete_type1 :
 // CHECK:         [[S:%.*]] = alloc_stack $T
 // CHECK-NOT:     init_existential_addr
diff --git a/test/SILOptimizer/simplify_apply.sil b/test/SILOptimizer/simplify_apply.sil
index 0e4614ee83a..25e5e8b2b2e 100644
--- a/test/SILOptimizer/simplify_apply.sil
+++ b/test/SILOptimizer/simplify_apply.sil
@@ -26,6 +26,20 @@ struct GenS<T> {
   var x: Int
 }
 
+protocol P2 {
+  associatedtype A: Q
+}
+
+protocol Q {
+}
+
+public struct S2: P2 {
+  var x: String
+  struct A: Q {
+    var y: String
+  }
+}
+
 sil @cl : $@convention(thin) () -> Int
 
 // CHECK-LABEL: sil [ossa] @thick_to_thin :
@@ -60,6 +74,7 @@ sil_vtable Bar {
 }
 
 sil @createit : $@convention(thin) <τ_0_0 where τ_0_0 : P> (@thick τ_0_0.Type, Int) -> @out τ_0_0
+sil @create2 : $@convention(thin) <τ_0_0 where τ_0_0 : Q> (Int) -> @out τ_0_0
 sil @useGenS : $@convention(thin) <τ_0_0 where τ_0_0 : P> (GenS<τ_0_0>) -> ()
 sil @returnGenS : $@convention(thin) <τ_0_0 where τ_0_0 : P> (@thick τ_0_0.Type) -> GenS<τ_0_0>
 
@@ -124,6 +139,22 @@ bb0(%0 : $Int):
   return %11
 }
 
+// TODO: we don't support dependent existential archetypes, yet. Once we do, change this test case.
+// CHECK-LABEL: sil [ossa] @dont_replace_dependent_archetype :
+// CHECK:         apply %{{[0-9]+}}<@opened
+// CHECK:       } // end sil function 'dont_replace_dependent_archetype'
+sil [ossa] @dont_replace_dependent_archetype : $@convention(thin) (Int) -> @out any Q {
+bb0(%0 : $*any Q, %1 : $Int):
+  %2 = metatype $@thick S2.Type
+  %3 = init_existential_metatype %2, $@thick any P2.Type
+  %4 = open_existential_metatype %3 to $@thick (@opened("0FC03D78-E9DB-11EF-B47C-0EA13E3AABB4", any P2) Self).Type
+  %6 = function_ref @create2 : $@convention(thin) <τ_0_0 where τ_0_0 : Q> (Int) -> @out τ_0_0
+  %7 = init_existential_addr %0, $(@opened("0FC03D78-E9DB-11EF-B47C-0EA13E3AABB4", any P2) Self).A
+  %8 = apply %6<(@opened("0FC03D78-E9DB-11EF-B47C-0EA13E3AABB4", any P2) Self).A>(%7, %1) : $@convention(thin) <τ_0_0 where τ_0_0 : Q> (Int) -> @out τ_0_0
+  %11 = tuple ()
+  return %11
+}
+
 // CHECK-LABEL: sil [ossa] @dont_replace_returned_embedded_archetype :
 // CHECK:         apply %{{[0-9]+}}<@opened
 // CHECK:       } // end sil function 'dont_replace_returned_embedded_archetype'