diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index a330b63faedcb..8d570aa307800 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -4946,21 +4946,19 @@ ProtocolDecl::findProtocolSelfReferences(const ValueDecl *value,
         return result;
     }
 
-    return ::findProtocolSelfReferences(this, type,
-                                        skipAssocTypes);
-  } else if (auto subscript = dyn_cast<SubscriptDecl>(value)) {
-    // Check the requirements of a generic subscript.
-    if (subscript->isGeneric()) {
-      if (auto result =
-            ::findProtocolSelfReferences(this,
-                                         subscript->getGenericSignature()))
-        return result;
-    }
-
     return ::findProtocolSelfReferences(this, type,
                                         skipAssocTypes);
   } else {
-    assert(isa<VarDecl>(value));
+    assert(isa<AbstractStorageDecl>(value));
+
+    if (auto *const subscript = dyn_cast<SubscriptDecl>(value)) {
+      // Check the requirements of a generic subscript.
+      if (subscript->isGeneric()) {
+        if (auto result = ::findProtocolSelfReferences(
+                this, subscript->getGenericSignature()))
+          return result;
+      }
+    }
 
     return ::findProtocolSelfReferences(this, type,
                                         skipAssocTypes);
@@ -4978,6 +4976,12 @@ bool ProtocolDecl::isAvailableInExistential(const ValueDecl *decl) const {
   if (selfKind.parameter || selfKind.other)
     return false;
 
+  // FIXME: Appropriately diagnose assignments instead.
+  if (auto *const storageDecl = dyn_cast<AbstractStorageDecl>(decl)) {
+    if (selfKind.result && storageDecl->supportsMutation())
+      return false;
+  }
+
   return true;
 }
 
diff --git a/test/decl/protocol/req/dynamic_self.swift b/test/decl/protocol/req/dynamic_self.swift
index 758a301ef6f22..ef4f33a100b1d 100644
--- a/test/decl/protocol/req/dynamic_self.swift
+++ b/test/decl/protocol/req/dynamic_self.swift
@@ -86,3 +86,39 @@ enum EError : P { // expected-error{{type 'EError' does not conform to protocol
   subscript() -> Int { 0 } // expected-note{{candidate has non-matching type '() -> Int'}}
   func f() -> Int { 0 } // expected-note{{candidate has non-matching type '() -> Int'}}
 }
+
+
+// Settable storage declaration requirements with a 'Self' result type may not
+// be used with an existential base.
+protocol P2 {
+  subscript() -> Self { get set }
+}
+protocol P3 {
+  var prop: Self { get set }
+}
+protocol P4 {
+  subscript<T: Sequence>() -> T where T.Element == Self { get set }
+}
+func takesP2P3P4(p2: P2, p3: P3, p4: P4) { }
+// expected-error@-1{{protocol 'P2' can only be used as a generic constraint because it has Self or associated type requirements}}
+// expected-error@-2{{protocol 'P3' can only be used as a generic constraint because it has Self or associated type requirements}}
+
+protocol P5 {
+}
+extension P5 {
+  var prop: Self {
+    get { self }
+    set { }
+  }
+
+  subscript() -> Self {
+    get { self }
+    set { }
+  }
+}
+func takesP5(p5: P5) {
+  _ = p5[]
+  // expected-error@-1{{member 'subscript' cannot be used on value of protocol type 'P5'; use a generic constraint instead}}
+  _ = p5.prop
+  // expected-error@-1{{member 'prop' cannot be used on value of protocol type 'P5'; use a generic constraint instead}}
+}