diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index cee993d42311c..7425d88b7ff5f 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -4281,8 +4281,25 @@ void AttributeChecker::visitImplementsAttr(ImplementsAttr *attr) {
   ProtocolDecl *PD = attr->getProtocol(DC);
 
   if (!PD) {
-    diagnose(attr->getLocation(), diag::implements_attr_non_protocol_type)
-      .highlight(attr->getProtocolTypeRepr()->getSourceRange());
+    // `ImplementsAttr::getProtocol()` returns `nullptr` both when a type fails
+    // to resolve, and when it resolves to something other than a protocol.
+    // Due to layering concerns, it doesn't resolve in a way that emits
+    // diagnostics.
+    //
+    // Distinguish between these situations by trying to resolve the type again.
+    // If it doesn't resolve, TypeResolution will have diagnosed the problem; if
+    // it does, it must have resolved to a non-protocol, so emit a diagnostic to
+    // that effect.
+    TypeResolutionOptions options(TypeResolverContext::None);
+    auto resolvedType = TypeResolution::resolveContextualType(
+        attr->getProtocolTypeRepr(), DC, options,
+        // Unbound generics and placeholders are not allowed within this attr.
+        /*unboundTyOpener*/ nullptr, /*placeholderHandler*/ nullptr,
+        /*packElementOpener*/ nullptr);
+
+    if (resolvedType && !resolvedType->hasError())
+      diagnose(attr->getLocation(), diag::implements_attr_non_protocol_type)
+        .highlight(attr->getProtocolTypeRepr()->getSourceRange());
     return;
   }
 
diff --git a/test/attr/attr_implements_bad_types.swift b/test/attr/attr_implements_bad_types.swift
index ea010f343c2d4..8bcc341280a83 100644
--- a/test/attr/attr_implements_bad_types.swift
+++ b/test/attr/attr_implements_bad_types.swift
@@ -21,5 +21,9 @@ struct S0 : NeedsF0 {  // expected-error {{type 'S0' does not conform to protoco
   @_implements(Equatable, ==(_:_:)) // expected-error {{containing type 'S0' does not conform to protocol 'Equatable'}}
   static func gg2(x:S0, y:S0) -> Bool {
   }
+
+  @_implements(NonexistentType, ff3()) // expected-error {{cannot find type 'NonexistentType' in scope}}
+  func gg3() {
+  }
 }