diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index f95ffb6e407..df5ea625221 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -2154,6 +2154,17 @@ ERROR(lazy_not_observable,none,
 ERROR(attr_for_debugger_support_only,none,
       "@LLDBDebuggerSupport may only be used when debugger support is on", ())
 
+// @_implements
+ERROR(implements_attr_protocol_lacks_member,none,
+      "protocol %0 has no member %1", (DeclName, DeclName))
+
+ERROR(implements_attr_non_protocol_type,none,
+      "non-protocol type in @_implements attribute", ())
+
+ERROR(implements_attr_protocol_not_conformed_to,none,
+      "containing type %0 does not conform to protocol %1",
+      (DeclName, DeclName))
+
 //------------------------------------------------------------------------------
 // Type Check Expressions
 //------------------------------------------------------------------------------
diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index 308adc3af32..77acfff9083 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -756,7 +756,6 @@ public:
     IGNORED_ATTR(WarnUnqualifiedAccess)
     IGNORED_ATTR(ShowInInterface)
     IGNORED_ATTR(ObjCMembers)
-    IGNORED_ATTR(Implements)
 #undef IGNORED_ATTR
 
   void visitAvailableAttr(AvailableAttr *attr);
@@ -797,6 +796,7 @@ public:
   void visitInlineableAttr(InlineableAttr *attr);
   
   void visitDiscardableResultAttr(DiscardableResultAttr *attr);
+  void visitImplementsAttr(ImplementsAttr *attr);
 };
 } // end anonymous namespace
 
@@ -1884,6 +1884,52 @@ void AttributeChecker::visitDiscardableResultAttr(DiscardableResultAttr *attr) {
   }
 }
 
+void AttributeChecker::visitImplementsAttr(ImplementsAttr *attr) {
+  TypeLoc &ProtoTypeLoc = attr->getProtocolType();
+  TypeResolutionOptions options;
+  options |= TR_AllowUnboundGenerics;
+
+  DeclContext *DC = D->getDeclContext();
+  Type T = TC.resolveType(ProtoTypeLoc.getTypeRepr(),
+                          DC, options);
+  ProtoTypeLoc.setType(T);
+
+  // Definite error-types were already diagnosed in resolveType.
+  if (!T || T->hasError())
+    return;
+
+  // Check that we got a ProtocolType.
+  if (auto PT = T->getAs<ProtocolType>()) {
+    ProtocolDecl *PD = PT->getDecl();
+
+    // Check that the ProtocolType has the specified member.
+    LookupResult R = TC.lookupMember(PD->getDeclContext(),
+                                     PT, attr->getMemberName());
+    if (!R) {
+      TC.diagnose(attr->getLocation(),
+                  diag::implements_attr_protocol_lacks_member,
+                  PD->getBaseName(), attr->getMemberName())
+        .highlight(attr->getMemberNameLoc().getSourceRange());
+    }
+
+    // Check that the decl we're decorating is a member of a type that actually
+    // conforms to the specified protocol.
+    NominalTypeDecl *NTD = DC->getAsNominalTypeOrNominalTypeExtensionContext();
+    SmallVector<ProtocolConformance *, 2> conformances;
+    if (!NTD->lookupConformance(DC->getParentModule(), PD, conformances)) {
+      TC.diagnose(attr->getLocation(),
+                  diag::implements_attr_protocol_not_conformed_to,
+                  NTD->getFullName(), PD->getFullName())
+        .highlight(ProtoTypeLoc.getTypeRepr()->getSourceRange());
+    }
+
+  } else {
+    TC.diagnose(attr->getLocation(),
+                diag::implements_attr_non_protocol_type)
+      .highlight(ProtoTypeLoc.getTypeRepr()->getSourceRange());
+  }
+}
+
 void TypeChecker::checkDeclAttributes(Decl *D) {
   AttributeChecker Checker(*this, D);