warn on empty OptionSet static constants

include/swift/AST/DiagnosticsSema.def

@@ -4423,6 +4423,12 @@ ERROR(property_delegate_type_not_usable_from_inline,none,
       "must be '@usableFromInline' or public",
       (bool, bool))
 
+WARNING(option_set_zero_constant,none,
+      "%0 %1 produces an empty option set",
+      (DescriptiveDeclKind, Identifier))
+NOTE(option_set_empty_set_init,none,
+      "use [] to silence this warning", ())
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

lib/Sema/TypeCheckDecl.cpp

@@ -478,6 +478,52 @@ static void checkInheritanceClause(
   }
 }
 
+// Check for static properties that produce empty option sets
+static void checkForEmptyOptionSet(const VarDecl *VD, TypeChecker &tc) {
+  if (!VD->isStatic() || !VD->isLet())
+    return;
+
+  auto DC = VD->getDeclContext();
+
+  auto *optionSetProto = tc.Context.getProtocol(KnownProtocolKind::OptionSet);
+  auto protocolConformance = tc.containsProtocol(DC->getSelfTypeInContext(), optionSetProto, DC, /*Flags*/None);
+  if (!protocolConformance)
+    return;
+
+  auto type = VD->getType();
+  if (!type->isEqual(DC->getSelfTypeInContext()))
+    return;
+
+  auto PBD = VD->getParentPatternBinding();
+  if (!PBD)
+    return;
+
+  for (auto entry : PBD->getPatternList()) {
+    if (entry.getPattern()->getSingleVar() != VD) continue;
+
+    auto ctor = dyn_cast<CallExpr>(entry.getInit());
+    if (!ctor) continue;
+    if (!isa<ConstructorDecl>(ctor->getCalledValue())) continue;
+
+    if (ctor->getNumArguments() != 1) continue;
+    auto argLabels = ctor->getArgumentLabels();
+    if (argLabels.front() != tc.Context.Id_rawValue) continue;
+
+    auto *args = cast<TupleExpr>(ctor->getArg());
+    auto intArg = dyn_cast<IntegerLiteralExpr>(args->getElement(0));
+    if (!intArg) continue;
+
+    auto val = intArg->getValue();
+    if (val != 0) continue;
+
+    auto loc = VD->getLoc();
+    tc.diagnose(loc, diag::option_set_zero_constant, DescriptiveDeclKind::StaticProperty, VD->getName());
+    auto range = ctor->getArg()->getSourceRange();
+    tc.diagnose(loc, diag::option_set_empty_set_init).fixItReplace(range, "([])");
+  }
+}
+
+
 /// Check the inheritance clauses generic parameters along with any
 /// requirements stored within the generic parameter list.
 static void checkGenericParams(GenericParamList *genericParams,
@@ -2360,6 +2406,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     TC.checkDeclAttributes(VD);
 
+    checkForEmptyOptionSet(VD, TC);
+
     triggerAccessorSynthesis(TC, VD);
 
     // Under the Swift 3 inference rules, if we have @IBInspectable or
@@ -2383,7 +2431,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     if (VD->getAttrs().hasAttribute<DynamicReplacementAttr>())
       TC.checkDynamicReplacementAttribute(VD);
   }
-
+  
   void visitBoundVars(Pattern *P) {
     P->forEachVariable([&] (VarDecl *VD) { this->visitBoundVariable(VD); });
   }

test/Sema/option-set-empty.swift

@@ -0,0 +1,4 @@
+struct MyOptions: OptionSet {
+    var rawValue: Int
+    static let none = MyOptions(rawValue: 0) // expected-warning {{'static let' constant inside 'MyOptions' that conforms to OptionSet produces an empty option set}}
+}