warn on empty OptionSet static constants

include/swift/AST/DiagnosticsSema.def

@@ -4623,6 +4623,12 @@ NOTE(previous_function_builder_here, none,
 ERROR(function_builder_arguments, none,
       "function builder attributes cannot have arguments", ())
 
+WARNING(option_set_zero_constant,none,
+      "static property %0 produces an empty option set",
+      (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

@@ -482,6 +482,47 @@ 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;
+
+  if (!VD->getType()->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;
+    if (ctor->getArgumentLabels().front() != tc.Context.Id_rawValue) continue;
+
+    auto *args = cast<TupleExpr>(ctor->getArg());
+    auto intArg = dyn_cast<IntegerLiteralExpr>(args->getElement(0));
+    if (!intArg) continue;
+    if (intArg->getValue() != 0) continue;
+
+    auto loc = VD->getLoc();
+    tc.diagnose(loc, diag::option_set_zero_constant, VD->getName());
+    tc.diagnose(loc, diag::option_set_empty_set_init).fixItReplace(args->getSourceRange(), "([])");
+  }
+}
+
+
 /// Check the inheritance clauses generic parameters along with any
 /// requirements stored within the generic parameter list.
 static void checkGenericParams(GenericParamList *genericParams,
@@ -2210,6 +2251,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
           VD->diagnose(diag::dynamic_self_in_mutable_property);
       }
     }
+
+    checkForEmptyOptionSet(VD, TC);
 
     // Under the Swift 3 inference rules, if we have @IBInspectable or
     // @GKInspectable but did not infer @objc, warn that the attribute is

test/Sema/option-set-empty.swift

@@ -0,0 +1,6 @@
+// RUN: %target-typecheck-verify-swift
+
+struct MyOptions: OptionSet {
+    var rawValue: Int
+    static let none = MyOptions(rawValue: 0) // expected-warning {{static property 'none' produces an empty option set}} expected-note {{use [] to silence this warning}}{{32-45=([])}}
+}