Sema: Ban potential unavailability on enum cases with associated values

This cannot be supported for the same reasons struct and class stored
properties with potential unavailability cannot be supported.

It is possible that some users relied on this, because it worked in
some cases where the type metadata for the potentially unavailable
type was not actually needed for anything (eg, if the type was @Frozen
and trivial, or a simple class reference). If this becomes an issue
we can downgrade this to a warning.

Note that just like the previous fix for stored properties, we do
still allow writing an '@available' attribute on the enum element if
the OS version is not any newer than the deployment target. This allows
annotating APIs with the OS version where they were introduced.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -5178,6 +5178,10 @@ ERROR(availability_stored_property_no_potential,
       none, "stored properties cannot be marked potentially unavailable with "
       "'@available'", ())
 
+ERROR(availability_enum_element_no_potential,
+      none, "enum cases with associated values cannot be marked potentially unavailable with "
+      "'@available'", ())
+
 ERROR(availability_protocol_requires_version,
       none, "protocol %0 requires %1 to be available in %2 %3 and newer",
       (Identifier, DeclName, StringRef, llvm::VersionTuple))

lib/Sema/TypeCheckAttr.cpp

@@ -3418,6 +3418,12 @@ TypeChecker::diagnosticIfDeclCannotBePotentiallyUnavailable(const Decl *D) {
     // for potential unavailability.
     if (!VD->isStatic() && !VD->getDeclContext()->isModuleScopeContext())
       return diag::availability_stored_property_no_potential;
+
+  } else if (auto *EED = dyn_cast<EnumElementDecl>(D)) {
+    // An enum element with an associated value cannot be potentially
+    // unavailable.
+    if (EED->hasAssociatedValues())
+      return diag::availability_enum_element_no_potential;
   }
 
   return None;

test/Sema/availability_stored.swift

@@ -9,26 +9,52 @@
 struct NewStruct {}
 
 @available(macOS 50, *)
-struct GoodReference {
+struct GoodReferenceStruct {
   var x: NewStruct
 }
 
 @available(macOS 50, *)
-struct GoodNestedReference {
+struct GoodNestedReferenceStruct {
   struct Inner {
     var x: NewStruct
   }
 }
 
-struct BadReference1 {
+struct BadReferenceStruct1 {
   // expected-error@+1 {{stored properties cannot be marked potentially unavailable with '@available'}}
   @available(macOS 50, *)
   var x: NewStruct
 }
 
 @available(macOS 40, *)
-struct BadReference2 {
+struct BadReferenceStruct2 {
   // expected-error@+1 {{stored properties cannot be marked potentially unavailable with '@available'}}
   @available(macOS 50, *)
   var x: NewStruct
 }
+
+// The same behavior should hold for enum elements with payloads.
+@available(macOS 50, *)
+enum GoodReferenceEnum {
+  case x(NewStruct)
+}
+
+@available(macOS 50, *)
+enum GoodNestedReferenceEnum {
+  enum Inner {
+    case x(NewStruct)
+  }
+}
+
+enum BadReferenceEnum1 {
+  // expected-error@+1 {{enum cases with associated values cannot be marked potentially unavailable with '@available'}}
+  @available(macOS 50, *)
+  case x(NewStruct)
+}
+
+@available(macOS 40, *)
+enum BadReferenceEnum2 {
+  // expected-error@+1 {{enum cases with associated values cannot be marked potentially unavailable with '@available'}}
+  @available(macOS 50, *)
+  case x(NewStruct)
+}

test/Sema/availability_versions.swift

@@ -537,17 +537,17 @@ enum CompassPoint {
 
   case WithAvailableByEnumPayload(p : EnumIntroducedOn10_51)
 
+  // expected-error@+1 {{enum cases with associated values cannot be marked potentially unavailable with '@available'}}
   @available(OSX, introduced: 10.52)
   case WithAvailableByEnumElementPayload(p : EnumIntroducedOn10_52)
 
+  // expected-error@+1 2{{enum cases with associated values cannot be marked potentially unavailable with '@available'}}
   @available(OSX, introduced: 10.52)
   case WithAvailableByEnumElementPayload1(p : EnumIntroducedOn10_52), WithAvailableByEnumElementPayload2(p : EnumIntroducedOn10_52)
 
   case WithUnavailablePayload(p : EnumIntroducedOn10_52) // expected-error {{'EnumIntroducedOn10_52' is only available in macOS 10.52 or newer}}
-      // expected-note@-1 {{add @available attribute to enclosing case}}
 
-    case WithUnavailablePayload1(p : EnumIntroducedOn10_52), WithUnavailablePayload2(p : EnumIntroducedOn10_52) // expected-error 2{{'EnumIntroducedOn10_52' is only available in macOS 10.52 or newer}}
-      // expected-note@-1 2{{add @available attribute to enclosing case}}
+  case WithUnavailablePayload1(p : EnumIntroducedOn10_52), WithUnavailablePayload2(p : EnumIntroducedOn10_52) // expected-error 2{{'EnumIntroducedOn10_52' is only available in macOS 10.52 or newer}}
 }
 
 @available(OSX, introduced: 10.52)
@@ -1336,11 +1336,9 @@ enum EnumForFixit {
       // expected-note@-1 2{{add @available attribute to enclosing enum}} {{1-1=@available(macOS 10.51, *)\n}}
   case CaseWithUnavailablePayload(p: ClassAvailableOn10_51)
       // expected-error@-1 {{'ClassAvailableOn10_51' is only available in macOS 10.51 or newer}}
-      // expected-note@-2 {{add @available attribute to enclosing case}} {{3-3=@available(macOS 10.51, *)\n  }}
 
   case CaseWithUnavailablePayload2(p: ClassAvailableOn10_51), WithoutPayload
       // expected-error@-1 {{'ClassAvailableOn10_51' is only available in macOS 10.51 or newer}}
-      // expected-note@-2 {{add @available attribute to enclosing case}} {{3-3=@available(macOS 10.51, *)\n  }}
 
 }