Sema: Downgrade potentially unavailable enum cases to a warning in module interfaces

include/swift/AST/DiagnosticsSema.def

@@ -5354,6 +5354,10 @@ ERROR(availability_enum_element_no_potential,
       none, "enum cases with associated values cannot be marked potentially unavailable with "
       "'@available'", ())
 
+WARNING(availability_enum_element_no_potential_warn,
+        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))

include/swift/Basic/LangOptions.h

@@ -134,6 +134,9 @@ namespace swift {
     /// Should conformance availability violations be diagnosed as errors?
     bool EnableConformanceAvailabilityErrors = false;
 
+    /// Should potential unavailability on enum cases be downgraded to a warning?
+    bool WarnOnPotentiallyUnavailableEnumCase = false;
+
     /// Maximum number of typo corrections we are allowed to perform.
     /// This is disabled by default until we can get typo-correction working within acceptable performance bounds.
     unsigned TypoCorrectionLimit = 0;

include/swift/Option/FrontendOptions.td

@@ -453,6 +453,10 @@ def disable_conformance_availability_errors : Flag<["-"],
   "disable-conformance-availability-errors">,
   HelpText<"Diagnose conformance availability violations as warnings">;
 
+def warn_on_potentially_unavailable_enum_case : Flag<["-"],
+  "warn-on-potentially-unavailable-enum-case">,
+  HelpText<"Downgrade potential unavailability of enum case to a warning">;
+
 def report_errors_to_debugger : Flag<["-"], "report-errors-to-debugger">,
   HelpText<"Deprecated, will be removed in future versions.">;
 

lib/Driver/ToolChains.cpp

@@ -175,6 +175,10 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
     arguments.push_back("-aarch64-use-tbi");
   }
 
+  if (output.getPrimaryOutputType() == file_types::TY_SwiftModuleFile) {
+    arguments.push_back("-warn-on-potentially-unavailable-enum-case");
+  }
+
   // Enable or disable ObjC interop appropriately for the platform
   if (Triple.isOSDarwin()) {
     arguments.push_back("-enable-objc-interop");

lib/Frontend/CompilerInvocation.cpp

@@ -457,6 +457,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       = A->getOption().matches(OPT_enable_conformance_availability_errors);
   }
 
+  Opts.WarnOnPotentiallyUnavailableEnumCase |=
+      Args.hasArg(OPT_warn_on_potentially_unavailable_enum_case);
   if (auto A = Args.getLastArg(OPT_enable_access_control,
                                OPT_disable_access_control)) {
     Opts.EnableAccessControl

lib/Sema/TypeCheckAttr.cpp

@@ -3365,6 +3365,8 @@ Type TypeChecker::checkReferenceOwnershipAttr(VarDecl *var, Type type,
 
 Optional<Diag<>>
 TypeChecker::diagnosticIfDeclCannotBePotentiallyUnavailable(const Decl *D) {
+  auto *DC = D->getDeclContext();
+
   if (auto *VD = dyn_cast<VarDecl>(D)) {
     if (!VD->hasStorage())
       return None;
@@ -3377,14 +3379,23 @@ TypeChecker::diagnosticIfDeclCannotBePotentiallyUnavailable(const Decl *D) {
 
     // Globals and statics are lazily initialized, so they are safe
     // for potential unavailability.
-    if (!VD->isStatic() && !VD->getDeclContext()->isModuleScopeContext())
+    if (!VD->isStatic() && !DC->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;
+    if (EED->hasAssociatedValues()) {
+      auto &ctx = DC->getASTContext();
+      auto *SF = DC->getParentSourceFile();
+
+      if (SF->Kind == SourceFileKind::Interface ||
+          ctx.LangOpts.WarnOnPotentiallyUnavailableEnumCase) {
+        return diag::availability_enum_element_no_potential_warn;
+      } else {
+        return diag::availability_enum_element_no_potential;
+      }
+    }
   }
 
   return None;

test/Sema/Inputs/availability_enum_case_other.swift

@@ -0,0 +1,4 @@
+public enum Horse {
+  @available(macOS 100, *)
+  case kevin(Int)
+}

test/Sema/availability_enum_case.swift

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-build-swift -emit-module %S/Inputs/availability_enum_case_other.swift -emit-module-interface-path %t/availability_enum_case_other.swiftinterface -swift-version 5 -enable-library-evolution
+// RUN: %target-typecheck-verify-swift -I %t
+
+// RUN: %target-build-swift -emit-module %S/Inputs/availability_enum_case_other.swift -emit-module-interface-path %t/availability_enum_case_other.swiftinterface -swift-version 5 -enable-library-evolution -whole-module-optimization
+// RUN: %target-typecheck-verify-swift -I %t
+
+// REQUIRES: OS=macosx
+
+import availability_enum_case_other
+
+func ride(horse: Horse) {
+  // expected-note@-1 {{add @available attribute to enclosing global function}}
+
+  _ = Horse.kevin
+  // expected-error@-1 {{'kevin' is only available in macOS 100 or newer}}
+  // expected-note@-2 {{add 'if #available' version check}}
+}