[TypeChecker] Implement ExtensibleEnums feature exhaustivily handling

If an enum comes from a different module that has `ExtensibleEnums`
feature enabled, unless it requires either `@unknown default:` or
`@frozen` because it is allowed to introduce new cases in the future
versions of the module.

lib/AST/Decl.cpp

@@ -6696,8 +6696,22 @@ bool EnumDecl::hasOnlyCasesWithoutAssociatedValues() const {
 }
 
 bool EnumDecl::treatAsExhaustiveForDiags(const DeclContext *useDC) const {
-  return isFormallyExhaustive(useDC) ||
-         (useDC && getModuleContext()->inSamePackage(useDC->getParentModule()));
+  if (useDC) {
+    auto *enumModule = getModuleContext();
+    if (enumModule->inSamePackage(useDC->getParentModule()))
+      return true;
+
+    // If the module where enum is declared supports extensible enumerations
+    // and this enum is not explicitly marked as "@frozen", cross-module
+    // access cannot be exhaustive and requires `@unknown default:`.
+    if (enumModule->supportsExtensibleEnums() &&
+        !getAttrs().hasAttribute<FrozenAttr>()) {
+      if (useDC != enumModule->getDeclContext())
+        return false;
+    }
+  }
+
+  return isFormallyExhaustive(useDC);
 }
 
 bool EnumDecl::isFormallyExhaustive(const DeclContext *useDC) const {

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1154,13 +1154,19 @@ namespace {
         assert(defaultReason == RequiresDefault::No);
         Type subjectType = Switch->getSubjectExpr()->getType();
         bool shouldIncludeFutureVersionComment = false;
+        bool shouldDowngradeToWarning = true;
         if (auto *theEnum = subjectType->getEnumOrBoundGenericEnum()) {
+          auto *enumModule = theEnum->getParentModule();
           shouldIncludeFutureVersionComment =
-              theEnum->getParentModule()->isSystemModule();
+              enumModule->isSystemModule() ||
+              enumModule->supportsExtensibleEnums();
+          // Since the module enabled `ExtensibleEnums` feature they
+          // opted-in all of their clients into exhaustivity errors.
+          shouldDowngradeToWarning = !enumModule->supportsExtensibleEnums();
         }
         DE.diagnose(startLoc, diag::non_exhaustive_switch_unknown_only,
                     subjectType, shouldIncludeFutureVersionComment)
-          .warnUntilSwiftVersion(6);
+          .warnUntilSwiftVersionIf(shouldDowngradeToWarning, 6);
         mainDiagType = std::nullopt;
       }
         break;

test/ModuleInterface/extensible_enums.swift

@@ -0,0 +1,110 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/src)
+// RUN: split-file %s %t/src
+
+/// Build the library
+// RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
+// RUN:   -module-name Lib \
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -enable-experimental-feature ExtensibleEnums
+
+// Check that the errors are produced when using enums from module with `ExtensibleEnums` feature enabled.
+// RUN: %target-swift-frontend -typecheck %t/src/TestChecking.swift \
+// RUN:   -swift-version 5 -module-name Client -I %t \
+// RUN:   -verify
+
+// Test to make sure that if the library and client are in the same package enums are checked exhaustively
+
+/// Build the library
+// RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
+// RUN:   -module-name Lib \
+// RUN:   -package-name Test \
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -enable-experimental-feature ExtensibleEnums
+
+// Different module but the same package
+// RUN: %target-swift-frontend -typecheck %t/src/TestSamePackage.swift \
+// RUN:   -swift-version 5 -module-name Client -I %t \
+// RUN:   -package-name Test \
+// RUN:   -verify
+
+// REQUIRES: swift_feature_ExtensibleEnums
+
+//--- Lib.swift
+
+public enum E {
+  case a
+}
+
+@frozen
+public enum F {
+  case a
+  case b
+}
+
+func test_same_module(e: E, f: F) {
+  switch e { // Ok
+  case .a: break 
+  }
+
+  switch f { // Ok
+  case .a: break
+  case .b: break
+  }
+}
+
+//--- TestChecking.swift
+import Lib
+
+func test(e: E, f: F) {
+  // `E` is not marked as `@frozen` which means it gets new semantics
+
+  switch e {
+  // expected-error@-1 {{switch covers known cases, but 'E' may have additional unknown values, possibly added in future versions}}
+  // expected-note@-2 {{handle unknown values using "@unknown default"}}
+  case .a: break
+  }
+
+  switch e { // Ok (no warnings)
+  case .a: break
+  @unknown default: break
+  }
+
+  // `F` is marked as `@frozen` which means regular rules apply even with `ExtensibleEnums` feature enabled.  
+
+  switch f { // Ok (no errors because `F` is `@frozen`)
+  case .a: break
+  case .b: break
+  }
+
+  switch f { // expected-error {{switch must be exhaustive}} expected-note {{dd missing case: '.b'}}
+  case .a: break
+  }
+  
+  switch f { // expected-warning {{switch must be exhaustive}} expected-note {{dd missing case: '.b'}}
+  case .a: break
+  @unknown default: break
+  }
+}
+
+//--- TestSamePackage.swift
+import Lib
+
+func test_no_default(e: E, f: F) {
+  switch e { // Ok
+  case .a: break
+  }
+
+  switch e { // expected-warning {{switch must be exhaustive}} expected-note {{dd missing case: '.a'}}
+  @unknown default: break
+  }
+
+  switch f { // expected-error {{switch must be exhaustive}} expected-note {{dd missing case: '.b'}}
+  case .a: break
+  }
+
+  switch f { // expected-warning {{switch must be exhaustive}} expected-note {{dd missing case: '.b'}}
+  case .a: break
+  @unknown default: break
+  }
+}