[Sema] Remove @unknown default requirement for package enums.

include/swift/AST/Decl.h

@@ -4726,6 +4726,15 @@ class EnumDecl final : public NominalTypeDecl {
   /// \sa isEffectivelyExhaustive
   bool isFormallyExhaustive(const DeclContext *useDC) const;
 
+  /// True if \s isFormallyExhaustive is true or the use site's module belongs
+  /// to the same package as this enum's defining module. If in same package
+  /// even though `isFormallyExhaustive` is false, we can skip requiring
+  /// `@unknown default` at the use site switch stmts because package modules
+  /// are expected to be built together whether they are resiliently built or
+  /// not. Used for diagnostics during typechecks only; if
+  /// `isFormallyExhaustive` is false, it should be reflected in SILgen.
+  bool treatAsExhaustiveForDiags(const DeclContext *useDC) const;
+
   /// True if the enum can be exhaustively switched within a function defined
   /// within \p M, with \p expansion specifying whether the function is
   /// inlinable.

lib/AST/Decl.cpp

@@ -6559,6 +6559,11 @@ bool EnumDecl::hasOnlyCasesWithoutAssociatedValues() const {
   return !hasAssociatedValues;
 }
 
+bool EnumDecl::treatAsExhaustiveForDiags(const DeclContext *useDC) const {
+  return isFormallyExhaustive(useDC) ||
+         (useDC && getModuleContext()->inSamePackage(useDC->getParentModule()));
+}
+
 bool EnumDecl::isFormallyExhaustive(const DeclContext *useDC) const {
   // Enums explicitly marked frozen are exhaustive.
   if (getAttrs().hasAttribute<FrozenAttr>())
@@ -6576,14 +6581,10 @@ bool EnumDecl::isFormallyExhaustive(const DeclContext *useDC) const {
     return true;
 
   // Non-public, non-versioned enums are always exhaustive.
-  AccessScope accessScope = getFormalAccessScope(/*useDC*/nullptr,
-                                                 /*respectVersioned*/true);
-  // Both public and package enums should behave the same unless
-  // package enum is optimized with bypassing resilience checks.
+  AccessScope accessScope = getFormalAccessScope(/*useDC*/ nullptr,
+                                                 /*respectVersioned*/ true);
   if (!accessScope.isPublicOrPackage())
     return true;
-  if (useDC && bypassResilienceInPackage(useDC->getParentModule()))
-    return true;
 
   // All other checks are use-site specific; with no further information, the
   // enum must be treated non-exhaustively.
@@ -6615,11 +6616,13 @@ bool EnumDecl::isEffectivelyExhaustive(ModuleDecl *M,
   if (isObjC())
     return false;
 
-  // Otherwise, the only non-exhaustive cases are those that don't have a fixed
-  // layout.
-  assert(isFormallyExhaustive(M) == !isResilient(M,ResilienceExpansion::Maximal)
-         && "ignoring the effects of @inlinable, @testable, and @objc, "
-            "these should match up");
+  // Otherwise, the only non-exhaustive enums are those that don't have
+  // a fixed layout; however, they are treated as exhaustive if package
+  // optimization is enabled.
+  assert((isFormallyExhaustive(M) || bypassResilienceInPackage(M)) ==
+             !isResilient(M, ResilienceExpansion::Maximal) &&
+         "ignoring the effects of @inlinable, @testable, and @objc, "
+         "these should match up");
   return !isResilient(M, expansion);
 }
 

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -861,7 +861,7 @@ namespace {
                                              constElemSpaces);
               });
 
-          if (!E->isFormallyExhaustive(DC)) {
+          if (!E->treatAsExhaustiveForDiags(DC)) {
             arr.push_back(Space::forUnknown(/*allowedButNotRequired*/false));
           } else if (!E->getAttrs().hasAttribute<FrozenAttr>()) {
             arr.push_back(Space::forUnknown(/*allowedButNotRequired*/true));