[cxx-interop] Restrict the uses of anonymous types

lib/ClangImporter/ImportDecl.cpp

@@ -4546,6 +4546,10 @@ namespace {
         // FIXME: Temporarily unreachable because of check above.
         markAsVariant(result, *correctSwiftName);
 
+      if (decl->isAnonymousStructOrUnion())
+        Impl.markUnavailable(
+            result, "refer to the members of the anonymous type instead");
+
       return result;
     }
 

lib/IRGen/StructLayout.cpp

@@ -589,10 +589,17 @@ unsigned irgen::getNumFields(const NominalTypeDecl *target) {
 }
 
 bool irgen::isExportableField(Field field) {
-  if (field.getKind() == Field::Kind::Var &&
-      field.getVarDecl()->getClangDecl() &&
-      field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
-    return false;
+  if (field.getKind() == Field::Kind::Var) {
+    if (field.getVarDecl()->getClangDecl() &&
+        field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
+      return false;
+    // We should not be able to refer to anonymous types.
+    if (const auto *vd = dyn_cast_or_null<clang::ValueDecl>(
+            field.getVarDecl()->getClangDecl()))
+      if (const auto *rd = vd->getType()->getAsRecordDecl())
+        if (rd->isAnonymousStructOrUnion())
+          return false;
+  }
   // All other fields are exportable
   return true;
 }

test/Interop/Cxx/class/Inputs/simple-structs.h

@@ -17,6 +17,15 @@ struct HasPublicFieldsOnly {
   HasPublicFieldsOnly(int i1, int i2) : publ1(i1), publ2(i2) {}
 };
 
+struct HasAnonymousType {
+  HasAnonymousType(int a, int b, int c) : a(a), b(b), c(c) {}
+
+  struct {
+    int a, b;
+  };
+  int c;
+};
+
 struct HasPrivatePublicProtectedFields {
 private:
   int priv1;

test/Interop/Cxx/class/access-anonymous-field.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -swift-version 6 -cxx-interoperability-mode=upcoming-swift
+
+// CHECK: Foobar
+
+import SimpleStructs
+
+let s = HasAnonymousType(1, 2, 3)
+let _ = s.__Anonymous_field0 // expected-error {{'__Anonymous_field0' is unavailable: refer to the members of the anonymous type instead}}
+// Referring to the members of the anonymous type directly.
+let _ = s.a
+let _ = s.b
+let _ = s.c

test/Interop/Cxx/class/print-simple-structs.swift

@@ -24,6 +24,11 @@ func printCxxStructNested() {
     print(s)
 }
 
+func printCxxStructWithAnonType() {
+    let s = HasAnonymousType(1, 2, 3)
+    print(s)
+}
+
 printCxxStructPrivateFields() 
 // CHECK: HasPrivateFieldsOnly()
 
@@ -35,3 +40,6 @@ printCxxStructPrivatePublicProtectedFields()
 
 printCxxStructNested()
 // CHECK: Outer(publStruct: {{.*}}.HasPrivatePublicProtectedFields(publ1: 8, publ2: 12))
+
+printCxxStructWithAnonType()
+// CHECK: HasAnonymousType(c: 3)