Explicitly reject fields that are closed enums with implicit presence…

… in Java generators.

The normal case of this is already rejected by the .proto parser, this additional check covers legacy_closed_enum cases. Those cases will otherwise reach a CHECK-fail, this just makes for a clearer error message that this is simply a disallowed combination.

PiperOrigin-RevId: 696648744

src/google/protobuf/compiler/java/file.cc

@@ -240,6 +240,17 @@ bool FileGenerator::Validate(std::string* error) {
         << "name for the .proto file to be safe.";
   }
 
+  // Check that no field is a closed enum with implicit presence. For normal
+  // cases this will be rejected by protoc before the generator is invoked, but
+  // for cases like legacy_closed_enum it may reach the generator.
+  google::protobuf::internal::VisitDescriptors(*file_, [&](const FieldDescriptor& field) {
+    if (field.enum_type() != nullptr && !SupportUnknownEnumValue(&field) &&
+        !field.has_presence() && !field.is_repeated()) {
+      absl::StrAppend(error, "Field ", field.full_name(),
+                      " has a closed enum type with implicit presence.\n");
+    }
+  });
+
   // Print a warning if optimize_for = LITE_RUNTIME is used.
   if (file_->options().optimize_for() == FileOptions::LITE_RUNTIME &&
       !options_.enforce_lite) {

src/google/protobuf/compiler/java/generator_unittest.cc

@@ -65,6 +65,28 @@ TEST_F(JavaGeneratorTest, BasicError) {
       "foo.proto:4:7: Expected \"required\", \"optional\", or \"repeated\"");
 }
 
+TEST_F(JavaGeneratorTest, ImplicitPresenceLegacyClosedEnumDisallowed) {
+  CreateTempFile("foo.proto",
+                 R"schema(
+    edition = "2023";
+    import "third_party/java/protobuf/java_features.proto";
+    option features.field_presence = IMPLICIT;
+    enum Bar {
+      AAA = 0;
+    }
+    message Foo {
+      Bar bar = 1 [features.(pb.java).legacy_closed_enum = true];
+    }
+  )schema");
+
+  RunProtoc(
+      "protocol_compiler --proto_path=$tmpdir --java_out=$tmpdir foo.proto");
+
+  ExpectErrorSubstring(
+      "foo.proto: Field Foo.bar has a closed enum type with implicit "
+      "presence.");
+}
+
 
 }  // namespace
 }  // namespace java