# compile on 27 with preview, then run the class file on 27 and on 28
$ java --enable-preview Server     (27.0.1.0.2+1)
Response[user=Alice, order=42]
Runtime: 27.0.1.0.2+1
$ java --enable-preview Server     (28-beta+21-ea)
Error: LinkageError occurred while loading main class Server
	java.lang.UnsupportedClassVersionError: Server (class file version 71.65535) was compiled with preview features that are unsupported. This version of the Java Runtime only recognizes preview features for class file version 72.65535
$ java Server     (27.0.1.0.2+1)
Error: LinkageError occurred while loading main class Server
	java.lang.UnsupportedClassVersionError: Preview features are not enabled for Server (class file version 71.65535). Try running with '--enable-preview'

# the class file's minor version
  minor version: 65535
  major version: 71

# javac 28 asked for --release 27
$ javac --release 27 src/Server.java     (28-beta+21-ea)
src/Server.java:2: error: StructuredTaskScope is a preview API and is disabled by default.
import java.util.concurrent.StructuredTaskScope;
                           ^
  (use --enable-preview to enable preview APIs)
src/Server.java:3: error: StructuredTaskScope is a preview API and is disabled by default.
import java.util.concurrent.StructuredTaskScope.Subtask;
                           ^
  (use --enable-preview to enable preview APIs)
src/Server.java:3: error: Subtask is a preview API and is disabled by default.
import java.util.concurrent.StructuredTaskScope.Subtask;
                                               ^
  (use --enable-preview to enable preview APIs)
src/Server.java:22: error: StructuredTaskScope is a preview API and is disabled by default.
        try (var scope = StructuredTaskScope.open()) {   // 1. open
                         ^
  (use --enable-preview to enable preview APIs)
src/Server.java:23: error: Subtask is a preview API and is disabled by default.
            Subtask<String>  user  = scope.fork(this::findUser);    // 2. fork
            ^
  (use --enable-preview to enable preview APIs)
src/Server.java:24: error: Subtask is a preview API and is disabled by default.
            Subtask<Integer> order = scope.fork(this::fetchOrder);  //    fork
            ^
  (use --enable-preview to enable preview APIs)
6 errors

# javac 28 with --enable-preview --release 28 on the same file (nothing in it needs preview)
$ javac --enable-preview --release 28 src/Server.java     (28-beta+21-ea)
Note: src/Server.java uses preview features of Java SE 28.
Note: Recompile with -Xlint:preview for details.
exit=0

# where that Note comes from: the first lines of -Xlint:preview
$ javac --enable-preview --release 28 -Xlint:preview src/Server.java 2>&1 | head -3     (28-beta+21-ea)
warning: [preview] class file for /@/java.base/java/lang/Record.sig uses preview features of Java SE 28.
warning: [preview] class file for /@/java.base/java/time/Duration.sig uses preview features of Java SE 28.
warning: [preview] class file for /@/java.base/java/lang/Integer.sig uses preview features of Java SE 28.

# recompile on 28 without any flag and run on 28
$ javac src/Server.java && java Server     (28-beta+21-ea)
Response[user=Alice, order=42]
Runtime: 28-beta+21-ea
