Using javac versions 23 and 24-ea it is possible to compile the following program without passing `--enable-preview`:

public class Test {

    public static void main(String[] args) {
        new Test().d(true);
    }

    void d(Boolean b) {
        switch (b) {
            case true -> System.out.println("1");
            case false -> System.out.println("2");
        };
    }
}

When trying to run this program the following exception is thrown:

    Exception in thread "main" java.lang.BootstrapMethodError: bootstrap method initialization exception
         at java.base/java.lang.invoke.BootstrapMethodInvoker.invoke(BootstrapMethodInvoker.java:187)
         at java.base/java.lang.invoke.CallSite.makeSite(CallSite.java:316)
         at java.base/java.lang.invoke.MethodHandleNatives.linkCallSiteImpl(MethodHandleNatives.java:275)
         at java.base/java.lang.invoke.MethodHandleNatives.linkCallSite(MethodHandleNatives.java:265)
         at Test.d(Test.java:8)
         at Test.main(Test.java:4)
    Caused by: java.lang.IllegalArgumentException: label with illegal type found:
    class java.lang.Boolean
         at java.base/java.lang.runtime.SwitchBootstraps.verifyLabel(SwitchBootstraps.java:214)
         at java.base/java.lang.runtime.SwitchBootstraps.typeSwitch(SwitchBootstraps.java:188)
         at java.base/java.lang.invoke.BootstrapMethodInvoker.invoke(BootstrapMethodInvoker.java:143)
         ... 5 more

Only when running with --enable-preview the program executes as expected.

This confirms that the compiler should have checked the preview flag, so that execution without --enable-preview would signal the following, rather than the BootstrapMethodError:

    Error: LinkageError occurred while loading main class Test
         java.lang.UnsupportedClassVersionError: Preview features are not enabled for Test (class file version 67.65535). Try running with '--enable-preview'

The same observation also holds for other pairs of types like Long + long.

Mailing list thread: https://mail.openjdk.org/pipermail/compiler-dev/2025-January/029255.html