Preview flag required at runtime is not checked at compile time

[Stephan Herrmann]

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.

best,
Stephan