A DESCRIPTION OF THE PROBLEM :
The java.lang.invoke.ConstantBootstraps.explicitCast method does not behave as documented when converting a null value to a primitive type. Instead of throwing a ClassCastException as its specification mandates, it silently returns the primitive's default zero value (e.g., 0 for int).

The core of this issue lies in a conflict between the high-level contract of explicitCast and the behavior of the lower-level mechanism it uses for its implementation.

The explicitCast Contract
The Javadoc for java.lang.invoke.ConstantBootstraps.explicitCast provides a clear specification for this case. It states:
"If dstType is a primitive type, then, if the runtime type of value is a primitive wrapper type (such as Integer), a Java unboxing conversion is applied ... If the runtime type of value is not a primitive wrapper type a ClassCastException is thrown."

The input value in the test case is null. The runtime type of null is not a primitive wrapper type. Therefore, according to this contract, a ClassCastException is required.

The Underlying Implementation's Behavior

The observed behavior (returning 0) stems from explicitCast delegating the conversion to java.lang.invoke.MethodHandles.explicitCastArguments. The documentation for MethodHandles.explicitCastArguments describes a different handling for null:

"If T0 is a reference and T1 a primitive, and if the reference is null at runtime, a zero value is introduced."

This perfectly explains why 0 is returned. The explicitCast method's implementation relies on explicitCastArguments, which has a specific, documented rule to convert a null reference to a primitive's default value. The problem is that explicitCast uses this mechanism without adding the check required to fulfill its own, more stringent, documented contract.

Maybe clarify JavaDoc is helpful


---------- BEGIN SOURCE ----------
import java.lang.invoke.ConstantBootstraps;

public class ExplicitCastBug {
    public static void main(String[] args) {
        System.out.println("Attempting to cast null to int via ConstantBootstraps.explicitCast...");
        try {
            // This call should throw ClassCastException according to the documentation.
            Object result = ConstantBootstraps.explicitCast(null, null, int.class, null);
            
            // This line is reached, which is incorrect.
            System.out.println("BUG: The call succeeded and returned: " + result);
            System.out.println("Expected a ClassCastException, but none was thrown.");
        } catch (ClassCastException e) {
            // This block is never reached.
            System.out.println("SUCCESS: Correctly caught the expected ClassCastException.");
            e.printStackTrace();
        }
    }
}
---------- END SOURCE ----------