ADDITIONAL SYSTEM INFORMATION :
wsl2-ubuntu 22.04
Linux 5.15.167.4-microsoft-standard-WSL2 #1 SMP Tue Nov 5 00:21:55 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

javac 21.0.8



A DESCRIPTION OF THE PROBLEM :
javac incorrectly allows assignment of a generic method's return value — whose type is a reference type constrained by U extends IntegralConstraint<U> — to a primitive int, even though no valid unboxing or conversion path exists.

The Eclipse Compiler for Java (ECJ) correctly rejects this code with a compilation error, while javac accepts it with only an unchecked cast warning.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compling following code using javac in JDK21.
```
javac Test.java
```

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The compliation should fail and report such error message:
```
1. ERROR in Test.java (at line 5)
        int result = checker.checkIntegral(42);
                     ^^^^^^^^^^^^^^^^^^^^^^^^^
Type mismatch: cannot convert from IntegralConstraint<IntegralConstraint<U>> to int
```
ACTUAL -
The compliation is successful with 1 warnings.
```
Test.java:32: warning: [unchecked] unchecked cast
        return (U) new MyInteger(value);
                   ^
  required: U
  found: MyInteger
  where U is a type-variable:
    U extends IntegralConstraint<U> declared in method <U>checkIntegral(int)
1 warning
```

---------- BEGIN SOURCE ----------
public class Test {

    public static void main(String[] args) {
        IntegralChecker<MyInteger> checker = new IntegralChecker<>();
        int result = checker.checkIntegral(42);
        System.out.println(result);
    }
}

interface IntegralConstraint<T> {

    boolean isIntegral();
}

class MyInteger implements IntegralConstraint<MyInteger> {

    private final int value;

    public MyInteger(int value) {
        this.value = value;
    }

    @Override
    public boolean isIntegral() {
        return true;
    }
}


class IntegralChecker<T extends IntegralConstraint<T>> {
    public <U extends IntegralConstraint<U>> U checkIntegral(int value) {
        return (U) new MyInteger(value);
    }
}
---------- END SOURCE ----------