A DESCRIPTION OF THE PROBLEM :
An exception occurs in the compiler when it encounters a nested interface that extends its enclosing interface, and the latter is local.

Affected javac versions: 20.0.2, 21.0.1, 23.0.2

Compiles OK with javac versions: 19.0.2, 17.0.8

Compiling the attached source with javac 20.0.2 fails with:

java.lang.NullPointerException: Cannot invoke "com.sun.tools.javac.util.List.nonEmpty()" because "l" is null
        at jdk.compiler/com.sun.tools.javac.comp.Lower.freevarDefs(Lower.java:1467)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.freevarDefs(Lower.java:1459)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2216)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:810)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2098)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2240)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:810)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2098)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:70)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.visitBlock(TreeTranslator.java:167)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitBlock(Lower.java:3609)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1088)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2098)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.visitMethodDef(TreeTranslator.java:150)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitMethodDefInternal(Lower.java:2775)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitMethodDef(Lower.java:2691)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:912)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2098)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2240)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:810)
        at jdk.compiler/com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2098)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translate(Lower.java:2117)
        at jdk.compiler/com.sun.tools.javac.comp.Lower.translateTopLevelClass(Lower.java:4210)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1625)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1440)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:960)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:317)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
        at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:64)
        at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:50)

Equivalent stack traces are generated for other affected versions.


REGRESSION : Last worked in version 19

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Define a local interface B.
2. Define interface/class/enum/record C nested in B and extending/implementing B.
3. Compile.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The program compiles successfully.
ACTUAL -
Compilation fails.

---------- BEGIN SOURCE ----------
public class A {

    void m() {
        interface B {
            // Compilation error
            interface C extends B {}

            // OK
            // class C implements B {}

            // OK
            // interface C {}
        }

        // OK
        // interface C extends B {}
    }

    // OK
    interface B {
        interface C extends B {}
    }

}
---------- END SOURCE ----------