Encountered while using jshell, javac swallows the exception because it already reported errors.

If a variable of an unknown type is referenced in a(n implicit) yield statement, the variable has type null.

For example, pasting

I m(I i, int x) {
   return switch (x) {
        default -> i;
   };
}

into jshell will crash the

The relevant part of the stacktrace:
Caused by: java.lang.AssertionError
        at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
        at jdk.compiler/com.sun.tools.javac.util.Assert.checkNonNull(Assert.java:62)
        at jdk.compiler/com.sun.tools.javac.util.ListBuffer.append(ListBuffer.java:127)
        at jdk.compiler/com.sun.tools.javac.comp.Attr$1.visitYield(Attr.java:1637)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCYield.accept(JCTree.java:1677)
        at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:50)
        at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:58)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.lambda$visitSwitchExpression$7(Attr.java:1644)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.handleSwitch(Attr.java:1843)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitSwitchExpression(Attr.java:1629)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCSwitchExpression.accept(JCTree.java:1399)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:662)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitReturn(Attr.java:2468)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCReturn.accept(JCTree.java:1728)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:662)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:736)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStats(Attr.java:755)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1439)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1092)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:662)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:736)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1229)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:916)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:662)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:736)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:5648)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClass(Attr.java:5536)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClass(Attr.java:5360)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attrib(Attr.java:5299)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1358)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1331)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.analyze(JavacTaskImpl.java:404)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.lambda$analyze$1(JavacTaskImpl.java:379)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.invocationHelper(JavacTaskImpl.java:152)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.analyze(JavacTaskImpl.java:379)
        ... 19 more