Reported here:
https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-June/078695.html

Code like this:
    Object o = null;
    var value = switch(o) {
          default -> 0;
          case String s -> 0;
    };
    System.out.println(value);

Will crash javac with:
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.check(Assert.java:46)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.handleSwitch(Gen.java:1310)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.doHandleSwitchExpression(Gen.java:1238)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitSwitchExpression(Gen.java:1202)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCSwitchExpression.accept(JCTree.java:1381)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genExpr(Gen.java:877)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitLetExpr(Gen.java:2376)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$LetExpr.accept(JCTree.java:3288)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genExpr(Gen.java:877)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitVarDef(Gen.java:1081)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:1028)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:610)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:645)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:631)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStats(Gen.java:682)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitBlock(Gen.java:1097)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1092)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:610)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:645)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genMethod(Gen.java:967)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitMethodDef(Gen.java:930)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:922)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:610)
at jdk.compiler/com.sun.tools.javac.jvm.Gen.genClass(Gen.java:2415)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.genCode(JavaCompiler.java:737)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1617)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1585)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:946)
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)

Per the current specification, the default case can be at any position, and javac should compile the code OK.