RFR: 8258897: wrong translation of capturing local classes inside nested lambdas

[Jia Yanwei]

On Fri, 27 Nov 2020 12:32:21 GMT, Jia Yanwei <github.com+3896345+hltj at openjdk.org> wrote:

> **Issue:**
> `javac` crashes with a NPE when compiling such code:
> - a local class captured a reference type local variable of a enclosed lambda (which contains multiple local variables with different types)
> - instance the local class in a nested lambda
> 
> Affects all versions from Java 8 to 16-ea, here is a example output with OpenJDK 15.0.1:
> 
> An exception has occurred in the compiler (15.0.1). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
> java.lang.NullPointerException: Cannot read field "sym" because "this.lvar[1]" is null
>     at jdk.compiler/com.sun.tools.javac.jvm.Code.emitop0(Code.java:571)
>     at jdk.compiler/com.sun.tools.javac.jvm.Items$LocalItem.load(Items.java:399)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genArgs(Gen.java:889)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitNewClass(Gen.java:1942)
>     at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1800)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genExpr(Gen.java:864)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitExec(Gen.java:1723)
>     at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1532)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:632)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:618)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStats(Gen.java:669)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitBlock(Gen.java:1084)
>     at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1047)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:632)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genMethod(Gen.java:954)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitMethodDef(Gen.java:917)
>     at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:893)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
>     at jdk.compiler/com.sun.tools.javac.jvm.Gen.genClass(Gen.java:2395)
>     at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.genCode(JavaCompiler.java:756)
>     at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1644)
>     at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1612)
>     at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:973)
>     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:59)
>     at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:45)
> printing javac parameters to: /tmp/javac.20201126_141249.args
> 
> The minimal reproducing code is:
> 
>  java
> class Main {
>     static Runnable runnable = () -> {
>         boolean b0 = false;
>         String s0 = "hello";
> 
>         class Local {
>             int i = s0.length();
>         }
> 
>         Runnable dummy = () -> new Local();
>     };
> }
> 
> If the captured variable is a primitive type, the code will compile but cause a runtime crash at startup. e.g.:
> 
>  java
> public class CaptureInt {
>     static Runnable runnable = () -> {
>         boolean b0 = false;
>         int i0 = 5;
> 
>         class Local {
>             int i = i0 + 2;
>         }
> 
>         Runnable dummy = () -> new Local();
>     };
> 
>     public static void main(String args[]) {
>     }
> }
> 
> **Reason & Solution:**
> 
> During compilation, the captured variable was correctly synthetized as a synthetic argument in the synthetic methods for the nested lambda after `desugar()`. 
> But the synthetic argument was not used when loading free variables, and it always use the original symbol for the original variable as if not in a nested lambda.
> My experimental solution is substituting the symbols in free variables that were captured as synthetic arguments in nested lambda.
> Welcome any advice and better solution.

I had signed OCA, but named as `jyw`, the email and full name are identical.

-------------

PR: https://git.openjdk.java.net/jdk/pull/1479