RFR: 8258897: wrong translation of capturing local classes inside nested lambdas [v5]

[Jia Yanwei]

> **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 & Experimental 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.
> I'm not sure whether the code style as well as the solution itself are appropriate. Welcome any advice and better solution.

Jia Yanwei has updated the pull request incrementally with one additional commit since the last revision:

  tweak the test comments

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1479/files
  - new: https://git.openjdk.java.net/jdk/pull/1479/files/810b909d..f142d14e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1479&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1479&range=03-04

  Stats: 12 lines in 2 files changed: 6 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1479.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1479/head:pull/1479

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