Bug in com.sun.tools.javac.tree.Pretty

[Jonathan Gibbons]

Matthew,

Thank you. This is being tracked as 8024415.  It should show up soon on 
bugs.sun.com.

-- Jon

On 09/05/2013 11:10 PM, Matthew Dempsky wrote:
> Hi Java compiler developers,
>
> I noticed a bug in javac's JCTree pretty printer: it mishandles the 
> precedence rules when pretty printing JCConditional trees.  The 
> "condition" subexpression should be pretty printed at precedence 
> condPrec+1 instead of condPrec.
>
> I've put together a short test program demonstrating this bug, 
> included below.  It constructs and pretty prints two different nested 
> conditional trees, but they generate identical output.
>
> Additionally, but less severely, the "true" subexpression can be 
> printed at noPrec without introducing any ambiguity; e.g., "x ? y = 1 
> : z" will parse the same as "x ? (y = 1) : z" anyway, so it's 
> unnecessary (but also not incorrect) for Pretty to print the latter.
>
> I believe (but have not verified) that these can both be fixed by 
> changing Pretty.visitConditional() to:
>
>         printExpr(tree.cond, TreeInfo.condPrec + 1);
>             print(" ? ");
>         printExpr(tree.truepart);
>         print(" : ");
>         printExpr(tree.falsepart, TreeInfo.condPrec);
>
> I discovered this bug in what I believe to be jdk7u40, but it looks 
> like it's still present in jdk8 langtools at tip: 
> http://hg.openjdk.java.net/jdk8/jdk8/langtools/file/tip/src/share/classes/com/sun/tools/javac/tree/Pretty.java
>
> Thanks
>
>
> Test program:
>
> package prettybug;
>
> import com.sun.tools.javac.file.JavacFileManager;
> import com.sun.tools.javac.tree.Pretty;
> import com.sun.tools.javac.tree.TreeMaker;
> import com.sun.tools.javac.util.Context;
>
> import java.io.PrintWriter;
>
> public class Bug {
>   public static void main(String[] args) throws Exception {
>     Context ctx = new Context();
> JavacFileManager.preRegister(ctx);
>     TreeMaker maker = TreeMaker.instance(ctx);
>
>     PrintWriter writer = new PrintWriter(System.out);
>     Pretty pretty = new Pretty(writer, true);
>
> writer.println("These two trees should print differently:");
>
>     pretty.printExpr(
> maker.Conditional(
> maker.Conditional(
> maker.Literal(1),
> maker.Literal(2),
> maker.Literal(3)),
> maker.Literal(4),
> maker.Literal(5)));
>     writer.println();
>
>     pretty.printExpr(
> maker.Conditional(
> maker.Literal(1),
> maker.Literal(2),
> maker.Conditional(
> maker.Literal(3),
> maker.Literal(4),
> maker.Literal(5))));
>     writer.println();
>
>     writer.println();
>
> writer.println("This expression is unambiguous even without the 
> parentheses:");
>
>     pretty.printExpr(
> maker.Conditional(
> maker.Literal(1),
>         maker.Assign(
> maker.Literal(2),
> maker.Literal(3)),
> maker.Literal(4)));
>     writer.println();
>
>     writer.flush();
>   }
> }

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