Some (but not all) ambiguity errors make Rascal Document Symbols crash

[sungshik]

Context

Consider the following two modules:

module Ambiguous1

str s = "<x<0>><x<1>>";

module Ambiguous2

tuple[str] f(tuple[str] tup) = <tup<0>> when _ := tup;

Both modules give rise to an ambiguity error in the REPL, as reported in usethesource/rascal#2122 and usethesource/rascal#2125. However, in the UI, VS Code reports no errors for module Ambiguous1, while it reports the following error for Ambiguous2 (each time the module is changed, regardless of the change, as long as the definition of f is kept):

Exception thrown

Undeclared field: signature for Tree
Advice: |https://www.rascal-mpl.org/docs/Rascal/Errors/CompileTimeErrors/UndeclaredField|

Stacktrace:

org.rascalmpl.interpreter.staticErrors.UndeclaredField: Undeclared field: signature for Tree
Advice: |https://www.rascal-mpl.org/docs/Rascal/Errors/CompileTimeErrors/UndeclaredField|
	at DocumentSymbols.?(/lang/rascal/lsp/DocumentSymbols.rsc:65)
	at org.rascalmpl.interpreter.result.ConstructorResult.fieldAccess(ConstructorResult.java:120)
	at org.rascalmpl.interpreter.result.ConcreteSyntaxResult.fieldAccess(ConcreteSyntaxResult.java:84)
	at org.rascalmpl.semantics.dynamic.Expression$FieldAccess.interpret(Expression.java:869)
	at org.rascalmpl.semantics.dynamic.Expression$FieldAccess.interpret(Expression.java:866)
	at org.rascalmpl.semantics.dynamic.Statement$Expression.interpret(Statement.java:365)
	at org.rascalmpl.interpreter.StringTemplateConverter$Visitor$IndentingAppend.interpret(StringTemplateConverter.java:108)
	at org.rascalmpl.semantics.dynamic.Statement$NonEmptyBlock.interpret(Statement.java:759)
	at org.rascalmpl.semantics.dynamic.Statement$For.interpret(Statement.java:468)
	at org.rascalmpl.semantics.dynamic.Literal$String.interpret(Literal.java:237)
	at org.rascalmpl.semantics.dynamic.Expression$Literal.interpret(Expression.java:1596)
	at org.rascalmpl.semantics.dynamic.Expression$CallOrTree.interpret(Expression.java:502)
	at org.rascalmpl.semantics.dynamic.Expression$List.interpret(Expression.java:1521)
	at org.rascalmpl.semantics.dynamic.Statement$Expression.interpret(Statement.java:365)
	at org.rascalmpl.semantics.dynamic.Statement$Assignment.interpret(Statement.java:205)
	at org.rascalmpl.interpreter.utils.Cases.matchAndEval(Cases.java:443)
	at org.rascalmpl.interpreter.utils.Cases$DefaultBlock.matchAndEval(Cases.java:271)
	at org.rascalmpl.interpreter.TraversalEvaluator.applyCases(TraversalEvaluator.java:645)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseTop(TraversalEvaluator.java:669)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:140)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseListOnce(TraversalEvaluator.java:510)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:167)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseADTOnce(TraversalEvaluator.java:353)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:163)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseListOnce(TraversalEvaluator.java:510)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:167)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseADTOnce(TraversalEvaluator.java:353)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:163)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseListOnce(TraversalEvaluator.java:510)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:167)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseADTOnce(TraversalEvaluator.java:353)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:163)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseListOnce(TraversalEvaluator.java:510)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:167)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseADTOnce(TraversalEvaluator.java:353)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:163)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseListOnce(TraversalEvaluator.java:510)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:167)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseADTOnce(TraversalEvaluator.java:353)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverseOnce(TraversalEvaluator.java:163)
	at org.rascalmpl.interpreter.TraversalEvaluator.traverse(TraversalEvaluator.java:122)
	at org.rascalmpl.semantics.dynamic.Visit$GivenStrategy.interpret(Visit.java:132)
	at org.rascalmpl.semantics.dynamic.Statement$Visit.interpret(Statement.java:1022)
	at org.rascalmpl.interpreter.result.RascalFunction.runBody(RascalFunction.java:384)
	at org.rascalmpl.interpreter.result.RascalFunction.call(RascalFunction.java:325)
	at org.rascalmpl.interpreter.result.AbstractFunction.call(AbstractFunction.java:198)
	at org.rascalmpl.interpreter.Evaluator.call(Evaluator.java:769)
	at org.rascalmpl.interpreter.Evaluator.call(Evaluator.java:752)
	at org.rascalmpl.interpreter.Evaluator.call(Evaluator.java:558)
	at org.rascalmpl.vscode.lsp.rascal.RascalLanguageServices.lambda$getDocumentSymbols$10(RascalLanguageServices.java:203)
	at org.rascalmpl.vscode.lsp.util.EvaluatorUtil.lambda$runEvaluator$0(EvaluatorUtil.java:96)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:642)
	at java.base/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:829)

[jurgenvinju]

The second example makes the top-level function decapitation ambiguous. So the amb node is where the pattern for function declaration matches. Next thing the signature field is projected which doesn't exist on the amb node.

The first example has the ambiguity nested deeper, where it can do less harm.

Best solution would be the start the visit with a generic amb node case: case amb(set[Tree] alts) and either ignore it altogether, or pick one alternative to continue, or put an ambiguous DocumentSymbol in the hierarchie.

[jurgenvinju]

For consistency with the behavior of error trees, we could make sure that errors caused by ambiguity nodes are wrapped in a general "Ambiguous" exception with the current exception as consequence (not cause). That way tools can catch them to make them robust against ambiguity in the same way we can make them robust against error recovery. @PieterOlivier wdyt?