Ambiguity when combining tuple projection and when clause

[sungshik]

Describe the bug

An ambiguity may arise when tuple projection is combined with (pattern matching in) a when clause.

To Reproduce

First (no problem yet), the following function consumes a unary tuple and produces the same unary tuple in a contrived way:

rascal>tuple[str] f1(tuple[str] tup) = <tup<0>>;
tuple[str] (tuple[str]): function(|prompt:///|(0,41,<1,0>,<1,41>))

rascal>f1(<"foo">)
tuple[str]: <"foo">

Next (no problem still), the following function is the same as f1, except it has a superfluous when clause:

rascal>tuple[str] f2(tuple[str] tup) = <tup<0>> when <_> := tup;
tuple[str] (tuple[str]): function(|prompt:///|(0,57,<1,0>,<1,57>))

rascal>f2(<"foo">)
tuple[str]: <"foo">

Last (problem), the following function is the same as f2, except the when clause is more liberal (but equally superfluous):

rascal>tuple[str] f3(tuple[str] tup) = <tup<0>> when _ := tup;
Unexpected failure during parsing of current command: 
Ambiguous code (internal error), tuple[str] f3(tuple[str] tup) = <tup<0>> when _ := tup;

Expected behavior

No ambiguity error

Stack traces

org.rascalmpl.interpreter.asserts.Ambiguous: Ambiguous code (internal error), tuple[str] f3(tuple[str] tup) = <tup<0>> when _ := tup;
        at org.rascalmpl.parser.uptr.UPTRNodeFactory.buildAmbiguityNode(UPTRNodeFactory.java:76)
        at org.rascalmpl.parser.uptr.UPTRNodeFactory.createAmbiguityNode(UPTRNodeFactory.java:85)
        at org.rascalmpl.parser.uptr.UPTRNodeFactory.createAmbiguityNode(UPTRNodeFactory.java:23)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:183)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.buildAlternative(SortContainerNodeFlattener.java:90)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:65)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherAlternatives(SortContainerNodeFlattener.java:53)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:166)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.buildAlternative(SortContainerNodeFlattener.java:90)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:65)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherAlternatives(SortContainerNodeFlattener.java:53)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:166)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.buildAlternative(SortContainerNodeFlattener.java:90)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:65)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:71)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:71)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherAlternatives(SortContainerNodeFlattener.java:53)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:166)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:79)
        at org.rascalmpl.parser.gtd.SGTDBF.buildResult(SGTDBF.java:1306)
        at org.rascalmpl.parser.gtd.SGTDBF.parse(SGTDBF.java:1245)
        at org.rascalmpl.parser.gtd.SGTDBF.parse(SGTDBF.java:1249)
        at org.rascalmpl.parser.gtd.SGTDBF.parse(SGTDBF.java:1261)
        at org.rascalmpl.interpreter.Evaluator.parseCommand(Evaluator.java:1052)
        at org.rascalmpl.interpreter.Evaluator.parseCommand(Evaluator.java:1042)
        at org.rascalmpl.repl.RascalInterpreterREPL.isStatementComplete(RascalInterpreterREPL.java:176)
        at org.rascalmpl.repl.BaseRascalREPL.handleInput(BaseRascalREPL.java:105)
        at org.rascalmpl.vscode.lsp.terminal.LSPTerminalREPL$1.handleInput(LSPTerminalREPL.java:233)
        at org.rascalmpl.repl.BaseREPL.handleInput(BaseREPL.java:179)
        at org.rascalmpl.repl.BaseREPL.run(BaseREPL.java:346)
        at org.rascalmpl.vscode.lsp.terminal.LSPTerminalREPL.main(LSPTerminalREPL.java:320)