Ambiguity between syntax constructor sort and abstract data type sort in visit typecheck _not reported_

[jurgenvinju]

Describe the bug

import ParseTree;
import lang::rascal::\syntax::Rascal;

private Symbol removeConditionals(Symbol sym) = visit(sym) {
  case conditional(s, _) => s
};

This generates:

[INFO] Warnings and errors for lang/rascal/grammar/ConcreteSyntax.rsc
[ERROR] lang/rascal/grammar/ConcreteSyntax.rsc:068:007: A pattern of type Replacement cannot be replaced by Expression

The cause is:

syntax Replacement
	= unconditional: Expression replacementExpression 
	| conditional: Expression replacementExpression "when" {Expression ","}+ conditions ;

and

data Symbol // <19>
     = \conditional(Symbol symbol, set[Condition] conditions);

Both conditional constructors have two arguments, so they might indeed match the pattern. In this case a warning or error regarding the ambiguous constructor is expected:

Ambiguous constructor conditional could be either X or Y

But the type-checker currently forgets accidentally about the Symbol version.

I'm not sure if the visit is essential in this example. I think it does require an ambiguous constructor on the pattern side. I'm fixing the code in the standard library to avoid this issue for now.