[antlr-interest] [C Target][3.1.3] Parser crashes when looking up a scoped attribute outside of a scope context.

[register]

I noticed that the parser crashes whenever a scoped attribute is referenced
outside of a scope context.
This kind of situation arises when a rule B that is referencing a scoped
attribute defined in a rule A may match on its own without requiring a
previous match to A.

In my specific case the crash occurs because the following code is
generated:
scfSetAttrAsIntRef(curAttr,intRef, (SCOPE_TOP(record))->curRecord);.

Since SCOPE_TOP(record) is null I get an access violation.

One can obviously avoid this problem defining rules that are able to
discriminate between the outside-scope and in-scope invocation of B. However
I see a lot of redundancy in this approach so I propose to modify the
generated code in the following way:

scfSetAttrAsIntRef(curAttr,intRef, (SCOPE_TOP(record))==0 ? 0 :
(SCOPE_TOP(record))->curRecord);

Doing so one has two benefits:

- The code doesn't crash.

- Discrimination between the in-scope and outside-scope invocation can be
detected and handled inside actions.

Greets,

Luca
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