missing children in visitor pattern with cpp-runtime of ANTLR

[chakpongchung]

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• I have reproduced my issue using the latest version of ANTLR
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Please include the following information:

• target information: C++
• smallest possible grammar and code that reproduces the behavior

Please see the bug description here and the minimal code to reproduce the problem.
opencypher/openCypher#529

• description of the expected behavior and actual behavior

The expected behavior is that the parent node mentioned should have more than one child. The actual behavior has only one child.

Pointers to suspicious code regions are also very welcome.

The problem is only present in the visitor pattern code when using antlr4-cpp-runtime (4.9.3)