If the query below was passed by stdin, the Parser (v3.0.2) reacted with an error message like ..."/graphql/June2018/#Document".

        query { 
           person(name: "Joe") { ...personFields } 
        }

        fragment personFields on Person { id  }

Note the blank line before the fragment definition. I found this while testing a main loop more or less copied from the today example. Therein, the statement

        while (std::getline(std::cin, line)) {input.append(line);}

effectively strips newline characters.

When feeding the query via stdin, the query resulted in an error -- but not when the query was passed as a filename to be read via peg::parseFile(argv[1]) directly. Yet the real problem seems to be hidden in the parser. The follwing input (with angle brackets denoting the exact content )

      <query{person(name:"Joe"){...personFields}}fragment personFields on Person {id}>

fails with boths variants (stdin, passing file name: premature EOF), whereas inserting a space between "} and "fragment" is sufficient to get the input accepted.

Since the grammar looks fine this behaviour may also be caused by a compiler bug. I used gcc-9.1. gcc-9.2 did not succeed because of some weired problems within a cppgraphqlgen header.