https://issues.dlang.org/show_bug.cgi?id=19028

          Issue ID: 19028
           Summary: Emit warning for identifiers starting with `__`
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: regnellday@protonmail.com

Identifiers starting with `__` are reserved for compiler and their behaviour is implementation-defined. But compilers do not show warning or error message when you try to use such names. Furthermore, some functions ignore these identifiers and may produce hidden bugs in a code. For example:

void main()
{
    writeln([__traits(allMembers, S)]); // will print only ["a"]
}

struct S
{
    int a;
    int __refCount;
}

Documentation of `allMembers` says nothing about this behaviour.

Links:
https://dlang.org/spec/lex.html#identifiers
https://dlang.org/spec/traits.html#allMembers
https://github.com/dlang/dmd/blob/2ce74fc1180a3b0ed5e5c29aa79bf3773af5d5b7/src/dmd/traits.d#L1385
https://forum.dlang.org/thread/yauflbmphapnnksckksu@forum.dlang.org

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