moc assumes all headers should only be included once

moc ignores #include if the header file has already been processed, but that is non-conformant. In particular, this leads to parser errors when including certain protobuf-generated code.

This is a simplified *.pb.h header:

#include <google/protobuf/port_def.inc>
#include <google/protobuf/port_undef.inc>
#include <google/protobuf/port_def.inc>

namespace example {
enum ExampleEnum : int {
    Add = 0,
    Explode PROTOBUF_DEPRECATED_ENUM = 1,
    Erase = 2
}
}

"port_def.inc" has a set of #define and "port_undef.inc" has corresponding #undef. Because moc ignores the second include of "port_def.inc", macros including PROTOBUF_DEPRECATED_ENUM are never re-defined and moc fails when parsing this enum.

The attached example reproduces this with moc test.h

If I understand correctly, traditional #ifdef include guards would already work. I think the restriction on re-including could be removed if moc supported #pragma once, which would solve the protobuf case.

As a workaround, we are currently patching moc to ignore port_undef.inc specifically.