[cxx-abi-dev] Adding consistency check for C++11 forward-declared enums?

[Kevin Fleming]

A discussion cropped up at C++Now today about the new forward declarations
of enumerations in C++11. Much like forward declarations of functions, a
forward declared enumeration consists of more than just a name; it also has
an underlying storage type. This provides an opportunity for the
enumeration's underlying type to be mismatch between a pair of TUs. If a
translation unit forward-declares the enumeration with a different
underlying type than the translation unit that defines the enumeration, any
functions in in the interface between those TUs will disagree on the amount
of data to be passed.

Even though there is no linker action require to 'resolve' forward-declared
enumerations, it seems like the existing name mangling mechanisms and
linker symbol resolution could be employed to provide a way for this
situation to be identified. If the enum-defining TU exported a symbol with
a suitably-mangled name of the enumeration, and the enum-consuming TU
attempted to import such a suitably-mangled name (even though none of the
object code in the consuming TU would ever reference the resolved symbol
address), the linker would be able to notify the developer of the
underlying type mismatch.

I see a long-tabled 'consistency checks' issue on the CXX-ABI pages that
seems to address similar issues, but I figured I'd at least broach the
subject to see if this is worth consideration.
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