The standard seems to imply that there is no restriction on the number of definitions of a variable if it is not odr-used (§3.2/3):

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program; no diagnostic required.

It does say that any variable can't be defined multiple times within a translation unit (§3.2/1):

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.

But I can't find a restriction for non-odr-used variables across the entire program. So why can't I compile something like the following:

// other.cpp
int x;

// main.cpp
int x;
int main() {}

Compiling and linking these files with g++ 4.6.3, I get a linker error for multiple definition of 'x'. To be honest, I expect this, but since x is not odr-used anywhere (as far as I can tell), I can't see how the standard restricts this. Or is it undefined behaviour?