Changing which line is commented...

The analogous case is that clang accepts

template struct nsRunnableMethodTraits;

template struct nsRunnableMethodTraits<void (C::)()> {
typedef int base_type;
};
template struct nsRunnableMethodTraits<void attribute ((regparm (0))) (C::)()> {
typedef int base_type;
};

and g++ rejects it:

nsUrlClassifierProxies.ii:7:27: error: redefinition of ‘struct nsRunnableMethodTraits<void (C::)()>’
nsUrlClassifierProxies.ii:3:27: error: previous definition of ‘struct nsRunnableMethodTraits<void (C::)()>’

Clang is behaving correctly here. A regparm-attributed member function pointer type is distinct from a non-regparm-attributed member function pointer type, so they'll have different template specializations. GCC's behavior appears to be inconsistent here, because the types are considered equivalent for the purposes of matching template arguments, so the regparm attribute associated with the first instantiation is the one that "sticks", even if subsequent instantiations don't have the regparm attribute.