Q_EXPORT_PLUGIN2 doesn't give a helpful error message

Using Q_EXPORT_PLUGIN2 gives the error
"error: invalid application of 'sizeof' to incomplete type 'QStaticAssertFailure<false>'"

When it should be saying something like "error: Using old plugin system". qplugin.h appears to have the right code for this:

#  define Q_EXPORT_PLUGIN2(PLUGIN, PLUGINCLASS)      \
    Q_STATIC_ASSERT_X(false, "Old plugin system used")

but Q_STATIC_ASSERT_X doesn't seem to be working right for me.