In general, TYPE_CANONICAL of a type strips all attributes.  An 
exception to this seems to be that TYPE_REF_CAN_ALIAS_ALL remains set on 
the TYPE_CANONICAL of a pointer/reference type even though its TREE_TYPE 
no longer has the may_alias attribute, and is inconsistent with 
"affects_type_identity" being false for may_alias.  This seems to have 
been a mistake in the patch that first introduced TYPE_CANONICAL, rather 
than a deliberate choice.

I'm also planning to fix 50800 in the front end, but this patch still 
seems like an improvement.

Tested x86-64-pc-linux-gnu, OK for trunk?