Allow pattern synonyms to optionally carry coerceability
Aside from their more interesting abilities, pattern synonyms can be used for name management. For example, if a module exports a type with a data constructor named
A, and also re-exports a type with a data constructor named
A, it can use a pattern synonym to rename the imported constructor. If the type is a
newtype, this pattern synonym will not carry coerceability. Sometimes, this is exactly what one would desire (an "unsafe" module breaking through a newtype abstraction may wish to allow importers to confine coerceability). Other times, it's not (pattern synonym just for name control). When defining a pattern synonym for a type in a module in which that type is coerceable, it would be nice to be able to tell GHC that the pattern synonym should carry coerceability.