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.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
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| Architecture |