Require -XIncoherentInstances to write role annotations on class definitions
With a role annotation declaring a representational role for a class parameter, it is very easy to create instance incoherence. See this comment. The solution: require -XIncoherentInstances
when writing such a role annotation. This also has the knock-on effect (happily) of preventing such shenanigans in Safe Haskell.
Class role annotations asking GHC to infer roles or with nominal roles, although rather useless, will still be accepted without -XIncoherentInstances
.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.1-rc1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
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Blocking | |
CC | |
Operating system | |
Architecture |