UndecidableInstances not needed for equality constraints
Consider
instance (Op c, In c ~ Int) => Op (D2 c) where ...
Without -XUndecidableInstaces
we get
undec.hs:21:0:
Constraint is no smaller than the instance head
in the constraint: In c ~ Int
(Use -XUndecidableInstances to permit this)
But the undecidable-instance check is only needed for class constraints I think, so GHC is being over conservative. Let's fix this.
Trac metadata
Trac field | Value |
---|---|
Version | 6.12.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |