Don't warn about redundant constraints for necessary
In the attached example, GHC warns that the constraint on line 16 of Main.hs is redundant.
• Redundant constraint: e ~ FGCD r s
• In the type signature for:
foo :: (e ~ FGCD r e', e' ~ (e * (s / r)), e ~ FGCD r s) =>
Proxy s -> Proxy r -> ()
However, when I remove that constraint, I get the following error:
• Occurs check: cannot construct the infinite type:
t0 ~ Factored.FMul (FGCD r t0) (Factored.FDiv s r)
The type variable ‘t0’ is ambiguous
• In the ambiguity check for ‘foo’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature:
foo :: forall e e' r s.
(e ~ FGCD r e', e' ~ (e * (s / r))) => Proxy s -> Proxy r -> ()
I'm not entirely sure if the error is correct or not; I can't figure out what it's getting on about.
So: if the constraint is required to make foo
compile, then it is most certainly not redundant, and there shouldn't be a warning suggesting its removal. If the constraint really isn't required, then the error is a bug.
(I apologize for the second bug involving singletons in as many days; I simply don't know an easy way to remove that dependency.)
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |