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
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