Don't warn about redundant constraints for type equalities

With -Wall, the following code

{-# LANGUAGE TypeFamilies #-}

type family Foo x where
  Foo Double = Int

foo :: (RealFrac a, Integral b, b ~ Foo a) => a -> b
foo = round

produces the warning

• Redundant constraint: b ~ Foo a
• In the type signature for:
       foo :: (RealFrac a, Integral b, b ~ Foo a) => a -> b

Technically, I suppose this is correct in the sense that if we remove the constraint b ~ Foo a, foo still compiles. However, the meaning changes without the type equality, because the function I wrote is equivalent to foo :: (RealFrac a, Integral (Foo a)) => a -> Foo a (which requires -XFlexibleContexts), while the function it suggests is the much more general foo :: (RealFrac a, Integral b) => a -> b. Since this is the case, I think the warning is invalid.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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