strange "instance .. => .. => .. where ..."

I don't understand what's happening here:

GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
Prelude> data T a
Prelude> instance Eq a => Eq a => Eq (T a) where (==) = undefined

<interactive>:2:10: error:
    • Class ‘Eq’ does not have a method ‘==’
    • In the instance declaration for ‘Eq (T a)’

Is that even syntactically correct? (ghc-8.4.4 says "malformed instance") If so, then I'd assume that A => B => C means A && B => C, that is, (A,B) => C in this case, but it does not, since this works:

instance (Eq a, Eq a)=> Eq (T a) where (==) = undefined

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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