Quantified constraints
This wiki page summarises the state of play on the idea of allowing quantification in class constraints. For example
data Rose f a = Branch a (f (Rose f a))
instance (Eq a, forall b. (Eq b) => Eq (f b))
=> Eq (Rose f a)
where ...
The new bit is the forall
in the context of the instance declaration. This is allowed in GHC 8.6 and later using the QuantifiedConstraints
extension.
Here are some resources

Derivable type classes, Section 7, where the idea was first proposed (I think).

#2893, a ticket about the idea

Quantified class constraints, a Haskell 2017 paper that works out the idea in some detail, and a Reddit thread about it.
Status
Use Keyword = QuantifiedConstraints
to ensure that a ticket ends up on these lists.
Open Tickets:
#8516  Add (\>) representation and the Invariant class to GHC.Generics 

#13153  Several Traversable instances have an extra fmap 
#14317  Solve Coercible constraints over type constructors 
#14831  QuantifiedConstraints: Odd superclass constraint 
#14832  QuantifiedConstraints: Adding to the context causes failure 
#14860  QuantifiedConstraints: Can't quantify constraint involving type family 
#14877  QuantifiedConstraints: Can't deduce \`xx' from \`(xx =\> a, xx)' 
#14879  QuantifiedConstraints: Big error message + can't substitute (=\>) with a class alias 
#14896  QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified 
#14937  QuantifiedConstraints: Reify implication constraints from terms lacking them 
#14943  Make (=\>) polykinded (:: k \> k \> Constraint) 
#14958  QuantifiedConstraints: Doesn't apply implication for existential? 
#14968  QuantifiedConstraints: Can't be RHS of type family instances 
#14983  Have custom type errors imply Void 
#14995  QuantifiedConstraints: Incorrect pretty printing 
#15347  QuantifiedConstraints: Implication constraints with type families don't work 
#15351  QuantifiedConstraints ignore FunctionalDependencies 
#15409  Quantified constraints halfwork with equality constraints 
#15639  Surprising failure combining QuantifiedConstraints with Coercible 
#15888  Quantified constraints can be loopy 
#16139  GHC confused about type synonym kind with QuantifiedConstraints 
#16140  Cannot create type synonym for quantified constraint without ImpredicativeTypes 
#16173  Move \`Data.Profunctor\` from \`profunctors\` package to \`base\` 
#16245  GHC panic (No skolem info) with QuantifiedConstraints and strange scoping 
#16365  Inconsistency in quantified constraint solving 
Closed Tickets:
#2256  Incompleteness of type inference: must quantify over implication constraints 

#2893  Implement "Quantified constraints" proposal 
#5927  A typelevel "implies" constraint on Constraints 
#9123  Emit quantified Coercible constraints in GeneralizedNewtypeDeriving 
#12245  Deriving Data at higher kinds 
#14070  Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’ 
#14733  Won't use (forall xx. f xx) with XQuantifiedConstraints 
#14734  QuantifiedConstraints conflated with impredicative polymorphism? 
#14735  GHC Panic with QuantifiedConstraints 
#14744  Nonexhaustive patterns in case in GHCi with quantified class contexts 
#14748  Infer context for Data instance of (data Foo f = Foo (f Bool) (f Int)) 
#14799  QuantifiedConstraints: Problems with Typeable 
#14822  XQuantifiedConstraints: Turn termlevel entailments (:) into constraints (=\>) 
#14833  QuantifiedConstraints: GHC can't deduce (() :: Constraint)? 
#14835  QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b" 
#14840  QuantifiedConstraints: Can't define class alias 
#14861  QuantifiedConstraints: Can't use forall'd variable in context 
#14863  QuantifiedConstraints: Can't deduce \`c' from \`(a, b)' and \`a \ b \ c' 
#14878  Can't witness transitivity ((.)) of isomorphism of Constraints 
#14883  QuantifiedConstraints don't kick in when used in TypeApplications 
#14897  QuantifiedConstraints: Can't print type of quantified constraint 
#14942  QuantifiedConstraints: GHC can't infer 
#14961  QuantifiedConstraints: introducing classes through equality constraints fails 
#14993  QuantifiedConstraints and principal types 
#15008  Type synonyms with hidden, determined type variables 
#15231  UndecidableInstances validity checking is wrong in the presence of QuantifiedConstraints 
#15244  Ambiguity checks in QuantifiedConstraints 
#15290  QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar" 
#15316  Regarding coherence and implication loops in presence of QuantifiedConstraints 
#15334  (forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x) 
#15359  Quantified constraints do not work with equality constraints 
#15507  Deriving with QuantifiedConstraints is unable to penetrate type families 
#15593  QuantifiedConstraints: trouble with type family 
#15625  GHC panic, with QuantifiedConstraints 
#15635  Implication introduction for quantified constraints 
#15636  Implication constraint priority breaks default class implementations 
#15943  "ASSERT failed" with quantified constraints 
#15974  QuantifiedConstraints: Spurious error involving superclass constraints 
#16123  QuantifiedConstraints fails to deduce trivial constraint 