From the Haskell 2010 report chapter 4,

• Class and instance declarations:

| 	class [scontext =>] ...
| 	instance [scontext =>] ...

• Normal type signatures:

	vars :: [context =>] ...

Notice the difference between scontext (//with// s) and context (without s).

scontext 	→ 	simpleclass
	| 	( simpleclass1 , … , simpleclassn ) 	    (n ≥ 0)
simpleclass 	→ 	qtycls tyvar

context 	→ 	class
	| 	( class1 , … , classn ) 	    (n ≥ 0)
class 	→ 	qtycls tyvar
	| 	qtycls ( tyvar atype1 … atypen ) 	    (n ≥ 1)

GHC seems to ignore this difference, and happily accepts class Eq (a Int) => C a where. Hugs (Version: September 2006) reports for that same example:

Illegal Haskell 98 class constraint in class declaration
*** Constraint : Eq (a Int)