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If I say

{-# LANGUAGE Haskell2010 #-}

module Bug where

((f x)) y = x + y

GHC succeeds. This appears to go against the Haskell 2010 Report, Section 4.4.3:

decl 	→ 	(funlhs | pat) rhs
 
funlhs 	→ 	var apat { apat }
	| 	pat varop pat
	| 	'(' funlhs ')' apat { apat }
 
rhs 	→ 	= exp [where decls]
	| 	gdrhs [where decls] 

Note that a funlhs is allowed to have parentheses, but there must be an apat after the ). My example above has a ) after the ), and yet is accepted.

Similarly

(f) x y = x + y

is accepted, despite having no apats within the parentheses.

I propose to continue accepting this syntax, but to document this as a deviation from the standard.