Summary:
The RdrHsSyn.isFunLhs function has the following

  isFunLhs e = go e []
   where
     go (L loc (HsVar f)) es
          | not (isRdrDataCon f)   = return (Just (L loc f, False, es))
     go (L _ (HsApp f e)) es       = go f (e:es)
     go (L _ (HsPar e))   es@(_:_) = go e es

The treatment of HsPar means that any parentheses around an infix function will be discarded.

e.g.

  (f =*= g) sa i = f (toF sa i) =^= g (toG sa i)

will lose the ( before f and the closing one after g

Test Plan: ./validate

Reviewers: hvr, austin

Reviewed By: austin

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D832

GHC Trac Issues: #10269