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    Testsuite: add ImpredicativeTypes to T7861 (#7861) · 4c96e7cf
    Thomas Miedema authored and Ben Gamari's avatar Ben Gamari committed
    The test was failing with:
    
        T7861: T7861.hs:15:13:
            Cannot instantiate unification variable ‘t0’
            with a type involving foralls: A a0 -> a0
              GHC doesn't yet support impredicative polymorphism
            In the first argument of ‘seq’, namely ‘f’
            In a stmt of a 'do' block: f `seq` print "Hello 2"
    
    It requires ImpredicativeTypes, at least since 7.8, because we
    instantiate seq's type (c->d->d) with f's type (c:= (forall b. a) -> a),
    which is polymorphic (it has foralls).
    
    I simplified the test a bit by removing the type synonym, and verified
    that ghc-7.6.3 still panics on this test.
    
    Reviewers: simonpj, austin, bgamari
    
    Reviewed By: bgamari
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D1080
    
    GHC Trac Issues: #7861
    4c96e7cf