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    Add "quantified constraint" context in error message, fix #15231. · 91822e4e
    Tao He authored
    This patch adds "quantified constraint" context in error message when
    UndecidableInstances checking fails for quantified constraints.
    See Trac #15231:comment#1.
    
    This patch also pretty-prints the instance head for better error messages.
    
    Test Plan: make test TEST="T15231"
    
    Reviewers: bgamari, simonpj
    
    Reviewed By: simonpj
    
    Subscribers: simonpj, rwbarton, thomie, carter
    
    GHC Trac Issues: #15231
    
    Differential Revision: https://phabricator.haskell.org/D4819
    91822e4e