[GHC] #15767: "StgCmmEnv: variable not found" with FunctionalDependencies and FlexibleContexts

[GHC]

#15767: "StgCmmEnv: variable not found" with FunctionalDependencies and
FlexibleContexts
-------------------------------------+-------------------------------------
        Reporter:  roland            |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  typecheck/should_fail/T15767
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by bgamari):

 I'm afraid I'm seeing some spurious test output changes when cherry-
 picking this to `ghc-8.6`. For instance,
 {{{#!patch
 diff -uw "./rename/should_fail/mc13.run/mc13.stderr.normalised"
 "./rename/should_fail/mc13.run/mc13.comp.stderr.normalised"
 --- ./rename/should_fail/mc13.run/mc13.stderr.normalised        2018-10-30
 14:54:22.690690790 -0400
 +++ ./rename/should_fail/mc13.run/mc13.comp.stderr.normalised   2018-10-30
 14:54:22.690690790 -0400
 @@ -1,2 +1,18 @@

 -mc13.hs:12:37: Variable not in scope: f :: [a] -> m a
 +mc13.hs:12:16:
 +     Ambiguous type variable ‘m0’ arising from a statement in a monad
 comprehension
 +      prevents the constraint ‘(Monad m0)’ from being solved.
 +      Relevant bindings include output :: m0 () (bound at mc13.hs:12:1)
 +      Probable fix: use a type annotation to specify what ‘m0’ should be.
 +      These potential instances exist:
 +        instance Monad IO -- Defined in ‘GHC.Base’
 +        instance Monad Maybe -- Defined in ‘GHC.Base’
 +        instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
 +        ...plus one other
 +        ...plus two instances involving out-of-scope types
 +        (use -fprint-potential-instances to see them all)
 +     In a stmt of a monad comprehension: then f
 +      In the expression: [() | f <- functions, then f]
 +      In an equation for ‘output’: output = [() | f <- functions, then f]
 +
 +mc13.hs:12:37: Variable not in scope: f :: [a] -> m0 a
 }}}

 and similar new errors in `TEST="T12529 T12921 mc13 mc14"`. Given that
 these are just error message regressions I'm going to go ahead and accept
 them (and perhaps we can fix them later if anyone knows the cause off-
 hand).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15767#comment:5>
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