[GHC] #5978: Type error in one function causes wrong type error report in another function in the presence of functionally dependent types

[GHC]

#5978: Type error in one function causes wrong type error report in another
function in the presence of functionally dependent types
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    Reporter:  Lemming                  |       Owner:                           
        Type:  bug                      |      Status:  new                      
    Priority:  normal                   |   Milestone:                           
   Component:  Compiler (Type checker)  |     Version:  7.4.1                    
    Keywords:                           |          Os:  Unknown/Multiple         
Architecture:  Unknown/Multiple         |     Failure:  GHC rejects valid program
  Difficulty:  Unknown                  |    Testcase:                           
   Blockedby:                           |    Blocking:                           
     Related:                           |  
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Comment(by Lemming):

 Sorry ... monoFoo is correct and monoBar is incorrect.
 But when I compile that module GHC reports two type errors: One for
 monoFoo and one for monoBar.
 If I remove monoBar, then monoFoo is accepted by GHC.

 The result of monoBar is Double, thus fromB=Double.
 The argument of polyBar in monoBar is of type Float, thus fromA=Float.
 From the functional dependencies it follows toB=Bool and toA=Char.
 Thus id would need type Char -> Bool, which is a type error and this one
 is correctly spotted by GHC.
 But additionally also a type error for monoBar is reported, although
 monoBar is correct.
 It has type Float and it follows to=Char, but the 'to' type is not used in
 monoBar.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5978#comment:2>
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