[GHC] #5138: TypeSynonymInstances should be enabled by default

[GHC]

#5138: TypeSynonymInstances should be enabled by default
---------------------------------+------------------------------------------
    Reporter:  mcandre           |       Owner:                                    
        Type:  feature request   |      Status:  new                               
    Priority:  normal            |   Component:  Compiler                          
     Version:  7.0.2             |    Keywords:  type synonyms instance declaration
    Testcase:                    |   Blockedby:                                    
          Os:  Unknown/Multiple  |    Blocking:                                    
Architecture:  Unknown/Multiple  |     Failure:  GHC rejects valid program         
---------------------------------+------------------------------------------
 Source: https://github.com/mcandre/genetics

 With {{{ {-# LANGUAGE TypeSynonymInstances #-} }}}, my genetic algorithm
 code compiles and runs perfectly.

 {{{
 $ make
 $ ./helloga
 ...
 hellowobld
 helloworyd
 helloporld
 hellyworld
 helloworrd
 hellowofld
 hellpworld
 Best candidate: helloworld
 }}}

 Without it, Haskell refuses to declare an instance on Strings.

 {{{
 $ make
 ghc --make -O2 -fforce-recomp helloga.hs
 [1 of 2] Compiling Genetics         ( Genetics.hs, Genetics.o )
 [2 of 2] Compiling Main             ( helloga.hs, helloga.o )

 helloga.hs:27:10:
     Illegal instance declaration for `Gene String'
       (All instance types must be of the form (T t1 ... tn)
        where T is not a synonym.
        Use -XTypeSynonymInstances if you want to disable this.)
     In the instance declaration for `Gene String'
 make: *** [all] Error 1
 }}}

 The fix is easy to discover and apply, but this is my first typeclass,
 nothing complicated. As a newbie I prefer that it "just works" without my
 having to use the special compile flag.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5138>
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