[GHC] #16331: REGRESSION: --supported-languages lies about supported languages, again

[GHC]

#16331: REGRESSION: --supported-languages lies about supported languages, again
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           Reporter:  hvr            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 To make it short, recent version of GHC broke the very thing we fixed in
 #11102 for GHC 8.0. This is utterly frustrating to me. ;-(


 Basically, if GHC advertises an extension, it's supposed to work without
 jumping through hoops if you enable it via `{-# LANGUAGE ... #-}` or
 `-X`-flags. Otherwise this breaks the whole idea of `other-extensions` and
 related cabal spec features which use `--supported-languages` to infer
 whether e.g. GHC when invoked with `-XTemplateHaskell` will succeed.

 However, consider the `Main.hs` below

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}

 main = $undefined
 }}}

 Unfortuantely now GHC lies unconditionally about supporting TH, thereby
 undermining the infrastructure we setup in and for #11102:

 {{{
 $ ghc --supported-languages | grep ^TemplateHaskell$
 TemplateHaskell

 $ ghc --make Foo.hs
 [1 of 1] Compiling Main             ( Foo.hs, Foo.o )
 ghc-stage2: this operation requires -fexternal-interpreter
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16331>
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