REGRESSION: --supported-languages lies about supported languages, again

To make it short, recent version of GHC broke the very thing we fixed in #11102 (closed) 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

{-# LANGUAGE TemplateHaskell #-}

main = $undefined

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

$ 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

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture