Install GHC by default for upgrade command (fixes #797)

ChangeLog.md

@@ -10,6 +10,7 @@ Other enhancements:
 * Added the `--docker-env` argument, to set environment variables in Docker container.
 * Set `LC_ALL` to UTF-8 encoding for builds to avoid "commitBuffer: invalid argument" errors from GHC [#793](https://github.com/commercialhaskell/stack/issues/793)
 * Enable translitation for encoding on stdout and stderr [#824](https://github.com/commercialhaskell/stack/issues/824)
+* By default, `stack upgrade` automatically installs GHC as necessary [#797](https://github.com/commercialhaskell/stack/issues/797)
 
 * Added the `ghc-options` field to stack.yaml [#796](https://github.com/commercialhaskell/stack/issues/796)
 * Added the `extra-path` field to stack.yaml

src/Stack/Upgrade.hs

@@ -11,6 +11,7 @@ import           Control.Monad.Reader        (MonadReader, asks)
 import           Control.Monad.Trans.Control
 import           Data.Foldable               (forM_)
 import qualified Data.Map                    as Map
+import           Data.Monoid                 ((<>))
 import qualified Data.Set                    as Set
 import           Network.HTTP.Client.Conduit (HasHttpManager, getHttpManager)
 import           Path
@@ -80,7 +81,9 @@ upgrade gitRepo mresolver = withSystemTempDirectory "stack-upgrade" $ \tmp' -> d
     forM_ mdir $ \dir -> liftIO $ do
         bconfig <- runStackLoggingT manager logLevel terminal reExec $ do
             lc <- loadConfig
-                configMonoid
+                (configMonoid <> mempty
+                    { configMonoidInstallGHC = Just True
+                    })
                 (Just $ dir </> $(mkRelFile "stack.yaml"))
             lcLoadBuildConfig lc mresolver
         envConfig1 <- runStackT manager logLevel bconfig terminal reExec $ setupEnv $ Just