Fix ghci crash when starting with -fno-implicit-import-qualified

`ghci -fno-implicit-import-qualified` didn't start with error message:

```
GHCi, version 8.6.2: http://www.haskell.org/ghc/  :? for help

<interactive>:1:6: error:
    Not in scope: ‘System.IO.hSetBuffering’
        No module named ‘System.IO’ is imported.

...
```

This change fixes it and update test T2452 to cover this.

Test Plan:
  TEST=T2452 make accept

harbormaster build runs

Reviewers: simonmar, bgamari, RyanGlScott

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5452

5 files changed, 24 insertions, 13 deletions

diff --git a/ghc/GHCi/UI/Monad.hs b/ghc/GHCi/UI/Monad.hs
index a3c21d8c01..969111b214 100644
--- a/ghc/GHCi/UI/Monad.hs
+++ b/ghc/GHCi/UI/Monad.hs
@@ -458,15 +458,16 @@ mkEvalWrapper progname args =
     "(System.Environment.withArgs " ++ show args ++ " m)"
 
 compileGHCiExpr :: GhcMonad m => String -> m ForeignHValue
-compileGHCiExpr expr = do
-  hsc_env <- getSession
-  let dflags = hsc_dflags hsc_env
-      -- RebindableSyntax can wreak havoc with GHCi in several ways
-      -- (see #13385 and #14342 for examples), so we take care to disable it
-      -- for the duration of running expressions that are internal to GHCi.
-      no_rb_hsc_env =
-        hsc_env { hsc_dflags = xopt_unset dflags LangExt.RebindableSyntax }
-  setSession no_rb_hsc_env
-  res <- GHC.compileExprRemote expr
-  setSession hsc_env
-  pure res
+compileGHCiExpr expr =
+  withTempSession mkTempSession $ GHC.compileExprRemote expr
+  where
+    mkTempSession hsc_env = hsc_env
+      { hsc_dflags = (hsc_dflags hsc_env)
+          -- RebindableSyntax can wreak havoc with GHCi in several ways
+          -- (see #13385 and #14342 for examples), so we take care to disable it
+          -- for the duration of running expressions that are internal to GHCi.
+          `xopt_unset` LangExt.RebindableSyntax
+          -- We heavily depend on -fimplicit-import-qualified to compile expr
+          -- with fully qualified names without imports.
+          `gopt_set` Opt_ImplicitImportQualified
+      }
diff --git a/testsuite/tests/ghci/scripts/T2452.script b/testsuite/tests/ghci/scripts/T2452.script
index 28d2bdc580..ef60f82e7a 100644
--- a/testsuite/tests/ghci/scripts/T2452.script
+++ b/testsuite/tests/ghci/scripts/T2452.script
@@ -1,2 +1,5 @@
+:t System.IO.hPutStrLn
+:set -fimplicit-import-qualified
+:t System.IO.hPutStrLn
 :set -fno-implicit-import-qualified
 :t System.IO.hPutStrLn
diff --git a/testsuite/tests/ghci/scripts/T2452.stderr b/testsuite/tests/ghci/scripts/T2452.stderr
index 663742f67c..99b0acbd21 100644
--- a/testsuite/tests/ghci/scripts/T2452.stderr
+++ b/testsuite/tests/ghci/scripts/T2452.stderr
@@ -2,3 +2,7 @@
 <interactive>:1:1: error:
     Not in scope: ‘System.IO.hPutStrLn’
     No module named ‘System.IO’ is imported.
+
+<interactive>:1:1: error:
+    Not in scope: ‘System.IO.hPutStrLn’
+    No module named ‘System.IO’ is imported.
diff --git a/testsuite/tests/ghci/scripts/T2452.stdout b/testsuite/tests/ghci/scripts/T2452.stdout
new file mode 100644
index 0000000000..b2dc3f68b2
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T2452.stdout
@@ -0,0 +1,2 @@
+System.IO.hPutStrLn
+  :: GHC.IO.Handle.Types.Handle -> String -> IO ()
diff --git a/testsuite/tests/ghci/scripts/all.T b/testsuite/tests/ghci/scripts/all.T
index c58f5872cf..13753cd3ec 100755
--- a/testsuite/tests/ghci/scripts/all.T
+++ b/testsuite/tests/ghci/scripts/all.T
@@ -108,7 +108,8 @@ test('ghci062', [extra_files(['ghci062/', 'ghci062/Test.hs']),
 		ghci_script, ['ghci062.script'])
 test('ghci063', normal, ghci_script, ['ghci063.script'])
 
-test('T2452', normal, ghci_script, ['T2452.script'])
+test('T2452', [extra_hc_opts("-fno-implicit-import-qualified")],
+    ghci_script, ['T2452.script'])
 test('T2766', normal, ghci_script, ['T2766.script'])
 
 test('T1914', [], ghci_script, ['T1914.script'])