Always set `safeInferred`, not only when it turns `False`wip/T19243

previously, `safeFlagCheck` would be happy to switch the `safeFlag` to
`False`, but not the other way around. This meant that after

    :set -XGeneralizedNewtypeDeriving
    :set -XNoGeneralizedNewtypeDeriving

in GHCi all loaded files would be still be infered as unsafe.

This fixes #19243.

This is a corner case, but somewhat relevant once ghci by default starts
with `GeneralizedNewtypeDeriving` on (due to GHC2021).

5 files changed, 17 insertions, 4 deletions

diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 8e92fc974f..0c09ce7ec2 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -2003,10 +2003,9 @@ safeFlagCheck _ dflags | safeLanguageOn dflags = (dflagsUnset, warns)
            ++ str]
 
 safeFlagCheck cmdl dflags =
-  case (safeInferOn dflags) of
-    True | safeFlags -> (dflags', warn)
-    True             -> (dflags' { safeInferred = False }, warn)
-    False            -> (dflags', warn)
+  case safeInferOn dflags of
+    True   -> (dflags' { safeInferred = safeFlags }, warn)
+    False  -> (dflags', warn)
 
   where
     -- dynflags and warn for when -fpackage-trust by itself with no safe
diff --git a/testsuite/tests/safeHaskell/ghci/T19243.hs b/testsuite/tests/safeHaskell/ghci/T19243.hs
new file mode 100644
index 0000000000..b32102030e
--- /dev/null
+++ b/testsuite/tests/safeHaskell/ghci/T19243.hs
@@ -0,0 +1,4 @@
+{-# LANGUAGE Haskell2010 #-}
+module T19234 where
+class A a
+instance A Int
diff --git a/testsuite/tests/safeHaskell/ghci/T19243.script b/testsuite/tests/safeHaskell/ghci/T19243.script
new file mode 100644
index 0000000000..ad2fc64777
--- /dev/null
+++ b/testsuite/tests/safeHaskell/ghci/T19243.script
@@ -0,0 +1,5 @@
+:set -XGeneralizedNewtypeDeriving
+:set -XNoGeneralizedNewtypeDeriving
+:load T19243
+-- The following should report safe
+:info A
diff --git a/testsuite/tests/safeHaskell/ghci/T19243.stdout b/testsuite/tests/safeHaskell/ghci/T19243.stdout
new file mode 100644
index 0000000000..93828deb9b
--- /dev/null
+++ b/testsuite/tests/safeHaskell/ghci/T19243.stdout
@@ -0,0 +1,4 @@
+type A :: * -> Constraint
+class A a
+  	-- Defined at T19243.hs:3:1
+instance [safe] A Int -- Defined at T19243.hs:4:10
diff --git a/testsuite/tests/safeHaskell/ghci/all.T b/testsuite/tests/safeHaskell/ghci/all.T
index 040cc288f9..26f72d20b8 100644
--- a/testsuite/tests/safeHaskell/ghci/all.T
+++ b/testsuite/tests/safeHaskell/ghci/all.T
@@ -19,4 +19,5 @@ test('p17', normalise_version("bytestring"), ghci_script, ['p17.script'])
 # 7172
 test('p18', normalise_version("bytestring"), ghci_script, ['p18.script'])
 test('T12509', [extra_hc_opts('-XSafe')], ghci_script, ['T12509.script'])
+test('T19243', normal, ghci_script, ['T19243.script'])