Disallow -XDerivingVia when -XSafe is on (#19786)

Since `GeneralizedNewtypeDeriving` is considered unsafe, `DerivingVia`
should be as well.

(cherry picked from commit 0281dae8b3fe3384939c415ae72ca2440b3cafb3)

3 files changed, 12 insertions, 0 deletions

diff --git a/testsuite/tests/safeHaskell/safeLanguage/SafeLang19.hs b/testsuite/tests/safeHaskell/safeLanguage/SafeLang19.hs
new file mode 100644
index 0000000000..915876a838
--- /dev/null
+++ b/testsuite/tests/safeHaskell/safeLanguage/SafeLang19.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE Haskell2010 #-}
+{-# LANGUAGE Safe, DerivingVia #-}
+
+-- | Tests that Safe disables DerivingVia (#19786)
+module SafeLang19 where
+
+f :: Int
+f = 1
diff --git a/testsuite/tests/safeHaskell/safeLanguage/SafeLang19.stderr b/testsuite/tests/safeHaskell/safeLanguage/SafeLang19.stderr
new file mode 100644
index 0000000000..1dbe65d156
--- /dev/null
+++ b/testsuite/tests/safeHaskell/safeLanguage/SafeLang19.stderr
@@ -0,0 +1,3 @@
+
+SafeLang19.hs:2:20: warning:
+    -XDerivingVia is not allowed in Safe Haskell; ignoring -XDerivingVia
diff --git a/testsuite/tests/safeHaskell/safeLanguage/all.T b/testsuite/tests/safeHaskell/safeLanguage/all.T
index de4a9b6908..ac15d88a21 100644
--- a/testsuite/tests/safeHaskell/safeLanguage/all.T
+++ b/testsuite/tests/safeHaskell/safeLanguage/all.T
@@ -32,6 +32,7 @@ test('SafeLang16', normal, compile, [''])
 test('SafeLang17', [], multimod_compile_fail, ['SafeLang17', ''])
 
 test('SafeLang18', normal, compile, [''])
+test('SafeLang19', normal, compile, [''])
 
 # Test building a package, that trust values are set correctly
 # and can be changed correctly