Don't suppress unimplemented type family warnings with DeriveAnyClass

Summary:
For some asinine reason, we were suppressing warnings when
deriving associated type family instances with `DeriveAnyClass`. That seems
like a bad idea. Let's not do that.

Along the way, I noticed that the error contexts associated with these
newly emitted warnings were less than ideal, so I did some minor refactoring
to improve the story there.

Fixes #14094

Test Plan: ./validate

Reviewers: bgamari, austin

Subscribers: rwbarton, thomie

GHC Trac Issues: #14094

Differential Revision: https://phabricator.haskell.org/D3828

5 files changed, 47 insertions, 7 deletions

diff --git a/testsuite/tests/deriving/should_compile/T14094.hs b/testsuite/tests/deriving/should_compile/T14094.hs
new file mode 100644
index 0000000000..29fa693e97
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T14094.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# OPTIONS_GHC -Wall #-}
+module Bug where
+
+class C a where
+  type T a
+  data D a
+  m :: a
+
+instance C Int
+deriving instance C Bool
diff --git a/testsuite/tests/deriving/should_compile/T14094.stderr b/testsuite/tests/deriving/should_compile/T14094.stderr
new file mode 100644
index 0000000000..b323a775f5
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T14094.stderr
@@ -0,0 +1,26 @@
+
+T14094.hs:12:1: warning: [-Wmissing-methods (in -Wdefault)]
+    • No explicit associated type or default declaration for ‘T’
+    • In the instance declaration for ‘C Int’
+
+T14094.hs:12:1: warning: [-Wmissing-methods (in -Wdefault)]
+    • No explicit associated type or default declaration for ‘D’
+    • In the instance declaration for ‘C Int’
+
+T14094.hs:12:10: warning: [-Wmissing-methods (in -Wdefault)]
+    • No explicit implementation for
+        ‘m’
+    • In the instance declaration for ‘C Int’
+
+T14094.hs:13:1: warning: [-Wmissing-methods (in -Wdefault)]
+    • No explicit associated type or default declaration for ‘T’
+    • In the instance declaration for ‘C Bool’
+
+T14094.hs:13:1: warning: [-Wmissing-methods (in -Wdefault)]
+    • No explicit associated type or default declaration for ‘D’
+    • In the instance declaration for ‘C Bool’
+
+T14094.hs:13:1: warning: [-Wmissing-methods (in -Wdefault)]
+    • No explicit implementation for
+        ‘m’
+    • In the instance declaration for ‘C Bool’
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 5b69565c52..65c6d7284e 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -95,3 +95,4 @@ test('T13813', normal, compile, [''])
 test('T13919', normal, compile, [''])
 test('T13998', normal, compile, [''])
 test('T14045b', normal, compile, [''])
+test('T14094', normal, compile, [''])
diff --git a/testsuite/tests/deriving/should_fail/T10598_fail3.stderr b/testsuite/tests/deriving/should_fail/T10598_fail3.stderr
index a987a4993d..c3f4e123b7 100644
--- a/testsuite/tests/deriving/should_fail/T10598_fail3.stderr
+++ b/testsuite/tests/deriving/should_fail/T10598_fail3.stderr
@@ -1,5 +1,4 @@
 
-T10598_fail3.hs:1:1: error:
-    Generic instances can only be derived in Safe Haskell using the stock strategy.
-    In the following instance:
-      instance [safe] Generic T
+T10598_fail3.hs:8:36: error:
+    • Generic instances can only be derived in Safe Haskell using the stock strategy.
+    • In the instance declaration for ‘Generic T’
diff --git a/testsuite/tests/deriving/should_fail/T8165_fail2.stderr b/testsuite/tests/deriving/should_fail/T8165_fail2.stderr
index 4c925f52a3..5e19173a33 100644
--- a/testsuite/tests/deriving/should_fail/T8165_fail2.stderr
+++ b/testsuite/tests/deriving/should_fail/T8165_fail2.stderr
@@ -1,5 +1,6 @@
 
 T8165_fail2.hs:9:12: error:
-    The type family application ‘T Loop’
-      is no smaller than the instance head
-    (Use UndecidableInstances to permit this)
+    • The type family application ‘T Loop’
+        is no smaller than the instance head
+      (Use UndecidableInstances to permit this)
+    • In the instance declaration for ‘C Loop’