Add "quantified constraint" context in error message, fix #15231.

This patch adds "quantified constraint" context in error message when
UndecidableInstances checking fails for quantified constraints.
See Trac #15231:comment#1.

This patch also pretty-prints the instance head for better error messages.

Test Plan: make test TEST="T15231"

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #15231

Differential Revision: https://phabricator.haskell.org/D4819

3 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/quantified-constraints/T15231.hs b/testsuite/tests/quantified-constraints/T15231.hs
new file mode 100644
index 0000000000..7b7834d1f2
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T15231.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+module Bug where
+
+import Data.Kind
+
+data ECC :: Constraint -> Type -> Type
+
+class Y a
+class Z a
+
+instance  c         => Y (ECC c a)
+instance (c => Z a) => Z (ECC c a)
diff --git a/testsuite/tests/quantified-constraints/T15231.stderr b/testsuite/tests/quantified-constraints/T15231.stderr
new file mode 100644
index 0000000000..afb7ac3a25
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T15231.stderr
@@ -0,0 +1,7 @@
+
+T15231.hs:15:10: error:
+    • Variable ‘c’ occurs more often
+        in the constraint ‘c’ than in the instance head ‘Z a’
+      (Use UndecidableInstances to permit this)
+    • In the quantified constraint ‘c => Z a’
+      In the instance declaration for ‘Z (ECC c a)’
diff --git a/testsuite/tests/quantified-constraints/all.T b/testsuite/tests/quantified-constraints/all.T
index e546a7c495..75fcf8c772 100644
--- a/testsuite/tests/quantified-constraints/all.T
+++ b/testsuite/tests/quantified-constraints/all.T
@@ -9,3 +9,4 @@ test('T14863', normal, compile, [''])
 test('T14961', normal, compile, [''])
 test('T9123a', normal, compile, [''])
 test('T15244', normal, compile, [''])
+test('T15231', normal, compile_fail, [''])