Defer errors in derived instances

Fixes Trac #9576.  Turned out to be pretty easy.

4 files changed, 22 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_run/T9576.hs b/testsuite/tests/deriving/should_run/T9576.hs
new file mode 100644
index 0000000000..b80de9cc0e
--- /dev/null
+++ b/testsuite/tests/deriving/should_run/T9576.hs
@@ -0,0 +1,9 @@
+{-# OPTIONS_GHC -fdefer-type-errors #-}
+
+module Main where
+
+data Foo = MkFoo
+data Bar = MkBar Foo deriving Show
+
+main = do { print True; print (MkBar MkFoo) }
+
diff --git a/testsuite/tests/deriving/should_run/T9576.stderr b/testsuite/tests/deriving/should_run/T9576.stderr
new file mode 100644
index 0000000000..6f8bf7f4e7
--- /dev/null
+++ b/testsuite/tests/deriving/should_run/T9576.stderr
@@ -0,0 +1,11 @@
+T9576: T9576.hs:6:31:
+    No instance for (Show Foo) arising from a use of ‘showsPrec’
+    In the second argument of ‘(.)’, namely ‘(showsPrec 11 b1)’
+    In the second argument of ‘showParen’, namely
+      ‘((.) (showString "MkBar ") (showsPrec 11 b1))’
+    In the expression:
+      showParen ((a >= 11)) ((.) (showString "MkBar ") (showsPrec 11 b1))
+    When typechecking the code for  ‘showsPrec’
+      in a derived instance for ‘Show Bar’:
+      To see the code I am typechecking, use -ddump-deriv
+(deferred type error)
diff --git a/testsuite/tests/deriving/should_run/T9576.stdout b/testsuite/tests/deriving/should_run/T9576.stdout
new file mode 100644
index 0000000000..0ca95142bb
--- /dev/null
+++ b/testsuite/tests/deriving/should_run/T9576.stdout
@@ -0,0 +1 @@
+True
diff --git a/testsuite/tests/deriving/should_run/all.T b/testsuite/tests/deriving/should_run/all.T
index 572f95bacd..21c1962ed1 100644
--- a/testsuite/tests/deriving/should_run/all.T
+++ b/testsuite/tests/deriving/should_run/all.T
@@ -36,4 +36,5 @@ test('T5628', exit_code(1), compile_and_run, [''])
 test('T5712', normal, compile_and_run, [''])
 test('T7931', normal, compile_and_run, [''])
 test('T8280', normal, compile_and_run, [''])
+test('T9576', exit_code(1), compile_and_run, [''])