Improved error message on data constructor return types

testsuite/tests/ghc-regress/gadt/gadt11.stderr

-
-gadt11.hs:9:2:
-    Data constructor `B1' returns type `X' instead of its parent type
-    In the definition of data constructor `B1'
-    In the data type declaration for `B'
+
+gadt11.hs:9:2:
+    Data constructor `B1' returns type `X []'
+      instead of an instance of its parent type `B a'
+    In the definition of data constructor `B1'
+    In the data type declaration for `B'

testsuite/tests/ghc-regress/typecheck/should_fail/tcfail155.stderr

-
-tcfail155.hs:8:5:
-    Data constructor `P' returns type `L2' instead of its parent type
-    In the definition of data constructor `P'
-    In the data type declaration for `T'
+
+tcfail155.hs:8:5:
+    Data constructor `P' returns type `L2'
+      instead of an instance of its parent type `T a'
+    In the definition of data constructor `P'
+    In the data type declaration for `T'

testsuite/tests/ghc-regress/typecheck/should_fail/tcfail176.stderr

-
-tcfail176.hs:7:20:
-    Data constructor `Bug' returns type `Maybe'
-      instead of its parent type
-    In the definition of data constructor `Bug'
-    In the newtype declaration for `Bug'
+
+tcfail176.hs:7:20:
+    Data constructor `Bug' returns type `Maybe a'
+      instead of an instance of its parent type `Bug a'
+    In the definition of data constructor `Bug'
+    In the newtype declaration for `Bug'