Better warning message for orphan instances (Ticket #9178)

Including a test case.

compiler/iface/MkIface.lhs

@@ -879,6 +879,13 @@ instOrphWarn :: DynFlags -> PrintUnqualified -> ClsInst -> WarnMsg
 instOrphWarn dflags unqual inst
   = mkWarnMsg dflags (getSrcSpan inst) unqual $
     hang (ptext (sLit "Orphan instance:")) 2 (pprInstanceHdr inst)
+    $$ text "To avoid this"
+    $$ nest 4 (vcat posibilities)
+  where
+    posibilities =
+      text "move the instance declaration to the module of the class or of the type, or" :
+      text "wrap the type with a newtype and declare the instance on the new type." :
+      []
 
 ruleOrphWarn :: DynFlags -> PrintUnqualified -> Module -> IfaceRule -> WarnMsg
 ruleOrphWarn dflags unqual mod rule

testsuite/tests/warnings/should_compile/Makefile

+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk

testsuite/tests/warnings/should_compile/T9178.hs

+
+
+module T9178 where
+
+import T9178DataType
+
+
+instance Show T9178_Type where
+         show _ = undefined
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testsuite/tests/warnings/should_compile/T9178.stderr

+[1 of 2] Compiling T9178DataType    ( T9178DataType.hs, T9178DataType.o )
+[2 of 2] Compiling T9178            ( T9178.hs, T9178.o )
+
+T9178.hs:8:10: Warning:
+    Orphan instance: instance Show T9178_Type
+    To avoid this
+        move the instance declaration to the module of the class or of the type, or
+        wrap the type with a newtype and declare the instance on the new type.

testsuite/tests/warnings/should_compile/T9178DataType.hs

+
+
+module T9178DataType where
+
+data T9178_Type

testsuite/tests/warnings/should_compile/all.T

+test('T9178', extra_clean(['T9178.o', 'T9178DataType.o',
+			   'T9178.hi', 'T9178DataType.hi']),
+	      multimod_compile, ['T9178', '-Wall'])
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