Add test for T7878

testsuite/tests/generics/T7878.hs

+module T7878C where
+
+import T7878A

testsuite/tests/generics/T7878A.hs

+{-# LANGUAGE DeriveGeneric #-}
+module T7878A where
+
+import T7878B
+import GHC.Generics
+
+data Test deriving Generic

testsuite/tests/generics/T7878A.hs-boot

+module T7878A where
+
+data Test -- The panic occurs regardless of whether this line is included

testsuite/tests/generics/T7878B.hs

+module T7878B where
+
+import {-# SOURCE #-} T7878A

testsuite/tests/generics/all.T

@@ -24,4 +24,9 @@ test('GenCannotDoRep1_4', normal, compile_fail, [''])
 test('GenCannotDoRep1_5', normal, compile_fail, [''])
 test('GenCannotDoRep1_6', normal, compile_fail, [''])
 test('GenCannotDoRep1_7', normal, compile_fail, [''])
-test('GenCannotDoRep1_8', normal, compile_fail, [''])
\ No newline at end of file
+test('GenCannotDoRep1_8', normal, compile_fail, [''])
+
+test('T7878', extra_clean(['T7878A.o'     ,'T7878A.hi'
+                          ,'T7878A.o-boot','T7878A.hi-boot'
+                          ,'T7878B.o'     ,'T7878B.hi']), 
+              multimod_compile, ['T7878', '-v0'])