Do not generate derived instances in TH brackets

See Note [Deriving inside TH brackets] in TcInstDcls
Fixes Trac #6005 (again)

1 files changed, 22 insertions, 1 deletions

diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs
index 6db2692fe1..776689084f 100644
--- a/compiler/typecheck/TcInstDcls.lhs
+++ b/compiler/typecheck/TcInstDcls.lhs
@@ -386,8 +386,12 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls
                         -- more errors still
 
        ; traceTc "tcDeriving" empty
+       ; th_stage <- getStage   -- See Note [Deriving inside TH brackets ]
        ; (gbl_env, deriv_inst_info, deriv_binds)
-              <- tcDeriving tycl_decls inst_decls deriv_decls
+              <- if isBrackStage th_stage 
+                 then return (gbl_env, emptyBag, emptyValBindsOut)
+                 else tcDeriving tycl_decls inst_decls deriv_decls
+
 
        -- Check that if the module is compiled with -XSafe, there are no
        -- hand written instances of Typeable as then unsafe casts could be
@@ -443,6 +447,23 @@ addFamInsts fam_insts thing_inside
     things = map ATyCon tycons ++ map ACoAxiom axioms 
 \end{code}
 
+Note [Deriving inside TH brackets]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Given a declaration bracket
+  [d| data T = A | B deriving( Show ) |]
+
+there is really no point in generating the derived code for deriving(
+Show) and then type-checking it. This will happen at the call site
+anyway, and the type check should never fail!  Moreover (Trac #6005)
+the scoping of the generated code inside the bracket does not seem to 
+work out.  
+
+The easy solution is simply not to generate the derived instances at
+all.  (A less brutal solution would be to generate them with no
+bindings.)  This will become moot when we shift to the new TH plan, so 
+the brutal solution will do.
+
+
 Note [Instance declaration cycles]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 With -XDataKinds we can get this