Commit d50cb3ed authored by Simon Peyton Jones's avatar Simon Peyton Jones
Browse files

Do not generate derived instances in TH brackets

See Note [Deriving inside TH brackets] in TcInstDcls
Fixes Trac #6005 (again)
parent d9d35d8c
......@@ -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
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
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