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-rw-r--r--compiler/GHC/Tc/Deriv/Generics.hs1
-rw-r--r--compiler/GHC/Tc/Deriv/Infer.hs10
-rw-r--r--compiler/GHC/Tc/Deriv/Utils.hs3
-rw-r--r--compiler/GHC/Tc/Gen/Expr.hs3
-rw-r--r--compiler/GHC/Tc/Gen/Head.hs4
-rw-r--r--compiler/GHC/Tc/Gen/Pat.hs3
-rw-r--r--compiler/GHC/Tc/TyCl.hs3
-rw-r--r--compiler/GHC/Tc/TyCl/Build.hs2
-rw-r--r--compiler/GHC/Tc/TyCl/Utils.hs3
9 files changed, 20 insertions, 12 deletions
diff --git a/compiler/GHC/Tc/Deriv/Generics.hs b/compiler/GHC/Tc/Deriv/Generics.hs
index 3d71c25b7d..eee7496b6f 100644
--- a/compiler/GHC/Tc/Deriv/Generics.hs
+++ b/compiler/GHC/Tc/Deriv/Generics.hs
@@ -119,6 +119,7 @@ following constraints are satisfied.
alpha-renaming).
(b) D cannot have a "stupid context".
+ See Note [The stupid context] in GHC.Core.DataCon.
(c) The right-hand side of D cannot include existential types, universally
quantified types, or "exotic" unlifted types. An exotic unlifted type
diff --git a/compiler/GHC/Tc/Deriv/Infer.hs b/compiler/GHC/Tc/Deriv/Infer.hs
index 7276bfde83..2466931219 100644
--- a/compiler/GHC/Tc/Deriv/Infer.hs
+++ b/compiler/GHC/Tc/Deriv/Infer.hs
@@ -194,13 +194,9 @@ inferConstraintsStock (DerivInstTys { dit_cls_tys = cls_tys
]
-- Stupid constraints from DatatypeContexts. Note that we
-- must gather these constraints from the data constructors,
- -- not from the parent type constructor, as the latter can
- -- lead to redundant constraints in some cases. For example,
- -- the derived Eq instance for:
- --
- -- data Show a => T a = MkT deriving Eq
- --
- -- Should not have Show in the instance context (#20501).
+ -- not from the parent type constructor, as the latter could
+ -- lead to redundant constraints due to thinning.
+ -- See Note [The stupid context] in GHC.Core.DataCon.
stupid_theta =
[ substTyWith (dataConUnivTyVars data_con)
all_rep_tc_args
diff --git a/compiler/GHC/Tc/Deriv/Utils.hs b/compiler/GHC/Tc/Deriv/Utils.hs
index d97db525eb..dfd1b557a7 100644
--- a/compiler/GHC/Tc/Deriv/Utils.hs
+++ b/compiler/GHC/Tc/Deriv/Utils.hs
@@ -930,6 +930,9 @@ cond_functorOK allowFunctions allowExQuantifiedLastTyVar _ _ rep_tc
| null tc_tvs
= NotValid $ DerivErrMustHaveSomeParameters rep_tc
+ -- We can't handle stupid contexts that mention the last type argument,
+ -- so error out if we encounter one.
+ -- See Note [The stupid context] in GHC.Core.DataCon.
| not (null bad_stupid_theta)
= NotValid $ DerivErrMustNotHaveClassContext rep_tc bad_stupid_theta
diff --git a/compiler/GHC/Tc/Gen/Expr.hs b/compiler/GHC/Tc/Gen/Expr.hs
index c9e9129251..6739e9a375 100644
--- a/compiler/GHC/Tc/Gen/Expr.hs
+++ b/compiler/GHC/Tc/Gen/Expr.hs
@@ -790,7 +790,8 @@ tcExpr expr@(RecordUpd { rupd_expr = record_expr, rupd_flds = Left rbnds }) res_
-- Typecheck the bindings
; rbinds' <- tcRecordUpd con1 con1_arg_tys' rbinds
- -- STEP 6: Deal with the stupid theta
+ -- STEP 6: Deal with the stupid theta.
+ -- See Note [The stupid context] in GHC.Core.DataCon.
; let theta' = substThetaUnchecked scrut_subst (conLikeStupidTheta con1)
; instStupidTheta RecordUpdOrigin theta'
diff --git a/compiler/GHC/Tc/Gen/Head.hs b/compiler/GHC/Tc/Gen/Head.hs
index bfaba5efcc..821b118ded 100644
--- a/compiler/GHC/Tc/Gen/Head.hs
+++ b/compiler/GHC/Tc/Gen/Head.hs
@@ -905,7 +905,9 @@ being able to reconstruct the exact original program.
Note [Instantiating stupid theta]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider a data type with a "stupid theta":
+Consider a data type with a "stupid theta" (see
+Note [The stupid context] in GHC.Core.DataCon):
+
data Ord a => T a = MkT (Maybe a)
We want to generate an Ord constraint for every use of MkT; but
diff --git a/compiler/GHC/Tc/Gen/Pat.hs b/compiler/GHC/Tc/Gen/Pat.hs
index a09d77b6f7..a235c43236 100644
--- a/compiler/GHC/Tc/Gen/Pat.hs
+++ b/compiler/GHC/Tc/Gen/Pat.hs
@@ -1311,7 +1311,8 @@ tcConArg penv (arg_pat, Scaled arg_mult arg_ty)
addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
-- Instantiate the "stupid theta" of the data con, and throw
--- the constraints into the constraint set
+-- the constraints into the constraint set.
+-- See Note [The stupid context] in GHC.Core.DataCon.
addDataConStupidTheta data_con inst_tys
| null stupid_theta = return ()
| otherwise = instStupidTheta origin inst_theta
diff --git a/compiler/GHC/Tc/TyCl.hs b/compiler/GHC/Tc/TyCl.hs
index 5dfa4cec86..17b6e2fc43 100644
--- a/compiler/GHC/Tc/TyCl.hs
+++ b/compiler/GHC/Tc/TyCl.hs
@@ -3312,7 +3312,8 @@ dataDeclChecks tc_name new_or_data mctxt cons
; let gadt_syntax = consUseGadtSyntax cons
; checkTc (gadtSyntax_ok || not gadt_syntax) (badGadtDecl tc_name)
- -- Check that the stupid theta is empty for a GADT-style declaration
+ -- Check that the stupid theta is empty for a GADT-style declaration.
+ -- See Note [The stupid context] in GHC.Core.DataCon.
; checkTc (null stupid_theta || not gadt_syntax) (badStupidTheta tc_name)
-- Check that a newtype has exactly one constructor
diff --git a/compiler/GHC/Tc/TyCl/Build.hs b/compiler/GHC/Tc/TyCl/Build.hs
index 7206fe70bd..4b44069950 100644
--- a/compiler/GHC/Tc/TyCl/Build.hs
+++ b/compiler/GHC/Tc/TyCl/Build.hs
@@ -192,6 +192,8 @@ buildDataCon fam_envs src_name declared_infix prom_info src_bangs impl_bangs
-- the type variables mentioned in the arg_tys
-- ToDo: Or functionally dependent on?
-- This whole stupid theta thing is, well, stupid.
+--
+-- See Note [The stupid context] in GHC.Core.DataCon.
mkDataConStupidTheta :: TyCon -> [Type] -> [TyVar] -> [PredType]
mkDataConStupidTheta tycon arg_tys univ_tvs
| null stupid_theta = [] -- The common case
diff --git a/compiler/GHC/Tc/TyCl/Utils.hs b/compiler/GHC/Tc/TyCl/Utils.hs
index 1ce9ef8f82..347a7e57ff 100644
--- a/compiler/GHC/Tc/TyCl/Utils.hs
+++ b/compiler/GHC/Tc/TyCl/Utils.hs
@@ -906,7 +906,8 @@ mkOneRecordSelector all_cons idDetails fl has_sel
|| has_sel == NoFieldSelectors
sel_ty | is_naughty = unitTy -- See Note [Naughty record selectors]
| otherwise = mkForAllTys (tyVarSpecToBinders data_tvbs) $
- mkPhiTy (conLikeStupidTheta con1) $ -- Urgh!
+ -- Urgh! See Note [The stupid context] in GHC.Core.DataCon
+ mkPhiTy (conLikeStupidTheta con1) $
-- req_theta is empty for normal DataCon
mkPhiTy req_theta $
mkVisFunTyMany data_ty $
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