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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-11-08 19:11:58 -0500 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2021-11-09 19:55:07 -0500 |
| commit | bdd7b2be6e61f24d467e7b4881b7309d524e2034 (patch) | |
| tree | c126bbff4ca33303908eb9174b56642bd916eb29 /compiler/GHC/Tc/Deriv | |
| parent | 983a99f0730b230367a327c5ef28cb5ec5dfedc2 (diff) | |
| download | haskell-bdd7b2be6e61f24d467e7b4881b7309d524e2034.tar.gz | |
Flesh out Note [The stupid context] and reference it
`Note [The stupid context]` in `GHC.Core.DataCon` talks about stupid contexts
from `DatatypeContexts`, but prior to this commit, it was rather outdated.
This commit spruces it up and references it from places where it is relevant.
Diffstat (limited to 'compiler/GHC/Tc/Deriv')
| -rw-r--r-- | compiler/GHC/Tc/Deriv/Generics.hs | 1 | ||||
| -rw-r--r-- | compiler/GHC/Tc/Deriv/Infer.hs | 10 | ||||
| -rw-r--r-- | compiler/GHC/Tc/Deriv/Utils.hs | 3 |
3 files changed, 7 insertions, 7 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 |