Use injectiveVarsOfType to catch dodgy type family instance binders (#17008)

Previously, we detected dodgy type family instances binders by
expanding type synonyms (via `exactTyCoVarsOfType`) and looking for
type variables on the RHS that weren't mentioned on the (expanded)
LHS. But this doesn't account for type families (like the example
in #17008), so we instead use `injectiveVarsOfType` to only count
LHS type variables that are in injective positions. That way, the `a`
in `type instance F (x :: T a) = a` will not count if `T` is a type
synonym _or_ a type family.

Along the way, I moved `exactTyCoVarsOfType` to `TyCoFVs` to live
alongside its sibling functions that also compute free variables.

Fixes #17008.

1 files changed, 0 insertions, 72 deletions

diff --git a/compiler/typecheck/TcType.hs b/compiler/typecheck/TcType.hs
index c55d16ae9a..7fe947678a 100644
--- a/compiler/typecheck/TcType.hs
+++ b/compiler/typecheck/TcType.hs
@@ -957,78 +957,6 @@ isTyFamFree :: Type -> Bool
 -- ^ Check that a type does not contain any type family applications.
 isTyFamFree = null . tcTyFamInsts
 
-{-
-************************************************************************
-*                                                                      *
-          The "exact" free variables of a type
-*                                                                      *
-************************************************************************
-
-Note [Silly type synonym]
-~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider
-  type T a = Int
-What are the free tyvars of (T x)?  Empty, of course!
-
-exactTyCoVarsOfType is used by the type checker to figure out exactly
-which type variables are mentioned in a type.  It only matters
-occasionally -- see the calls to exactTyCoVarsOfType.
-
-Historical note: years and years ago this function was used during
-generalisation -- see #1813.  But that code has long since died.
--}
-
-exactTyCoVarsOfType :: Type -> TyCoVarSet
--- Find the free type variables (of any kind)
--- but *expand* type synonyms.  See Note [Silly type synonym] above.
-exactTyCoVarsOfType ty
-  = go ty
-  where
-    go ty | Just ty' <- tcView ty = go ty'  -- This is the key line
-    go (TyVarTy tv)         = goVar tv
-    go (TyConApp _ tys)     = exactTyCoVarsOfTypes tys
-    go (LitTy {})           = emptyVarSet
-    go (AppTy fun arg)      = go fun `unionVarSet` go arg
-    go (FunTy _ arg res)    = go arg `unionVarSet` go res
-    go (ForAllTy bndr ty)   = delBinderVar (go ty) bndr `unionVarSet` go (binderType bndr)
-    go (CastTy ty co)       = go ty `unionVarSet` goCo co
-    go (CoercionTy co)      = goCo co
-
-    goMCo MRefl    = emptyVarSet
-    goMCo (MCo co) = goCo co
-
-    goCo (Refl ty)            = go ty
-    goCo (GRefl _ ty mco)     = go ty `unionVarSet` goMCo mco
-    goCo (TyConAppCo _ _ args)= goCos args
-    goCo (AppCo co arg)     = goCo co `unionVarSet` goCo arg
-    goCo (ForAllCo tv k_co co)
-      = goCo co `delVarSet` tv `unionVarSet` goCo k_co
-    goCo (FunCo _ co1 co2)   = goCo co1 `unionVarSet` goCo co2
-    goCo (CoVarCo v)         = goVar v
-    goCo (HoleCo h)          = goVar (coHoleCoVar h)
-    goCo (AxiomInstCo _ _ args) = goCos args
-    goCo (UnivCo p _ t1 t2)  = goProv p `unionVarSet` go t1 `unionVarSet` go t2
-    goCo (SymCo co)          = goCo co
-    goCo (TransCo co1 co2)   = goCo co1 `unionVarSet` goCo co2
-    goCo (NthCo _ _ co)      = goCo co
-    goCo (LRCo _ co)         = goCo co
-    goCo (InstCo co arg)     = goCo co `unionVarSet` goCo arg
-    goCo (KindCo co)         = goCo co
-    goCo (SubCo co)          = goCo co
-    goCo (AxiomRuleCo _ c)   = goCos c
-
-    goCos cos = foldr (unionVarSet . goCo) emptyVarSet cos
-
-    goProv UnsafeCoerceProv     = emptyVarSet
-    goProv (PhantomProv kco)    = goCo kco
-    goProv (ProofIrrelProv kco) = goCo kco
-    goProv (PluginProv _)       = emptyVarSet
-
-    goVar v = unitVarSet v `unionVarSet` go (varType v)
-
-exactTyCoVarsOfTypes :: [Type] -> TyVarSet
-exactTyCoVarsOfTypes tys = mapUnionVarSet exactTyCoVarsOfType tys
-
 anyRewritableTyVar :: Bool    -- Ignore casts and coercions
                    -> EqRel   -- Ambient role
                    -> (EqRel -> TcTyVar -> Bool)