Fix bad interaction between withDict and the Specialiserwip/T21575

This MR fixes a bad bug, where the withDict was inlined too
vigorously, which in turn made the type-class Specialiser generate
a bogus specialisation, because it saw the same overloaded function
applied to two /different/ dictionaries.

Solution: inline `withDict` later.  See (WD8) of Note [withDict]
in GHC.HsToCore.Expr

See #21575, which is fixed by this change.

2 files changed, 5 insertions, 4 deletions

diff --git a/compiler/GHC/Core/Coercion.hs b/compiler/GHC/Core/Coercion.hs
index 235e8c65fb..98aa337026 100644
--- a/compiler/GHC/Core/Coercion.hs
+++ b/compiler/GHC/Core/Coercion.hs
@@ -1708,9 +1708,8 @@ has to do in its full generality.  See #18413.
 %************************************************************************
 -}
 
--- | If @co :: T ts ~ rep_ty@ then:
---
--- > instNewTyCon_maybe T ts = Just (rep_ty, co)
+-- | If `instNewTyCon_maybe T ts = Just (rep_ty, co)`
+--   then `co :: T ts ~R# rep_ty`
 --
 -- Checks for a newtype, and for being saturated
 instNewTyCon_maybe :: TyCon -> [Type] -> Maybe (Type, Coercion)
diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs
index e50751c2a0..6514b68aaf 100644
--- a/compiler/GHC/Core/TyCon.hs
+++ b/compiler/GHC/Core/TyCon.hs
@@ -1184,7 +1184,9 @@ data AlgTyConRhs
                         -- See Note [Newtype eta]
         nt_co :: CoAxiom Unbranched,
                              -- The axiom coercion that creates the @newtype@
-                             -- from the representation 'Type'.
+                             -- from the representation 'Type'.  The axiom witnesses
+                             -- a representational coercion:
+                             --   nt_co :: N ty1 ~R# rep_tys
 
                              -- See Note [Newtype coercions]
                              -- Invariant: arity = #tvs in nt_etad_rhs;