Postpone associated tyfam default checks until after typecheckingwip/T18648

Previously, associated type family defaults were validity-checked
during typechecking. Unfortunately, the error messages that these
checks produce run the risk of printing knot-tied type constructors,
which will cause GHC to diverge. In order to preserve the current
error message's descriptiveness, this patch postpones these validity
checks until after typechecking, which are now located in the new
function `GHC.Tc.Validity.checkValidAssocTyFamDeflt`.

Fixes #18648.

1 files changed, 13 insertions, 2 deletions

diff --git a/compiler/GHC/Core/Class.hs b/compiler/GHC/Core/Class.hs
index f5d24aaf3c..57e6defca6 100644
--- a/compiler/GHC/Core/Class.hs
+++ b/compiler/GHC/Core/Class.hs
@@ -8,7 +8,7 @@
 module GHC.Core.Class (
         Class,
         ClassOpItem,
-        ClassATItem(..),
+        ClassATItem(..), ATValidityInfo(..),
         ClassMinimalDef,
         DefMethInfo, pprDefMethInfo,
 
@@ -97,10 +97,21 @@ type DefMethInfo = Maybe (Name, DefMethSpec Type)
 
 data ClassATItem
   = ATI TyCon         -- See Note [Associated type tyvar names]
-        (Maybe (Type, SrcSpan))
+        (Maybe (Type, ATValidityInfo))
                       -- Default associated type (if any) from this template
                       -- Note [Associated type defaults]
 
+-- | Information about an associated type family default implementation. This
+-- is used solely for validity checking.
+-- See @Note [Type-checking default assoc decls]@ in "GHC.Tc.TyCl".
+data ATValidityInfo
+  = NoATVI               -- Used for associated type families that are imported
+                         -- from another module, for which we don't need to
+                         -- perform any validity checking.
+
+  | ATVI SrcSpan [Type]  -- Used for locally defined associated type families.
+                         -- The [Type] are the LHS patterns.
+
 type ClassMinimalDef = BooleanFormula Name -- Required methods
 
 data ClassBody