Check generic-default method for ambiguity

Fixes Trac #7497 and #12151.   In some earlier upheaval I introduced
a bug in the ambiguity check for genreric-default method.

This patch fixes it.  But in fixing it I realised that the
sourc-location of any such error message was bogus, so I fixed
that too, which involved a slightly wider change; see the
comments with TcMethInfo.

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/vectorise/Vectorise/Type/TyConDecl.hs b/compiler/vectorise/Vectorise/Type/TyConDecl.hs
index a75391eca5..7aa79215d5 100644
--- a/compiler/vectorise/Vectorise/Type/TyConDecl.hs
+++ b/compiler/vectorise/Vectorise/Type/TyConDecl.hs
@@ -13,6 +13,8 @@ import Type
 import TyCon
 import DataCon
 import DynFlags
+import BasicTypes( DefMethSpec(..) )
+import SrcLoc( SrcSpan, noSrcSpan )
 import Var
 import Name
 import Outputable
@@ -124,6 +126,13 @@ vectMethod id defMeth ty
       ; return  (Var.varName id', ty', defMethSpecOfDefMeth defMeth)
       }
 
+-- | Convert a `DefMethInfo` to a `DefMethSpec`, which discards the name field in
+--   the `DefMeth` constructor of the `DefMeth`.
+defMethSpecOfDefMeth :: DefMethInfo -> Maybe (DefMethSpec (SrcSpan, Type))
+defMethSpecOfDefMeth Nothing = Nothing
+defMethSpecOfDefMeth (Just (_, VanillaDM))    = Just VanillaDM
+defMethSpecOfDefMeth (Just (_, GenericDM ty)) = Just (GenericDM (noSrcSpan, ty))
+
 -- |Vectorise the RHS of an algebraic type.
 --
 vectAlgTyConRhs :: TyCon -> AlgTyConRhs -> VM AlgTyConRhs