lib.types.defaultTypeMerge: add 'loc' to default type merge

lib/modules.nix

@@ -11,6 +11,7 @@ let
     concatLists
     concatMap
     concatStringsSep
+    defaultTypeMerge
     elem
     filter
     foldl'
@@ -749,7 +750,7 @@ let
     foldl' (res: opt:
       let t  = res.type;
           t' = opt.options.type;
-          mergedType = t.typeMerge t'.functor;
+          mergedType = defaultTypeMerge loc t.functor t'.functor;
           typesMergeable = mergedType != null;
           typeSet = if (bothHave "type") && typesMergeable
                        then { type = mergedType; }

lib/types.nix

@@ -82,7 +82,7 @@ rec {
 
   # Default type merging function
   # takes two type functors and return the merged type
-  defaultTypeMerge = f: f':
+  defaultTypeMerge = loc: f: f':
     let mergedWrapped = f.wrapped.typeMerge f'.wrapped.functor;
         mergedPayload = f.binOp f.payload f'.payload;
 
@@ -103,6 +103,8 @@ rec {
           Use either `functor.payload` or `functor.wrapped` but not both.
 
           If your code worked before remove `functor.payload` from the type definition.
+
+          Location: ${showOption loc}
         ''
       else
         # Has payload
@@ -182,7 +184,7 @@ rec {
     , # Function that merge type declarations.
       # internal, takes a functor as argument and returns the merged type.
       # returning null means the type is not mergeable
-      typeMerge ? defaultTypeMerge functor
+      typeMerge ? defaultTypeMerge ["unknown location"] functor
     , # The type functor.
       # internal, representation of the type as an attribute set.
       #   name: name of the type