lang: Update the Build signature to return a type

This returns the type with the arg names we'll actually use. This is
helpful so we can pass values to the right places. We have named edges
so you can actually see what's going on.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/funcs/simplepoly/simplepoly.go

@@ -128,6 +128,8 @@ func consistentArgs(fns []*types.FuncValue) ([]string, error) {
 	return seq, nil
 }
 
+var _ interfaces.PolyFunc = &WrappedFunc{} // ensure it meets this expectation
+
 // WrappedFunc is a scaffolding function struct which fulfills the boiler-plate
 // for the function API, but that can run a very simple, static, pure,
 // polymorphic function.
@@ -478,23 +480,32 @@ func (obj *WrappedFunc) Polymorphisms(partialType *types.Type, partialValues []t
 // specific statically typed version. It is usually run after Unify completes,
 // and must be run before Info() and any of the other Func interface methods are
 // used.
-func (obj *WrappedFunc) Build(typ *types.Type) error {
+func (obj *WrappedFunc) Build(typ *types.Type) (*types.Type, error) {
 	// typ is the KindFunc signature we're trying to build...
 
 	index, err := langutil.FnMatch(typ, obj.Fns)
 	if err != nil {
-		return err
+		return nil, err
 	}
-	obj.buildFunction(typ, index) // found match at this index
+	newTyp := obj.buildFunction(typ, index) // found match at this index
 
-	return nil
+	return newTyp, nil
 }
 
 // buildFunction builds our concrete static function, from the potentially
 // abstract, possibly variant containing list of functions.
-func (obj *WrappedFunc) buildFunction(typ *types.Type, ix int) {
-	obj.fn = obj.Fns[ix].Copy().(*types.FuncValue)
-	obj.fn.T = typ.Copy() // overwrites any contained "variant" type
+func (obj *WrappedFunc) buildFunction(typ *types.Type, ix int) *types.Type {
+	cp := obj.Fns[ix].Copy()
+	fn, ok := cp.(*types.FuncValue)
+	if !ok {
+		panic("unexpected type")
+	}
+	obj.fn = fn
+	if obj.fn.T == nil { // XXX: should this even ever happen? What about argnames here?
+		obj.fn.T = typ.Copy() // overwrites any contained "variant" type
+	}
+
+	return obj.fn.T
 }
 
 // Validate makes sure we've built our struct properly. It is usually unused for