Allow for return partials with falsy arguments (#9298)

Partials with returns values are parsed, then inserted into a
partial return wrapper via wrapInPartialReturnWrapper in order
to assign the return value via *contextWrapper.Set. The
predefined wrapper template for partials inserts a partial's nodes
into a "with" template action in order to set dot to a
*contextWrapper within the partial. However, because "with" is
skipped if its argument is falsy, partials with falsy arguments
were not being evaluated.

This replaces the "with" action in the partial wrapper with a
"range" action that isn't skipped if .Arg is falsy.

Fixes #7528

2 files changed, 10 insertions, 9 deletions

diff --git a/tpl/partials/partials.go b/tpl/partials/partials.go
index e8a8adc36..b0dc0a997 100644
--- a/tpl/partials/partials.go
+++ b/tpl/partials/partials.go
@@ -25,7 +25,6 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/gohugoio/hugo/common/hreflect"
 	texttemplate "github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -121,10 +120,6 @@ func (ns *Namespace) Include(name string, contextList ...interface{}) (interface
 	var w io.Writer
 
 	if info.HasReturn {
-		if !hreflect.IsTruthful(context) {
-			// TODO(bep) we need to fix this, but it is non-trivial.
-			return nil, errors.New("partial that returns a value needs a non-zero argument.")
-		}
 		// Wrap the context sent to the template to capture the return value.
 		// Note that the template is rewritten to make sure that the dot (".")
 		// and the $ variable points to Arg.
diff --git a/tpl/tplimpl/template_ast_transformers.go b/tpl/tplimpl/template_ast_transformers.go
index de9b5424f..33461dc7d 100644
--- a/tpl/tplimpl/template_ast_transformers.go
+++ b/tpl/tplimpl/template_ast_transformers.go
@@ -112,7 +112,11 @@ func getParseTree(templ tpl.Template) *parse.Tree {
 }
 
 const (
-	partialReturnWrapperTempl = `{{ $_hugo_dot := $ }}{{ $ := .Arg }}{{ with .Arg }}{{ $_hugo_dot.Set ("PLACEHOLDER") }}{{ end }}`
+	// We parse this template and modify the nodes in order to assign
+	// the return value of a partial to a contextWrapper via Set. We use
+	// "range" over a one-element slice so we can shift dot to the
+	// partial's argument, Arg, while allowing Arg to be falsy.
+	partialReturnWrapperTempl = `{{ $_hugo_dot := $ }}{{ $ := .Arg }}{{ range (slice .Arg) }}{{ $_hugo_dot.Set ("PLACEHOLDER") }}{{ end }}`
 )
 
 var partialReturnWrapper *parse.ListNode
@@ -125,16 +129,18 @@ func init() {
 	partialReturnWrapper = templ.Tree.Root
 }
 
+// wrapInPartialReturnWrapper copies and modifies the parsed nodes of a
+// predefined partial return wrapper to insert those of a user-defined partial.
 func (c *templateContext) wrapInPartialReturnWrapper(n *parse.ListNode) *parse.ListNode {
 	wrapper := partialReturnWrapper.CopyList()
-	withNode := wrapper.Nodes[2].(*parse.WithNode)
-	retn := withNode.List.Nodes[0]
+	rangeNode := wrapper.Nodes[2].(*parse.RangeNode)
+	retn := rangeNode.List.Nodes[0]
 	setCmd := retn.(*parse.ActionNode).Pipe.Cmds[0]
 	setPipe := setCmd.Args[1].(*parse.PipeNode)
 	// Replace PLACEHOLDER with the real return value.
 	// Note that this is a PipeNode, so it will be wrapped in parens.
 	setPipe.Cmds = []*parse.CommandNode{c.returnNode}
-	withNode.List.Nodes = append(n.Nodes, retn)
+	rangeNode.List.Nodes = append(n.Nodes, retn)
 
 	return wrapper
 }