tpl: Fix some baseof lookup issues

We were mistakingly using the templates (e.g. list.html) descriptor to resolve the base template and not the page,
which worked fine in most cases, but not all.

Fixes #13583

Author: bep

Diff for: tpl/templates/templates_integration_test.go

@@ -166,3 +166,31 @@ p3.current.Ancestors.Reverse: {{ with templates.Current }}{{ range .Ancestors.Re
 		"p2.current.Ancestors: _partials/p1.html|all.html",
 	)
 }
+
+func TestBaseOfIssue13583(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- content/_index.md --
+---
+title: "Home"
+outputs: ["html", "amp"]
+---
+title: "Home"
+-- layouts/baseof.html --
+layouts/baseof.html
+{{ block "main" . }}{{ end }}
+-- layouts/baseof.amp.html --
+layouts/baseof.amp.html
+{{ block "main" . }}{{ end }}
+-- layouts/home.html --
+{{ define "main" }}
+Home.
+{{ end }}
+
+`
+	b := hugolib.Test(t, files)
+	b.AssertFileContent("public/index.html", "layouts/baseof.html")
+	b.AssertFileContent("public/amp/index.html", "layouts/baseof.amp.html")
+}

Diff for: tpl/tplimpl/templatestore.go

@@ -301,7 +301,7 @@ func (ti *TemplInfo) String() string {
 	return ti.PathInfo.String()
 }
 
-func (ti *TemplInfo) findBestMatchBaseof(s *TemplateStore, k1 string, slashCountK1 int, best *bestMatch) {
+func (ti *TemplInfo) findBestMatchBaseof(s *TemplateStore, d1 TemplateDescriptor, k1 string, slashCountK1 int, best *bestMatch) {
 	if ti.baseVariants == nil {
 		return
 	}
@@ -310,11 +310,11 @@ func (ti *TemplInfo) findBestMatchBaseof(s *TemplateStore, k1 string, slashCount
 		slashCountK2 := strings.Count(k2, "/")
 		distance := slashCountK1 - slashCountK2
 
-		for d, vv := range v {
-			weight := s.dh.compareDescriptors(CategoryBaseof, ti.D, d)
+		for d2, vv := range v {
+			weight := s.dh.compareDescriptors(CategoryBaseof, d1, d2)
 			weight.distance = distance
 			if best.isBetter(weight, vv.Template) {
-				best.updateValues(weight, k2, d, vv.Template)
+				best.updateValues(weight, k2, d2, vv.Template)
 			}
 		}
 		return false, nil
@@ -538,7 +538,7 @@ func (s *TemplateStore) LookupPagesLayout(q TemplateQuery) *TemplInfo {
 		return m
 	}
 	best1.reset()
-	m.findBestMatchBaseof(s, key, slashCountKey, best1)
+	m.findBestMatchBaseof(s, q.Desc, key, slashCountKey, best1)
 	if best1.w.w1 <= 0 {
 		return nil
 	}