hugolib: Fix default date assignment for sections

See #5784

hugolib/page_test.go

@@ -448,7 +448,7 @@ categories: ["cool stuff"]
 
 	b := newTestSitesBuilder(t)
 	b.WithSimpleConfigFile().WithContent("page.md", pageContent)
-	b.WithSimpleConfigFile().WithContent("blog/page.md", pageContent)
+	b.WithContent("blog/page.md", pageContent)
 
 	b.CreateSites().Build(BuildCfg{})
 
@@ -470,6 +470,34 @@ categories: ["cool stuff"]
 
 }
 
+func TestPageDatesSections(t *testing.T) {
+	t.Parallel()
+	assert := assert.New(t)
+
+	b := newTestSitesBuilder(t)
+	b.WithSimpleConfigFile().WithContent("no-index/page.md", `
+---
+title: Page
+date: 2017-01-15
+---
+`)
+	b.WithSimpleConfigFile().WithContent("with-index-no-date/_index.md", `---
+title: No Date
+---
+
+`)
+
+	b.CreateSites().Build(BuildCfg{})
+
+	assert.Equal(1, len(b.H.Sites))
+	s := b.H.Sites[0]
+
+	assert.Equal(2017, s.getPage("/").Date().Year())
+	assert.Equal(2017, s.getPage("/no-index").Date().Year())
+	assert.True(s.getPage("/with-index-no-date").Date().IsZero())
+
+}
+
 func TestCreateNewPage(t *testing.T) {
 	t.Parallel()
 	assertFunc := func(t *testing.T, ext string, pages page.Pages) {

hugolib/site_sections.go

@@ -162,6 +162,7 @@ func (s *Site) assembleSections() pageStatePages {
 			if currentSection != nil {
 				// A new section
 				currentSection.setPages(children)
+				currentSection.m.Dates = *dates
 			}
 
 			currentSection = p
@@ -176,6 +177,7 @@ func (s *Site) assembleSections() pageStatePages {
 		p.parent = currentSection
 		children = append(children, p)
 		dates.UpdateDateAndLastmodIfAfter(p)
+
 		return false
 	})