hugolib: Fix dates for sections with dates in front matter

Fixes #5854

hugolib/page_test.go

@@ -505,6 +505,14 @@ date: 2017-01-15
 title: No Date
 ---
 
+`)
+
+	// https://github.com/gohugoio/hugo/issues/5854
+	b.WithSimpleConfigFile().WithContent("with-index-date/_index.md", `---
+title: Date
+date: 2018-01-15
+---
+
 `)
 
 	b.CreateSites().Build(BuildCfg{})
@@ -515,6 +523,7 @@ title: No Date
 	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())
+	assert.Equal(2018, s.getPage("/with-index-date").Date().Year())
 
 }
 

hugolib/site_sections.go

@@ -162,12 +162,18 @@ func (s *Site) assembleSections() pageStatePages {
 			if currentSection != nil {
 				// A new section
 				currentSection.setPages(children)
+				if dates != nil {
 					currentSection.m.Dates = *dates
 				}
+			}
 
 			currentSection = p
 			children = make(page.Pages, 0)
+			dates = nil
+			// Use section's dates from front matter if set.
+			if resource.IsZeroDates(currentSection) {
 				dates = &resource.Dates{}
+			}
 
 			return false
 
@@ -176,15 +182,18 @@ func (s *Site) assembleSections() pageStatePages {
 		// Regular page
 		p.parent = currentSection
 		children = append(children, p)
+		if dates != nil {
 			dates.UpdateDateAndLastmodIfAfter(p)
+		}
 
 		return false
 	})
 
 	if currentSection != nil {
 		currentSection.setPages(children)
+		if dates != nil {
 			currentSection.m.Dates = *dates
-
+		}
 	}
 
 	// Build the sections hierarchy