tpl: Resolve layouts/all.html for all html output formats

Fixes #13587

Author: bep

Diff for: tpl/templates/templates_integration_test.go

@@ -194,3 +194,23 @@ Home.
 	b.AssertFileContent("public/index.html", "layouts/baseof.html")
 	b.AssertFileContent("public/amp/index.html", "layouts/baseof.amp.html")
 }
+
+func TestAllVsAmp(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- content/_index.md --
+---
+title: "Home"
+outputs: ["html", "amp"]
+---
+title: "Home"
+-- layouts/all.html --
+All.
+
+`
+	b := hugolib.Test(t, files)
+	b.AssertFileContent("public/index.html", "All.")
+	b.AssertFileContent("public/amp/index.html", "All.")
+}

Diff for: tpl/tplimpl/templatedescriptor.go

@@ -92,6 +92,7 @@ func (this TemplateDescriptor) doCompare(category Category, other TemplateDescri
 	if other.Kind != "" && other.Kind != this.Kind {
 		return w
 	}
+
 	if other.Layout != "" && other.Layout != layoutAll && other.Layout != this.Layout {
 		if isLayoutCustom(this.Layout) {
 			if this.Kind == "" {
@@ -108,6 +109,7 @@ func (this TemplateDescriptor) doCompare(category Category, other TemplateDescri
 			return w
 		}
 	}
+
 	if other.Lang != "" && other.Lang != this.Lang {
 		return w
 	}
@@ -120,7 +122,7 @@ func (this TemplateDescriptor) doCompare(category Category, other TemplateDescri
 		// We want e.g. home page in amp output format (media type text/html) to
 		// find a template even if one isn't specified for that output format,
 		// when one exist for the html output format (same media type).
-		if category != CategoryBaseof && (this.Kind == "" || (this.Kind != other.Kind && this.Layout != other.Layout)) {
+		if category != CategoryBaseof && (this.Kind == "" || (this.Kind != other.Kind && (this.Layout != other.Layout && other.Layout != layoutAll))) {
 			return w
 		}