tpl: Fix case issue in templates.Exists

Fixes #13684

tpl/templates/templates_integration_test.go

@@ -299,3 +299,19 @@ P2.
 	b := hugolib.Test(t, files)
 	b.AssertFileContent("public/index.html", "P1: P1.\nP2: foo bar")
 }
+
+func TestTemplateExistsCaseIssue13684(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/home.html --
+P1: {{ templates.Exists "_partials/MyPartial.html" }}|P1: {{ templates.Exists "_partials/mypartial.html" }}|
+-- layouts/_partials/MyPartial.html --
+MyPartial.
+
+`
+
+	b := hugolib.Test(t, files)
+	b.AssertFileContent("public/index.html", "P1: true|P1: true|")
+}

tpl/tplimpl/templatestore.go

@@ -712,6 +712,7 @@ func (s *TemplateStore) RefreshFiles(include func(fi hugofs.FileMetaInfo) bool)
 }
 
 func (s *TemplateStore) HasTemplate(templatePath string) bool {
+	templatePath = strings.ToLower(templatePath)
 	templatePath = paths.AddLeadingSlash(templatePath)
 	return s.templatesByPath.Contains(templatePath)
 }