resolves #891 allow backend to delegate to a registered backend using the syntax synthetic:delegate (PR #3100)

4 files changed, 28 insertions, 12 deletions

diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 98561088..40f8557c 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -63,6 +63,7 @@ Enhancements / Compliance::
   * drop verse table cell style (treat as normal table cell) (#3111)
   * log warning if footnoteref macro is found and compat mode is not enabled (#3114)
   * log debug message instead of warning if block style is unknown (#3092)
+  * allow backend to delegate to a registered backend using the syntax synthetic:delegate when using custom templates (e.g., slides:html) (#891)
 
 Improvements::
 
diff --git a/lib/asciidoctor/converter.rb b/lib/asciidoctor/converter.rb
index 91703a29..fbb452c3 100644
--- a/lib/asciidoctor/converter.rb
+++ b/lib/asciidoctor/converter.rb
@@ -213,22 +213,26 @@ module Converter
     # found, the built-in converter is returned or nil if no converter is found.
     #
     # backend - the String backend name.
-    # opts    - an optional Hash of options that get passed on to the converter's constructor. If the :template_dirs
-    #           key is found in the options Hash, this method returns a {CompositeConverter} that delegates to a
-    #           {TemplateConverter}. (optional, default: {})
+    # opts    - a Hash of options to customize creation; also passed to the converter's constructor:
+    #           :template_dirs - a String Array of directories used to instantiate a {TemplateConverter} (optional).
+    #           :delegate_backend - a backend String of the last converter in the {CompositeConverter} chain (optional).
     #
     # Returns the [Converter] instance.
     def create backend, opts = {}
-      template_dirs = opts[:template_dirs]
       if (converter = self.for backend)
         converter = converter.new backend, opts if ::Class === converter
-        if template_dirs && BackendTraits === converter && converter.supports_templates?
+        if (template_dirs = opts[:template_dirs]) && BackendTraits === converter && converter.supports_templates?
           CompositeConverter.new backend, (TemplateConverter.new backend, template_dirs, opts), converter, backend_traits_source: converter
         else
           converter
         end
-      elsif template_dirs
-        TemplateConverter.new backend, template_dirs, opts
+      elsif (template_dirs = opts[:template_dirs])
+        if (delegate_backend = opts[:delegate_backend]) && (converter = self.for delegate_backend)
+          converter = converter.new delegate_backend, opts if ::Class === converter
+          CompositeConverter.new backend, (TemplateConverter.new backend, template_dirs, opts), converter, backend_traits_source: converter
+        else
+          TemplateConverter.new backend, template_dirs, opts
+        end
       end
     end
 
diff --git a/lib/asciidoctor/document.rb b/lib/asciidoctor/document.rb
index 14afd9ad..5b48cf77 100644
--- a/lib/asciidoctor/document.rb
+++ b/lib/asciidoctor/document.rb
@@ -1116,7 +1116,7 @@ class Document < AbstractBlock
   # Internal: Create and initialize an instance of the converter for this document
   #--
   # QUESTION is there any additional information we should be passing to the converter?
-  def create_converter backend
+  def create_converter backend, delegate_backend
     converter_opts = { document: self, htmlsyntax: @attributes['htmlsyntax'] }
     if (template_dirs = (opts = @options)[:template_dirs] || opts[:template_dir])
       converter_opts[:template_dirs] = [*template_dirs]
@@ -1125,6 +1125,7 @@ class Document < AbstractBlock
       converter_opts[:template_engine_options] = opts[:template_engine_options]
       converter_opts[:eruby] = opts[:eruby]
       converter_opts[:safe] = @safe
+      converter_opts[:delegate_backend] = delegate_backend if delegate_backend
     end
     if (converter = opts[:converter])
       (Converter::CustomFactory.new backend => converter).create backend, converter_opts
@@ -1268,15 +1269,15 @@ class Document < AbstractBlock
       current_backend = @backend
       current_basebackend = (attrs = @attributes)['basebackend']
       current_doctype = @doctype
+      actual_backend, _, new_backend = new_backend.partition ':' if new_backend.include? ':'
       if new_backend.start_with? 'xhtml'
         attrs['htmlsyntax'] = 'xml'
         new_backend = new_backend.slice 1, new_backend.length
       elsif new_backend.start_with? 'html'
         attrs['htmlsyntax'] ||= 'html'
       end
-      if (resolved_backend = BACKEND_ALIASES[new_backend])
-        new_backend = resolved_backend
-      end
+      new_backend = BACKEND_ALIASES[new_backend] || new_backend
+      new_backend, delegate_backend = actual_backend, new_backend if actual_backend
       if current_doctype
         if current_backend
           attrs.delete %(backend-#{current_backend})
@@ -1291,7 +1292,7 @@ class Document < AbstractBlock
       # QUESTION should we defer the @backend assignment until after the converter is created?
       @backend = attrs['backend'] = new_backend
       # (re)initialize converter
-      if Converter::BackendTraits === (converter = create_converter new_backend)
+      if Converter::BackendTraits === (converter = create_converter new_backend, delegate_backend)
         new_basebackend = converter.basebackend
         new_filetype = converter.filetype
         if (htmlsyntax = converter.htmlsyntax)
diff --git a/test/converter_test.rb b/test/converter_test.rb
index 93db687d..57a7ddc7 100644
--- a/test/converter_test.rb
+++ b/test/converter_test.rb
@@ -154,6 +154,16 @@ context 'Converter' do
       assert_xpath '//aside/header/following-sibling::p[text()="Sidebar content"]', output, 1
     end
 
+    test 'should allow custom backend to emulate a known backend' do
+      doc = Asciidoctor.load 'content', backend: 'html5-tweaks:html', template_dir: (fixture_path 'custom-backends/haml'), template_cache: false
+      assert doc.basebackend? 'html'
+      assert_equal 'html5-tweaks', doc.backend
+      converter = doc.converter
+      assert_kind_of Asciidoctor::Converter::TemplateConverter, (converter.find_converter 'embedded')
+      refute_kind_of Asciidoctor::Converter::TemplateConverter, (converter.find_converter 'admonition')
+      assert_equal '<p>content</p>', doc.convert
+    end
+
     test 'should create template converter even when a converter is not registered for the specified backend' do
       input = 'paragraph content'
       output = convert_string_to_embedded input, backend: :unknown, template_dir: (fixture_path 'custom-backends/haml/html5-tweaks'), template_cache: false