tag/Tag: add Merge() which takes Tag pointers

src/input/IcyInputStream.cxx

@@ -82,17 +82,7 @@ IcyInputStream::ReadTag() noexcept
 		/* no change */
 		return nullptr;
 
-	if (input_tag == nullptr && icy_tag == nullptr)
+	return Tag::Merge(input_tag.get(), icy_tag.get());
-		/* no tag */
-		return nullptr;
-
-	if (input_tag == nullptr)
-		return std::make_unique<Tag>(*icy_tag);
-
-	if (icy_tag == nullptr)
-		return std::make_unique<Tag>(*input_tag);
-
-	return Tag::MergePtr(*input_tag, *icy_tag);
 }
 
 size_t

src/tag/Tag.cxx

@@ -81,6 +81,22 @@ Tag::Merge(std::unique_ptr<Tag> base, std::unique_ptr<Tag> add) noexcept
 	return MergePtr(*base, *add);
 }
 
+std::unique_ptr<Tag>
+Tag::Merge(const Tag *base, const Tag *add) noexcept
+{
+	if (base == nullptr && add == nullptr)
+		/* no tag */
+		return nullptr;
+
+	if (base == nullptr)
+		return std::make_unique<Tag>(*add);
+
+	if (add == nullptr)
+		return std::make_unique<Tag>(*base);
+
+	return MergePtr(*base, *add);
+}
+
 const char *
 Tag::GetValue(TagType type) const noexcept
 {

src/tag/Tag.hxx

@@ -132,6 +132,15 @@ struct Tag {
 	static std::unique_ptr<Tag> Merge(std::unique_ptr<Tag> base,
 					  std::unique_ptr<Tag> add) noexcept;
 
+	/**
+	 * Merges the data from two tags.  Any of the two may be nullptr.
+	 *
+	 * @return a newly allocated tag (or nullptr if both
+	 * parameters are nullptr)
+	 */
+	static std::unique_ptr<Tag> Merge(const Tag *base,
+					  const Tag *add) noexcept;
+
 	/**
 	 * Returns the first value of the specified tag type, or
 	 * nullptr if none is present in this tag object.