Consider class-provided citation engines

Do not attempt to load, nor allow to set, a different engine in this
case.

Fixes: #11022

src/BufferParams.cpp

@@ -2322,7 +2322,11 @@ bool BufferParams::writeLaTeX(otexstream & os, LaTeXFeatures & features,
 	}
 
 	// ... but before biblatex (see #7065)
-	if (features.mustProvide("biblatex")) {
+	if (features.mustProvide("biblatex")
+	    && !features.isProvided("biblatex-natbib")
+	    && !features.isProvided("natbib-internal")
+	    && !features.isProvided("natbib")
+	    && !features.isProvided("jurabib")) {
 		string delim = "";
 		string opts;
 		os << "\\usepackage";

src/LaTeXFeatures.cpp

@@ -1231,7 +1231,11 @@ string const LaTeXFeatures::getPackages() const
 	// Some classes load natbib themselves, but still allow (or even require)
 	// plain numeric citations (ReVTeX is such a case, see bug 5182).
 	// This special case is indicated by the "natbib-internal" key.
-	if (mustProvide("natbib") && !isProvided("natbib-internal")) {
+	if (mustProvide("natbib")
+	    && !isProvided("natbib-internal")
+	    && !isProvided("biblatex")
+	    && !isProvided("biblatex-natbib")
+	    && !isProvided("jurabib")) {
 		packages << "\\usepackage[";
 		if (params_.citeEngineType() == ENGINE_TYPE_NUMERICAL)
 			packages << "numbers";
@@ -1250,7 +1254,11 @@ string const LaTeXFeatures::getPackages() const
 	}
 
 	// jurabib -- we need version 0.6 at least.
-	if (mustProvide("jurabib")) {
+	if (mustProvide("jurabib")
+	    && !isProvided("natbib-internal")
+	    && !isProvided("natbib")
+	    && !isProvided("biblatex")
+	    && !isProvided("biblatex-natbib")) {
 		packages << "\\usepackage";
 		if (!params_.biblio_opts.empty())
 			packages << '[' << params_.biblio_opts << ']';

src/frontends/qt4/GuiDocument.cpp

@@ -2445,6 +2445,28 @@ void GuiDocument::biblioChanged()
 }
 
 
+void GuiDocument::checkPossibleCiteEngines()
+{
+	// Check if the class provides a specific engine,
+	// and if so, enforce this.
+	string force_engine;
+	if (documentClass().provides("natbib")
+	    || documentClass().provides("natbib-internal"))
+		force_engine = "natbib";
+	else if (documentClass().provides("jurabib"))
+		force_engine = "jurabib";
+	else if (documentClass().provides("biblatex"))
+		force_engine = "biblatex";
+	else if (documentClass().provides("biblatex-natbib"))
+		force_engine = "biblatex-natbib";
+
+	if (!force_engine.empty())
+		biblioModule->citeEngineCO->setCurrentIndex(
+			biblioModule->citeEngineCO->findData(toqstr(force_engine)));
+	biblioModule->citeEngineCO->setEnabled(force_engine.empty());
+}
+
+
 void GuiDocument::rescanBibFiles()
 {
 	if (isBiblatex())
@@ -3326,6 +3348,8 @@ void GuiDocument::paramsToDialog()
 	updateEngineType(documentClass().opt_enginetype(),
 		bp_.citeEngineType());
 
+	checkPossibleCiteEngines();
+
 	biblioModule->citeStyleCO->setCurrentIndex(
 		biblioModule->citeStyleCO->findData(bp_.citeEngineType()));
 

src/frontends/qt4/GuiDocument.h

@@ -296,6 +296,8 @@ private:
 	///
 	bool isBiblatex() const;
 	///
+	void checkPossibleCiteEngines();
+	///
 	BufferParams bp_;
 	/// List of names of available modules
 	std::list<modInfoStruct> moduleNames_;