Properly persist deduction guide templates in index

Turned out that index is missing information about template parameters of
deduction guide templates, and all affected names which are only available
via index could not be resolved. This happens to e.g. std::map<> which is
usually looked up via index populated from <map> header file.

Fix this by implementing ICPPTemplateParameterOwner and ICPPTemplateDefinition
interfaces in new CPPDeductionGuideTemplate which delegates missing resolution
calls to the function object which already carry required template information.

Closes #438

core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/CPPDeductionGuideTemplate.java

@@ -0,0 +1,47 @@
+/*******************************************************************************
+ * Copyright (c) 2023 Igor V. Kovalenko.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Igor V. Kovalenko - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.cdt.internal.core.dom.parser.cpp;
+
+import org.eclipse.cdt.core.dom.ast.IASTDeclarator;
+import org.eclipse.cdt.core.dom.ast.IBinding;
+import org.eclipse.cdt.core.dom.ast.cpp.ICPPFunction;
+import org.eclipse.cdt.core.dom.ast.cpp.ICPPTemplateDefinition;
+import org.eclipse.cdt.core.dom.ast.cpp.ICPPTemplateParameter;
+
+/**
+ * Represents c++17 deduction guide template.
+ */
+public class CPPDeductionGuideTemplate extends CPPDeductionGuide
+		implements ICPPTemplateDefinition, ICPPTemplateParameterOwner {
+
+	public CPPDeductionGuideTemplate(IASTDeclarator fnDecl, ICPPFunction functionBinding) {
+		super(fnDecl, functionBinding);
+	}
+
+	@Override
+	public ICPPTemplateParameter[] getTemplateParameters() {
+		if (functionBinding instanceof ICPPTemplateDefinition template) {
+			return template.getTemplateParameters();
+		}
+		return null;
+	}
+
+	@Override
+	public IBinding resolveTemplateParameter(ICPPTemplateParameter param) {
+		if (functionBinding instanceof ICPPTemplateParameterOwner owner) {
+			return owner.resolveTemplateParameter(param);
+		}
+		return null;
+	}
+}

core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/CPPVisitor.java

@@ -214,6 +214,7 @@
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPConstructor;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPConstructorTemplate;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPDeductionGuide;
+import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPDeductionGuideTemplate;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPEnumeration;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPEnumerator;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPField;
@@ -999,7 +1000,8 @@ private static IBinding createBinding(IASTDeclarator declarator) {
 			}
 
 			if (isDeductionGuide) {
-				binding = new CPPDeductionGuide(typeRelevantDtor, (ICPPFunction) binding);
+				binding = template ? new CPPDeductionGuideTemplate(typeRelevantDtor, (ICPPFunction) binding)
+						: new CPPDeductionGuide(typeRelevantDtor, (ICPPFunction) binding);
 			} else {
 				binding = CPPSemantics.checkDeclSpecifier(binding, name, parent);
 

core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/dom/cpp/PDOMCPPLinkage.java

@@ -853,7 +853,7 @@ PDOMBinding createBinding(PDOMNode parent, IBinding binding, long fileLocalRec)
 				pdomBinding = new PDOMCPPMethod(this, parent, (ICPPMethod) binding);
 			}
 		} else if (binding instanceof ICPPDeductionGuide guide) {
-			if (guide.getFunctionBinding() instanceof ICPPFunctionTemplate) {
+			if (guide instanceof ICPPTemplateDefinition) {
 				pdomBinding = new PDOMCPPDeductionGuideTemplate(this, parent, guide);
 			} else {
 				pdomBinding = new PDOMCPPDeductionGuide(this, parent, guide);
@@ -1145,7 +1145,7 @@ else if (binding instanceof ICPPTemplateNonTypeParameter)
 			// this must be before functions
 			return CPPMETHOD;
 		} else if (binding instanceof ICPPDeductionGuide guide) {
-			if (guide.getFunctionBinding() instanceof ICPPFunctionTemplate) {
+			if (guide instanceof ICPPTemplateDefinition) {
 				return CPP_DEDUCTION_GUIDE_TEMPLATE;
 			} else {
 				return CPP_DEDUCTION_GUIDE;