diff --git a/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java b/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java
index 27d75405018..a6fde67a74f 100644
--- a/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java
+++ b/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java
@@ -7351,4 +7351,17 @@ public class AST2TemplateTests extends AST2TestBase {
 	public void testVariadicNonTypeTemplateParameter_401400() throws Exception {
 		parseAndCheckBindings();
 	}
+	
+	//	template <typename... Args>
+	//	struct foo {
+	//		static constexpr int i = sizeof...(Args);
+	//	};
+	//	constexpr int bar = foo<int, double>::i;
+	public void testSizeofParameterPackOnTypeid_401973() throws Exception {
+		BindingAssertionHelper helper = new BindingAssertionHelper(getAboveComment(), true);
+		ICPPVariable bar = helper.assertNonProblem("bar", ICPPVariable.class);
+		Long barValue = bar.getInitialValue().numericalValue();
+		assertNotNull(barValue);
+		assertEquals(2,  barValue.longValue());
+	}
 }
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/core/dom/ast/IASTTypeIdExpression.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/core/dom/ast/IASTTypeIdExpression.java
index f2f0e0a4fb5..3c76114dcb5 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/core/dom/ast/IASTTypeIdExpression.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/core/dom/ast/IASTTypeIdExpression.java
@@ -145,7 +145,13 @@ public interface IASTTypeIdExpression extends IASTExpression {
 	 * @since 5.5
 	 */
 	public static final int op_is_trivial= 21;
-			
+
+	/**
+	 * <code>op_sizeofParameterPack</code> is used for sizeof... ( typeid ) expressions.
+	 * @since 5.5
+	 */
+	public static final int op_sizeofParameterPack = 22;
+	
 	/**
 	 * @deprecated constants should be declared here, to avoid using the same constant in different
 	 * interfaces.
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/GNUCPPSourceParser.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/GNUCPPSourceParser.java
index ccd86e57d21..8289a2fceb1 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/GNUCPPSourceParser.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/GNUCPPSourceParser.java
@@ -1279,19 +1279,9 @@ public class GNUCPPSourceParser extends AbstractGNUSourceCodeParser {
         	if (LTcatchEOF(2) == IToken.tELLIPSIS) {
         		int offset= consume().getOffset(); 									// sizeof
         		consume();															// ...
-        		consume(IToken.tLPAREN);											// (
-        		IASTName id= identifier();
-        		IASTIdExpression idexpr= nodeFactory.newIdExpression(id);
-        		setRange(idexpr, id);
-        		IASTUnaryExpression expr= nodeFactory.newUnaryExpression(IASTUnaryExpression.op_sizeofParameterPack, idexpr);
-        		final int lt1= LT(1);
-        		if (lt1 == IToken.tEOC) {
-        			setRange(expr, offset, calculateEndOffset(id));
-        		} else {
-        			final int endOffset = consume(IToken.tRPAREN).getEndOffset();	// )
-					setRange(expr, offset, endOffset);
-        		}
-        		return expr;
+        		return parseTypeidInParenthesisOrUnaryExpression(true, offset, 
+        				IASTTypeIdExpression.op_sizeofParameterPack, 
+        				IASTUnaryExpression.op_sizeofParameterPack, ctx, strat);        		
         	}
         	return parseTypeidInParenthesisOrUnaryExpression(false, consume().getOffset(),
         			IASTTypeIdExpression.op_sizeof, IASTUnaryExpression.op_sizeof, ctx, strat);
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/EvalUnaryTypeID.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/EvalUnaryTypeID.java
index 417c079a99f..ca19b9a1255 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/EvalUnaryTypeID.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/EvalUnaryTypeID.java
@@ -32,6 +32,7 @@ import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_is_standard_l
 import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_is_trivial;
 import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_is_union;
 import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_sizeof;
+import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_sizeofParameterPack;
 import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_typeid;
 import static org.eclipse.cdt.core.dom.ast.IASTTypeIdExpression.op_typeof;
 
@@ -92,6 +93,8 @@ public class EvalUnaryTypeID extends CPPDependentEvaluation {
 	@Override
 	public boolean isValueDependent() {
 		switch (fOperator) {
+		case op_sizeofParameterPack:
+			return true;
 		case op_sizeof:
 		case op_alignof:
 		case op_has_nothrow_copy:
@@ -130,6 +133,7 @@ public class EvalUnaryTypeID extends CPPDependentEvaluation {
 	private IType computeType(IASTNode point) {
 		switch (fOperator) {
 		case op_sizeof:
+		case op_sizeofParameterPack:
 		case op_alignof:
 			return CPPVisitor.get_SIZE_T(point);
 		case op_typeid:
@@ -191,6 +195,14 @@ public class EvalUnaryTypeID extends CPPDependentEvaluation {
 	@Override
 	public ICPPEvaluation instantiate(ICPPTemplateParameterMap tpMap, int packOffset,
 			ICPPClassSpecialization within, int maxdepth, IASTNode point) {
+		if (fOperator == op_sizeofParameterPack) {
+			int packSize = determinePackSize(tpMap);
+			if (packSize == CPPTemplates.PACK_SIZE_FAIL || packSize == CPPTemplates.PACK_SIZE_NOT_FOUND) {
+				return EvalFixed.INCOMPLETE;
+			} else if (packSize != CPPTemplates.PACK_SIZE_DEFER) {
+				return new EvalFixed(getTypeOrFunctionSet(point), getValueCategory(point), Value.create(packSize));
+			}
+		}
 		IType type = CPPTemplates.instantiateType(fOrigType, tpMap, packOffset, within, point);
 		if (type == fOrigType)
 			return this;