Consistently error when rest element isn't last in tuple type

Author: ahejlsberg

src/compiler/checker.ts

@@ -31121,6 +31121,7 @@ namespace ts {
         function checkTupleType(node: TupleTypeNode) {
             const elementTypes = node.elements;
             let seenOptionalElement = false;
+            let seenRestElement = false;
             const hasNamedElement = some(elementTypes, isNamedTupleMember);
             for (let i = 0; i < elementTypes.length; i++) {
                 const e = elementTypes[i];
@@ -31130,16 +31131,17 @@ namespace ts {
                 }
                 const flags = getTupleElementFlags(e);
                 if (flags & ElementFlags.Variadic) {
-                    if (!isArrayLikeType(getTypeFromTypeNode((<RestTypeNode | NamedTupleMember>e).type))) {
+                    const type = getTypeFromTypeNode((<RestTypeNode | NamedTupleMember>e).type);
+                    if (!isArrayLikeType(type)) {
                         error(e, Diagnostics.A_rest_element_type_must_be_an_array_type);
                         break;
                     }
+                    if (isArrayType(type) || isTupleType(type) && type.target.combinedFlags & ElementFlags.Rest) {
+                        seenRestElement = true;
+                    }
                 }
                 else if (flags & ElementFlags.Rest) {
-                    if (i !== elementTypes.length - 1) {
-                        grammarErrorOnNode(e, Diagnostics.A_rest_element_must_be_last_in_a_tuple_type);
-                        break;
-                    }
+                    seenRestElement = true;
                 }
                 else if (flags & ElementFlags.Optional) {
                     seenOptionalElement = true;
@@ -31148,6 +31150,10 @@ namespace ts {
                     grammarErrorOnNode(e, Diagnostics.A_required_element_cannot_follow_an_optional_element);
                     break;
                 }
+                if (seenRestElement && i !== elementTypes.length - 1) {
+                    grammarErrorOnNode(e, Diagnostics.A_rest_element_must_be_last_in_a_tuple_type);
+                    break;
+                }
             }
             forEach(node.elements, checkSourceElement);
         }