Pad tuple type initializers of parameter array binding elements

Author: ahejlsberg

src/compiler/checker.ts

@@ -24233,10 +24233,13 @@ namespace ts {
         function checkDeclarationInitializer(declaration: HasExpressionInitializer) {
             const initializer = getEffectiveInitializer(declaration)!;
             const type = getTypeOfExpression(initializer, /*cache*/ true);
+            const padded = isParameter(declaration) && declaration.name.kind === SyntaxKind.ArrayBindingPattern &&
+                isTupleType(type) && !type.target.hasRestElement && getTypeReferenceArity(type) < declaration.name.elements.length ?
+                padTupleType(type, declaration.name) : type;
             const widened = getCombinedNodeFlags(declaration) & NodeFlags.Const ||
                 isDeclarationReadonly(declaration) ||
                 isTypeAssertion(initializer) ||
-                isLiteralOfContextualType(type, getContextualType(initializer)) ? type : getWidenedLiteralType(type);
+                isLiteralOfContextualType(padded, getContextualType(initializer)) ? padded : getWidenedLiteralType(padded);
             if (isInJSFile(declaration)) {
                 if (widened.flags & TypeFlags.Nullable) {
                     reportImplicitAny(declaration, anyType);
@@ -24250,6 +24253,22 @@ namespace ts {
             return widened;
         }
 
+        function padTupleType(type: TupleTypeReference, pattern: ArrayBindingPattern) {
+            const patternElements = pattern.elements;
+            const arity = getTypeReferenceArity(type);
+            const elementTypes = arity ? type.typeArguments!.slice() : [];
+            for (let i = arity; i < patternElements.length; i++) {
+                const e = patternElements[i];
+                if (i < patternElements.length - 1 || !(e.kind === SyntaxKind.BindingElement && (<BindingElement>e).dotDotDotToken)) {
+                    elementTypes.push(!isOmittedExpression(e) && hasDefaultValue(e) ? getTypeFromBindingElement(e, /*includePatternInType*/ false, /*reportErrors*/ false) : anyType);
+                    if (!isOmittedExpression(e) && !hasDefaultValue(e)) {
+                        reportImplicitAny(e, anyType);
+                    }
+                }
+            }
+            return createTupleType(elementTypes, type.target.minLength, /*hasRestElement*/ false, type.target.readonly);
+        }
+
         function isLiteralOfContextualType(candidateType: Type, contextualType: Type | undefined): boolean {
             if (contextualType) {
                 if (contextualType.flags & TypeFlags.UnionOrIntersection) {