Simplify tuple destructuring logic

src/compiler/checker.ts

@@ -722,6 +722,7 @@ namespace ts {
             NoIndexSignatures = 1 << 0,
             Writing = 1 << 1,
             CacheSymbol = 1 << 2,
+            NoTupleBoundsCheck = 1 << 3,
         }
 
         const enum CallbackCheck {
@@ -5115,21 +5116,25 @@ namespace ts {
                 }
                 else if (isArrayLikeType(parentType)) {
                     const indexType = getLiteralType(index);
-                    const declaredType = getConstraintForLocation(getIndexedAccessType(parentType, indexType, declaration.name), declaration.name);
+                    const accessFlags = hasDefaultValue(declaration) ? AccessFlags.NoTupleBoundsCheck : 0;
+                    const declaredType = getConstraintForLocation(getIndexedAccessTypeOrUndefined(parentType, indexType, declaration.name, accessFlags) || errorType, declaration.name);
                     type = getFlowTypeOfDestructuring(declaration, declaredType);
                 }
                 else {
                     type = elementType;
                 }
             }
-            // In strict null checking mode, if a default value of a non-undefined type is specified, remove
-            // undefined from the final type.
-            if (strictNullChecks && declaration.initializer && !(getFalsyFlags(checkDeclarationInitializer(declaration)) & TypeFlags.Undefined)) {
-                type = getTypeWithFacts(type, TypeFacts.NEUndefined);
+            if (!declaration.initializer) {
+                return type;
             }
-            return declaration.initializer && !getEffectiveTypeAnnotationNode(walkUpBindingElementsAndPatterns(declaration)) ?
-                getUnionType([type, checkDeclarationInitializer(declaration)], UnionReduction.Subtype) :
-                type;
+            if (getEffectiveTypeAnnotationNode(walkUpBindingElementsAndPatterns(declaration))) {
+                // In strict null checking mode, if a default value of a non-undefined type is specified, remove
+                // undefined from the final type.
+                return strictNullChecks && !(getFalsyFlags(checkDeclarationInitializer(declaration)) & TypeFlags.Undefined) ?
+                    getTypeWithFacts(type, TypeFacts.NEUndefined) :
+                    type;
+            }
+            return getUnionType([getTypeWithFacts(type, TypeFacts.NEUndefined), checkDeclarationInitializer(declaration)], UnionReduction.Subtype);
         }
 
         function getTypeForDeclarationFromJSDocComment(declaration: Node) {
@@ -10131,7 +10136,7 @@ namespace ts {
                         propType;
                 }
                 if (everyType(objectType, isTupleType) && isNumericLiteralName(propName) && +propName >= 0) {
-                    if (accessNode && everyType(objectType, t => !(<TupleTypeReference>t).target.hasRestElement)) {
+                    if (accessNode && everyType(objectType, t => !(<TupleTypeReference>t).target.hasRestElement) && !(accessFlags & AccessFlags.NoTupleBoundsCheck)) {
                         const indexNode = getIndexNodeForAccessExpression(accessNode);
                         if (isTupleType(objectType)) {
                             error(indexNode, Diagnostics.Tuple_type_0_of_length_1_has_no_element_at_index_2,
@@ -19196,26 +19201,7 @@ namespace ts {
         function getArrayLiteralTupleTypeIfApplicable(elementTypes: Type[], contextualType: Type | undefined, hasRestElement: boolean, elementCount = elementTypes.length, readonly = false) {
             // Infer a tuple type when the contextual type is or contains a tuple-like type
             if (readonly || (contextualType && forEachType(contextualType, isTupleLikeType))) {
-                const minLength = elementCount - (hasRestElement ? 1 : 0);
-                const pattern = contextualType && contextualType.pattern;
-                // If array literal is contextually typed by a binding pattern or an assignment pattern, pad the resulting
-                // tuple type with the corresponding binding or assignment element types to make the lengths equal.
-                if (!hasRestElement && pattern && (pattern.kind === SyntaxKind.ArrayBindingPattern || pattern.kind === SyntaxKind.ArrayLiteralExpression)) {
-                    const patternElements = (<BindingPattern | ArrayLiteralExpression>pattern).elements;
-                    for (let i = elementCount; i < patternElements.length; i++) {
-                        const e = patternElements[i];
-                        if (hasDefaultValue(e)) {
-                            elementTypes.push((<TypeReference>contextualType).typeArguments![i]);
-                        }
-                        else if (i < patternElements.length - 1 || !(e.kind === SyntaxKind.BindingElement && (<BindingElement>e).dotDotDotToken || e.kind === SyntaxKind.SpreadElement)) {
-                            if (e.kind !== SyntaxKind.OmittedExpression) {
-                                error(e, Diagnostics.Initializer_provides_no_value_for_this_binding_element_and_the_binding_element_has_no_default_value);
-                            }
-                            elementTypes.push(strictNullChecks ? implicitNeverType : undefinedWideningType);
-                        }
-                    }
-                }
-                return createTupleType(elementTypes, minLength, hasRestElement, readonly);
+                return createTupleType(elementTypes, elementCount - (hasRestElement ? 1 : 0), hasRestElement, readonly);
             }
         }
 
@@ -23655,8 +23641,10 @@ namespace ts {
                     if (isArrayLikeType(sourceType)) {
                         // We create a synthetic expression so that getIndexedAccessType doesn't get confused
                         // when the element is a SyntaxKind.ElementAccessExpression.
-                        const elementType = getIndexedAccessType(sourceType, indexType, createSyntheticExpression(element, indexType));
-                        const type = getFlowTypeOfDestructuring(element, elementType);
+                        const accessFlags = hasDefaultValue(element) ? AccessFlags.NoTupleBoundsCheck : 0;
+                        const elementType = getIndexedAccessTypeOrUndefined(sourceType, indexType, createSyntheticExpression(element, indexType), accessFlags) || errorType;
+                        const assignedType = hasDefaultValue(element) ? getTypeWithFacts(elementType, TypeFacts.NEUndefined) : elementType;
+                        const type = getFlowTypeOfDestructuring(element, assignedType);
                         return checkDestructuringAssignment(element, type, checkMode);
                     }
                     return checkDestructuringAssignment(element, elementType, checkMode);
@@ -24245,10 +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);
@@ -24262,6 +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 && 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) {

tests/baselines/reference/declarationEmitDestructuringArrayPattern2.errors.txt

@@ -1,6 +1,6 @@
-tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,6): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
-tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,11): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
-tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,16): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,6): error TS2493: Tuple type '[]' of length '0' has no element at index '0'.
+tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,11): error TS2493: Tuple type '[]' of length '0' has no element at index '1'.
+tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,16): error TS2493: Tuple type '[]' of length '0' has no element at index '2'.
 
 
 ==== tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts (3 errors) ====
@@ -9,11 +9,11 @@ tests/cases/compiler/declarationEmitDestructuringArrayPattern2.ts(4,16): error T
     var [x11 = 0, y11 = ""] = [1, "hello"];
     var [a11, b11, c11] = [];
          ~~~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[]' of length '0' has no element at index '0'.
               ~~~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[]' of length '0' has no element at index '1'.
                    ~~~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[]' of length '0' has no element at index '2'.
 
     var [a2, [b2, { x12, y12: c2 }]=["abc", { x12: 10, y12: false }]] = [1, ["hello", { x12: 5, y12: true }]];
 

tests/baselines/reference/declarationEmitDestructuringArrayPattern2.js

@@ -22,7 +22,7 @@ var _m = [[x13, y13], { x: x13, y: y13 }], a3 = _m[0], b3 = _m[1];
 //// [declarationEmitDestructuringArrayPattern2.d.ts]
 declare var x10: number, y10: string, z10: boolean;
 declare var x11: number, y11: string;
-declare var a11: any, b11: any, c11: any;
+declare var a11: undefined, b11: undefined, c11: undefined;
 declare var a2: number, b2: string, x12: number, c2: boolean;
 declare var x13: number, y13: string;
 declare var a3: (string | number)[], b3: {

tests/baselines/reference/declarationEmitDestructuringArrayPattern2.types

@@ -20,10 +20,10 @@ var [x11 = 0, y11 = ""] = [1, "hello"];
 >"hello" : "hello"
 
 var [a11, b11, c11] = [];
->a11 : any
->b11 : any
->c11 : any
->[] : [undefined?, undefined?, undefined?]
+>a11 : undefined
+>b11 : undefined
+>c11 : undefined
+>[] : []
 
 var [a2, [b2, { x12, y12: c2 }]=["abc", { x12: 10, y12: false }]] = [1, ["hello", { x12: 5, y12: true }]];
 >a2 : number

tests/baselines/reference/declarationsAndAssignments.errors.txt

@@ -1,16 +1,16 @@
-tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(5,16): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(5,16): error TS2493: Tuple type '[number, string]' of length '2' has no element at index '2'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(22,17): error TS2353: Object literal may only specify known properties, and 'x' does not exist in type '{}'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(22,23): error TS2353: Object literal may only specify known properties, and 'y' does not exist in type '{}'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(23,25): error TS2353: Object literal may only specify known properties, and 'y' does not exist in type '{ x: any; }'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(24,19): error TS2353: Object literal may only specify known properties, and 'x' does not exist in type '{ y: any; }'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(28,28): error TS2353: Object literal may only specify known properties, and 'y' does not exist in type '{ x: any; }'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(29,22): error TS2353: Object literal may only specify known properties, and 'x' does not exist in type '{ y: any; }'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(58,9): error TS2403: Subsequent variable declarations must have the same type.  Variable 'y' must be of type 'string | number', but here has type 'string'.
-tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(62,10): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
-tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(62,13): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
-tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(62,16): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
-tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(63,13): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
-tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(63,16): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(62,10): error TS2493: Tuple type '[]' of length '0' has no element at index '0'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(62,13): error TS2493: Tuple type '[]' of length '0' has no element at index '1'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(62,16): error TS2493: Tuple type '[]' of length '0' has no element at index '2'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(63,13): error TS2493: Tuple type '[number]' of length '1' has no element at index '1'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(63,16): error TS2493: Tuple type '[number]' of length '1' has no element at index '2'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(67,9): error TS2461: Type '{}' is not an array type.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(68,9): error TS2461: Type '{ 0: number; 1: number; }' is not an array type.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(73,11): error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
@@ -29,7 +29,7 @@ tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(138,9):
         var [x, y] = [1, "hello"];
         var [x, y, z] = [1, "hello"];
                    ~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[number, string]' of length '2' has no element at index '2'.
         var [,, x] = [0, 1, 2];
         var x: number;
         var y: string;
@@ -103,16 +103,16 @@ tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(138,9):
     function f8() {
         var [a, b, c] = [];   // Error, [] is an empty tuple
              ~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[]' of length '0' has no element at index '0'.
                 ~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[]' of length '0' has no element at index '1'.
                    ~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[]' of length '0' has no element at index '2'.
         var [d, e, f] = [1];  // Error, [1] is a tuple
                 ~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[number]' of length '1' has no element at index '1'.
                    ~
-!!! error TS2525: Initializer provides no value for this binding element and the binding element has no default value.
+!!! error TS2493: Tuple type '[number]' of length '1' has no element at index '2'.
     }
 
     function f9() {

tests/baselines/reference/declarationsAndAssignments.types

@@ -23,8 +23,8 @@ function f0() {
     var [x, y, z] = [1, "hello"];
 >x : number
 >y : string
->z : any
->[1, "hello"] : [number, string, undefined?]
+>z : undefined
+>[1, "hello"] : [number, string]
 >1 : 1
 >"hello" : "hello"
 
@@ -255,16 +255,16 @@ function f8() {
 >f8 : () => void
 
     var [a, b, c] = [];   // Error, [] is an empty tuple
->a : any
->b : any
->c : any
->[] : [undefined?, undefined?, undefined?]
+>a : undefined
+>b : undefined
+>c : undefined
+>[] : []
 
     var [d, e, f] = [1];  // Error, [1] is a tuple
 >d : number
->e : any
->f : any
->[1] : [number, undefined?, undefined?]
+>e : undefined
+>f : undefined
+>[1] : [number]
 >1 : 1
 }