Infer array rest as tuple if possible

src/compiler/checker.ts

@@ -4555,11 +4555,11 @@ namespace ts {
                 const elementType = checkIteratedTypeOrElementType(parentType, pattern, /*allowStringInput*/ false, /*allowAsyncIterables*/ false);
                 const index = pattern.elements.indexOf(declaration);
                 if (declaration.dotDotDotToken) {
-                    // If the parent is a tuple type, the rest element has an array type with a union of the
+                    // If the parent is a tuple type, the rest element has a tuple type of the
                     // remaining tuple element types. Otherwise, the rest element has an array type with same
                     // element type as the parent type.
                     type = isTupleType(parentType) ?
-                        getArrayLiteralType((parentType.typeArguments || emptyArray).slice(index, getTypeReferenceArity(parentType))) :
+                        sliceTupleType(parentType, index) :
                         createArrayType(elementType);
                 }
                 else {
@@ -8526,6 +8526,20 @@ namespace ts {
             return links.resolvedType;
         }
 
+        function sliceTupleType(type: TupleTypeReference, index: number) {
+            const tuple = type.target;
+            if (tuple.hasRestElement) {
+                // don't slice off rest element
+                index = Math.min(index, getTypeReferenceArity(type) - 1);
+            }
+            return createTupleType(
+                (type.typeArguments || emptyArray).slice(index),
+                Math.max(0, tuple.minLength - index),
+                tuple.hasRestElement,
+                tuple.associatedNames && tuple.associatedNames.slice(index),
+            );
+        }
+
         function getTypeFromOptionalTypeNode(node: OptionalTypeNode): Type {
             const type = getTypeFromTypeNode(node.type);
             return strictNullChecks ? getOptionalType(type) : type;
@@ -21052,7 +21066,7 @@ namespace ts {
                         else {
                             checkGrammarForDisallowedTrailingComma(node.elements, Diagnostics.A_rest_parameter_or_binding_pattern_may_not_have_a_trailing_comma);
                             const type = isTupleType(sourceType) ?
-                                getArrayLiteralType((sourceType.typeArguments || emptyArray).slice(elementIndex, getTypeReferenceArity(sourceType))) :
+                                sliceTupleType(sourceType, elementIndex) :
                                 createArrayType(elementType);
                             return checkDestructuringAssignment(restExpression, type, checkMode);
                         }

tests/baselines/reference/declarationEmitDestructuring3.types

@@ -8,7 +8,7 @@ function bar([x, z, ...w]) { }
 function foo([x, ...y] = [1, "string", true]) { }
 >foo : ([x, ...y]?: [number, string, boolean]) => void
 >x : number
->y : (string | boolean)[]
+>y : [string, boolean]
 >[1, "string", true] : [number, string, boolean]
 >1 : 1
 >"string" : "string"

tests/baselines/reference/declarationEmitDestructuringArrayPattern4.js

@@ -22,10 +22,10 @@ var _f = [1, "hello", true], x19 = _f[0], y19 = _f[1], z19 = _f[2], a13 = _f.sli
 
 //// [declarationEmitDestructuringArrayPattern4.d.ts]
 declare var a5: number[];
-declare var x14: number, a6: number[];
-declare var x15: number, y15: number, a7: number[];
-declare var x16: number, y16: number, z16: number, a8: any[];
+declare var x14: number, a6: [number, number];
+declare var x15: number, y15: number, a7: [number];
+declare var x16: number, y16: number, z16: number, a8: [];
 declare var a9: (string | number | boolean)[];
-declare var x17: number, a10: (string | boolean)[];
-declare var x18: number, y18: string, a12: boolean[];
-declare var x19: number, y19: string, z19: boolean, a13: any[];
+declare var x17: number, a10: [string, boolean];
+declare var x18: number, y18: string, a12: [boolean];
+declare var x19: number, y19: string, z19: boolean, a13: [];

tests/baselines/reference/declarationEmitDestructuringArrayPattern4.types

@@ -8,7 +8,7 @@ var [...a5] = [1, 2, 3];
 
 var [x14, ...a6] = [1, 2, 3];
 >x14 : number
->a6 : number[]
+>a6 : [number, number]
 >[1, 2, 3] : [number, number, number]
 >1 : 1
 >2 : 2
@@ -17,7 +17,7 @@ var [x14, ...a6] = [1, 2, 3];
 var [x15, y15, ...a7] = [1, 2, 3];
 >x15 : number
 >y15 : number
->a7 : number[]
+>a7 : [number]
 >[1, 2, 3] : [number, number, number]
 >1 : 1
 >2 : 2
@@ -27,7 +27,7 @@ var [x16, y16, z16, ...a8] = [1, 2, 3];
 >x16 : number
 >y16 : number
 >z16 : number
->a8 : any[]
+>a8 : []
 >[1, 2, 3] : [number, number, number]
 >1 : 1
 >2 : 2
@@ -42,7 +42,7 @@ var [...a9] = [1, "hello", true];
 
 var [x17, ...a10] = [1, "hello", true];
 >x17 : number
->a10 : (string | boolean)[]
+>a10 : [string, boolean]
 >[1, "hello", true] : [number, string, boolean]
 >1 : 1
 >"hello" : "hello"
@@ -51,7 +51,7 @@ var [x17, ...a10] = [1, "hello", true];
 var [x18, y18, ...a12] = [1, "hello", true];
 >x18 : number
 >y18 : string
->a12 : boolean[]
+>a12 : [boolean]
 >[1, "hello", true] : [number, string, boolean]
 >1 : 1
 >"hello" : "hello"
@@ -61,7 +61,7 @@ var [x19, y19, z19, ...a13] = [1, "hello", true];
 >x19 : number
 >y19 : string
 >z19 : boolean
->a13 : any[]
+>a13 : []
 >[1, "hello", true] : [number, string, boolean]
 >1 : 1
 >"hello" : "hello"

tests/baselines/reference/declarationsAndAssignments.errors.txt

@@ -23,9 +23,15 @@ tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(106,5):
       Property 'x' is missing in type '{ y: false; }'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(138,6): error TS2322: Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(138,9): error TS2322: Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(158,16): error TS2403: Subsequent variable declarations must have the same type.  Variable 'a' must be of type 'number[]', but here has type '[number, number]'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(159,19): error TS2403: Subsequent variable declarations must have the same type.  Variable 'a' must be of type 'number[]', but here has type '[number]'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(160,22): error TS2403: Subsequent variable declarations must have the same type.  Variable 'a3' must be of type 'any[]', but here has type '[]'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(176,16): error TS2403: Subsequent variable declarations must have the same type.  Variable 'a1' must be of type '(string | boolean)[]', but here has type '[string, boolean]'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(177,19): error TS2403: Subsequent variable declarations must have the same type.  Variable 'a2' must be of type 'boolean[]', but here has type '[boolean]'.
+tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(178,22): error TS2403: Subsequent variable declarations must have the same type.  Variable 'a3' must be of type 'any[]', but here has type '[]'.
 
 
-==== tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts (22 errors) ====
+==== tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts (28 errors) ====
     function f0() {
         var [] = [1, "hello"];
         var [x] = [1, "hello"];
@@ -231,8 +237,14 @@ tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(138,9):
         var a3: any[];
         var [...a] = [1, 2, 3];
         var [x, ...a] = [1, 2, 3];
+                   ~
+!!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'a' must be of type 'number[]', but here has type '[number, number]'.
         var [x, y, ...a] = [1, 2, 3];
+                      ~
+!!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'a' must be of type 'number[]', but here has type '[number]'.
         var [x, y, z, ...a3] = [1, 2, 3];
+                         ~~
+!!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'a3' must be of type 'any[]', but here has type '[]'.
         [...a] = [1, 2, 3];
         [x, ...a] = [1, 2, 3];
         [x, y, ...a] = [1, 2, 3];
@@ -249,8 +261,14 @@ tests/cases/conformance/es6/destructuring/declarationsAndAssignments.ts(138,9):
         var a3: any[];
         var [...a0] = [1, "hello", true];
         var [x, ...a1] = [1, "hello", true];
+                   ~~
+!!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'a1' must be of type '(string | boolean)[]', but here has type '[string, boolean]'.
         var [x, y, ...a2] = [1, "hello", true];
+                      ~~
+!!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'a2' must be of type 'boolean[]', but here has type '[boolean]'.
         var [x, y, z, ...a3] = [1, "hello", true];
+                         ~~
+!!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'a3' must be of type 'any[]', but here has type '[]'.
         [...a0] = [1, "hello", true];
         [x, ...a1] = [1, "hello", true];
         [x, y, ...a2] = [1, "hello", true];

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment1ES5.types

@@ -148,7 +148,7 @@ var [,,,...c10] = [1, 2, 3, 4, "hello"];
 > : undefined
 > : undefined
 > : undefined
->c10 : (string | number)[]
+>c10 : [number, string]
 >[1, 2, 3, 4, "hello"] : [number, number, number, number, string]
 >1 : 1
 >2 : 2
@@ -159,7 +159,7 @@ var [,,,...c10] = [1, 2, 3, 4, "hello"];
 var [c11, c12, ...c13] = [1, 2, "string"];
 >c11 : number
 >c12 : number
->c13 : string[]
+>c13 : [string]
 >[1, 2, "string"] : [number, number, string]
 >1 : 1
 >2 : 2

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment1ES5iterable.types

@@ -148,7 +148,7 @@ var [,,,...c10] = [1, 2, 3, 4, "hello"];
 > : undefined
 > : undefined
 > : undefined
->c10 : (string | number)[]
+>c10 : [number, string]
 >[1, 2, 3, 4, "hello"] : [number, number, number, number, string]
 >1 : 1
 >2 : 2
@@ -159,7 +159,7 @@ var [,,,...c10] = [1, 2, 3, 4, "hello"];
 var [c11, c12, ...c13] = [1, 2, "string"];
 >c11 : number
 >c12 : number
->c13 : string[]
+>c13 : [string]
 >[1, 2, "string"] : [number, number, string]
 >1 : 1
 >2 : 2

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment1ES6.types

@@ -148,7 +148,7 @@ var [,,,...c10] = [1, 2, 3, 4, "hello"];
 > : undefined
 > : undefined
 > : undefined
->c10 : (string | number)[]
+>c10 : [number, string]
 >[1, 2, 3, 4, "hello"] : [number, number, number, number, string]
 >1 : 1
 >2 : 2
@@ -159,7 +159,7 @@ var [,,,...c10] = [1, 2, 3, 4, "hello"];
 var [c11, c12, ...c13] = [1, 2, "string"];
 >c11 : number
 >c12 : number
->c13 : string[]
+>c13 : [string]
 >[1, 2, "string"] : [number, number, string]
 >1 : 1
 >2 : 2

tests/baselines/reference/destructuringTuple.js

@@ -0,0 +1,14 @@
+//// [destructuringTuple.ts]
+declare var tuple: [boolean, number, ...string[]];
+
+const [a, b, c, ...rest] = tuple;
+
+declare var receiver: typeof tuple;
+
+[...receiver] = tuple;
+
+
+//// [destructuringTuple.js]
+"use strict";
+var a = tuple[0], b = tuple[1], c = tuple[2], rest = tuple.slice(3);
+receiver = tuple.slice(0);

tests/baselines/reference/destructuringTuple.symbols

@@ -0,0 +1,19 @@
+=== tests/cases/compiler/destructuringTuple.ts ===
+declare var tuple: [boolean, number, ...string[]];
+>tuple : Symbol(tuple, Decl(destructuringTuple.ts, 0, 11))
+
+const [a, b, c, ...rest] = tuple;
+>a : Symbol(a, Decl(destructuringTuple.ts, 2, 7))
+>b : Symbol(b, Decl(destructuringTuple.ts, 2, 9))
+>c : Symbol(c, Decl(destructuringTuple.ts, 2, 12))
+>rest : Symbol(rest, Decl(destructuringTuple.ts, 2, 15))
+>tuple : Symbol(tuple, Decl(destructuringTuple.ts, 0, 11))
+
+declare var receiver: typeof tuple;
+>receiver : Symbol(receiver, Decl(destructuringTuple.ts, 4, 11))
+>tuple : Symbol(tuple, Decl(destructuringTuple.ts, 0, 11))
+
+[...receiver] = tuple;
+>receiver : Symbol(receiver, Decl(destructuringTuple.ts, 4, 11))
+>tuple : Symbol(tuple, Decl(destructuringTuple.ts, 0, 11))
+

tests/baselines/reference/destructuringTuple.types

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/destructuringTuple.ts ===
+declare var tuple: [boolean, number, ...string[]];
+>tuple : [boolean, number, ...string[]]
+
+const [a, b, c, ...rest] = tuple;
+>a : boolean
+>b : number
+>c : string
+>rest : string[]
+>tuple : [boolean, number, ...string[]]
+
+declare var receiver: typeof tuple;
+>receiver : [boolean, number, ...string[]]
+>tuple : [boolean, number, ...string[]]
+
+[...receiver] = tuple;
+>[...receiver] = tuple : [boolean, number, ...string[]]
+>[...receiver] : (string | number | boolean)[]
+>...receiver : string | number | boolean
+>receiver : [boolean, number, ...string[]]
+>tuple : [boolean, number, ...string[]]
+

tests/baselines/reference/destructuringVariableDeclaration2.types

@@ -59,7 +59,7 @@ var [c1, c2, { c3: c4, c5 }, , ...c6] = [1, 2, { c3: 4, c5: 0 }];  // Error
 >c4 : number
 >c5 : number
 > : undefined
->c6 : any[]
+>c6 : []
 >[1, 2, { c3: 4, c5: 0 }] : [number, number, { c3: number; c5: number; }, undefined?]
 >1 : 1
 >2 : 2

tests/baselines/reference/restElementMustBeLast.types

@@ -1,6 +1,6 @@
 === tests/cases/conformance/types/rest/restElementMustBeLast.ts ===
 var [...a, x] = [1, 2, 3];  // Error, rest must be last element
->a : number[]
+>a : [number, number, number]
 >x : number
 >[1, 2, 3] : [number, number, number]
 >1 : 1
@@ -11,7 +11,7 @@ var [...a, x] = [1, 2, 3];  // Error, rest must be last element
 >[...a, x] = [1, 2, 3] : number[]
 >[...a, x] : number[]
 >...a : number
->a : number[]
+>a : [number, number, number]
 >x : number
 >[1, 2, 3] : number[]
 >1 : 1

tests/baselines/reference/restElementWithAssignmentPattern4.errors.txt

@@ -1,14 +1,9 @@
-tests/cases/conformance/es6/destructuring/restElementWithAssignmentPattern4.ts(3,10): error TS2322: Type 'string | number' is not assignable to type 'string'.
-  Type 'number' is not assignable to type 'string'.
-tests/cases/conformance/es6/destructuring/restElementWithAssignmentPattern4.ts(3,18): error TS2459: Type '(string | number)[]' has no property 'b' and no string index signature.
+tests/cases/conformance/es6/destructuring/restElementWithAssignmentPattern4.ts(3,18): error TS2459: Type '[string, number]' has no property 'b' and no string index signature.
 
 
-==== tests/cases/conformance/es6/destructuring/restElementWithAssignmentPattern4.ts (2 errors) ====
+==== tests/cases/conformance/es6/destructuring/restElementWithAssignmentPattern4.ts (1 errors) ====
     var a: string, b: number;
     var tuple: [string, number] = ["", 1];
     [...{ 0: a = "", b }] = tuple;
-             ~
-!!! error TS2322: Type 'string | number' is not assignable to type 'string'.
-!!! error TS2322:   Type 'number' is not assignable to type 'string'.
                      ~
-!!! error TS2459: Type '(string | number)[]' has no property 'b' and no string index signature.
+!!! error TS2459: Type '[string, number]' has no property 'b' and no string index signature.