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diff --git a/‎src/compiler/checker.ts
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+3-2Lines changed: 3 additions & 2 deletions b/‎src/compiler/checker.ts
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+3-2Lines changed: 3 additions & 2 deletions
diff --git a/‎src/compiler/factory.ts
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+26Lines changed: 26 additions & 0 deletions b/‎src/compiler/factory.ts
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+26Lines changed: 26 additions & 0 deletions
diff --git a/‎src/compiler/transformers/es2015.ts
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+27-11Lines changed: 27 additions & 11 deletions b/‎src/compiler/transformers/es2015.ts
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+27-11Lines changed: 27 additions & 11 deletions
diff --git a/‎src/compiler/types.ts
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+7-6Lines changed: 7 additions & 6 deletions b/‎src/compiler/types.ts
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+7-6Lines changed: 7 additions & 6 deletions
diff --git a/‎tests/baselines/reference/argumentExpressionContextualTyping.js
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+8-1Lines changed: 8 additions & 1 deletion b/‎tests/baselines/reference/argumentExpressionContextualTyping.js
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+8-1Lines changed: 8 additions & 1 deletion
diff --git a/‎tests/baselines/reference/arrayLiteralExpressionContextualTyping.js
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+10-3Lines changed: 10 additions & 3 deletions b/‎tests/baselines/reference/arrayLiteralExpressionContextualTyping.js
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+10-3Lines changed: 10 additions & 3 deletions
diff --git a/‎tests/baselines/reference/arrayLiteralSpread.js
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+16-9Lines changed: 16 additions & 9 deletions b/‎tests/baselines/reference/arrayLiteralSpread.js
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+16-9Lines changed: 16 additions & 9 deletions
diff --git a/‎tests/baselines/reference/arrayLiterals2ES5.js
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+20-13Lines changed: 20 additions & 13 deletions b/‎tests/baselines/reference/arrayLiterals2ES5.js
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+20-13Lines changed: 20 additions & 13 deletions
diff --git a/‎tests/baselines/reference/arrayLiterals3.js
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+10-3Lines changed: 10 additions & 3 deletions b/‎tests/baselines/reference/arrayLiterals3.js
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+10-3Lines changed: 10 additions & 3 deletions
diff --git a/‎tests/baselines/reference/callOverload.js
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+8-1Lines changed: 8 additions & 1 deletion b/‎tests/baselines/reference/callOverload.js
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+8-1Lines changed: 8 additions & 1 deletion
@@ -19080,8 +19080,8 @@ namespace ts {
         }
 
         function checkSpreadExpression(node: SpreadElement, checkMode?: CheckMode): Type {
-            if (languageVersion < ScriptTarget.ES2015 && compilerOptions.downlevelIteration) {
-                checkExternalEmitHelpers(node, ExternalEmitHelpers.SpreadIncludes);
+            if (languageVersion < ScriptTarget.ES2015) {
+                checkExternalEmitHelpers(node, compilerOptions.downlevelIteration ? ExternalEmitHelpers.SpreadIncludes : ExternalEmitHelpers.SpreadArrays);
             }
 
             const arrayOrIterableType = checkExpression(node.expression, checkMode);
@@ -31104,6 +31104,7 @@ namespace ts {
                 case ExternalEmitHelpers.Values: return "__values";
                 case ExternalEmitHelpers.Read: return "__read";
                 case ExternalEmitHelpers.Spread: return "__spread";
+                case ExternalEmitHelpers.SpreadArrays: return "__spreadArrays";
                 case ExternalEmitHelpers.Await: return "__await";
                 case ExternalEmitHelpers.AsyncGenerator: return "__asyncGenerator";
                 case ExternalEmitHelpers.AsyncDelegator: return "__asyncDelegator";
 
@@ -2657,6 +2657,7 @@ namespace ts {
             valuesHelper,
             readHelper,
             spreadHelper,
+            spreadArraysHelper,
             restHelper,
             decorateHelper,
             metadataHelper,
@@ -3693,6 +3694,31 @@ namespace ts {
         );
     }
 
+    export const spreadArraysHelper: UnscopedEmitHelper = {
+        name: "typescript:spreadArrays",
+        scoped: false,
+        text: `
+            var __spreadArrays = (this && this.__spreadArrays) || function () {
+                for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+                for (var r = Array(s), k = 0, i = 0; i < il; i++)
+                    for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+                        r[k] = a[j];
+                return r;
+            };`
+    };
+
+    export function createSpreadArraysHelper(context: TransformationContext, argumentList: ReadonlyArray<Expression>, location?: TextRange) {
+        context.requestEmitHelper(spreadArraysHelper);
+        return setTextRange(
+            createCall(
+                getHelperName("__spreadArrays"),
+                /*typeArguments*/ undefined,
+                argumentList
+            ),
+            location
+        );
+    }
+
     // Utilities
 
     export function createForOfBindingStatement(node: ForInitializer, boundValue: Expression): Statement {
 
@@ -3819,8 +3819,11 @@ namespace ts {
             // [source]
             //      [a, ...b, c]
             //
+            // [output (downlevelIteration)]
+            //      __spread([a], b, [c])
+            //
             // [output]
-            //      [a].concat(b, [c])
+            //      __spreadArrays([a], b, [c])
 
             // Map spans of spread expressions into their expressions and spans of other
             // expressions into an array literal.
@@ -3834,10 +3837,7 @@ namespace ts {
             if (compilerOptions.downlevelIteration) {
                 if (segments.length === 1) {
                     const firstSegment = segments[0];
-                    if (isCallExpression(firstSegment)
-                        && isIdentifier(firstSegment.expression)
-                        && (getEmitFlags(firstSegment.expression) & EmitFlags.HelperName)
-                        && firstSegment.expression.escapedText === "___spread") {
+                    if (isCallToHelper(firstSegment, "___spread" as __String)) {
                         return segments[0];
                     }
                 }
@@ -3846,17 +3846,33 @@ namespace ts {
             }
             else {
                 if (segments.length === 1) {
-                    const firstElement = elements[0];
-                    return needsUniqueCopy && isSpreadElement(firstElement) && firstElement.expression.kind !== SyntaxKind.ArrayLiteralExpression
-                        ? createArraySlice(segments[0])
-                        : segments[0];
+                    const firstSegment = segments[0];
+                    if (!needsUniqueCopy
+                        || isPackedArrayLiteral(firstSegment)
+                        || isCallToHelper(firstSegment, "___spreadArrays" as __String)) {
+                        return segments[0];
+                    }
                 }
 
-                // Rewrite using the pattern <segment0>.concat(<segment1>, <segment2>, ...)
-                return createArrayConcat(segments.shift()!, segments);
+                return createSpreadArraysHelper(context, segments);
             }
         }
 
+        function isPackedElement(node: Expression) {
+            return !isOmittedExpression(node);
+        }
+
+        function isPackedArrayLiteral(node: Expression) {
+            return isArrayLiteralExpression(node) && every(node.elements, isPackedElement);
+        }
+
+        function isCallToHelper(firstSegment: Expression, helperName: __String) {
+            return isCallExpression(firstSegment)
+                && isIdentifier(firstSegment.expression)
+                && (getEmitFlags(firstSegment.expression) & EmitFlags.HelperName)
+                && firstSegment.expression.escapedText === helperName;
+        }
+
         function partitionSpread(node: Expression) {
             return isSpreadElement(node)
                 ? visitSpanOfSpreads
 
@@ -5339,12 +5339,13 @@ namespace ts {
         Values = 1 << 8,                // __values (used by ES2015 for..of and yield* transformations)
         Read = 1 << 9,                  // __read (used by ES2015 iterator destructuring transformation)
         Spread = 1 << 10,               // __spread (used by ES2015 array spread and argument list spread transformations)
-        Await = 1 << 11,                // __await (used by ES2017 async generator transformation)
-        AsyncGenerator = 1 << 12,       // __asyncGenerator (used by ES2017 async generator transformation)
-        AsyncDelegator = 1 << 13,       // __asyncDelegator (used by ES2017 async generator yield* transformation)
-        AsyncValues = 1 << 14,          // __asyncValues (used by ES2017 for..await..of transformation)
-        ExportStar = 1 << 15,           // __exportStar (used by CommonJS/AMD/UMD module transformation)
-        MakeTemplateObject = 1 << 16,   // __makeTemplateObject (used for constructing template string array objects)
+        SpreadArrays = 1 << 11,         // __spreadArrays (used by ES2015 array spread and argument list spread transformations)
+        Await = 1 << 12,                // __await (used by ES2017 async generator transformation)
+        AsyncGenerator = 1 << 13,       // __asyncGenerator (used by ES2017 async generator transformation)
+        AsyncDelegator = 1 << 14,       // __asyncDelegator (used by ES2017 async generator yield* transformation)
+        AsyncValues = 1 << 15,          // __asyncValues (used by ES2017 for..await..of transformation)
+        ExportStar = 1 << 16,           // __exportStar (used by CommonJS/AMD/UMD module transformation)
+        MakeTemplateObject = 1 << 17,   // __makeTemplateObject (used for constructing template string array objects)
         FirstEmitHelper = Extends,
         LastEmitHelper = MakeTemplateObject,
 
 
@@ -19,6 +19,13 @@ baz(["string", 1, true, ...array]);  // Error
 foo(o);                              // Error because x has an array type namely (string|number)[]
 
 //// [argumentExpressionContextualTyping.js]
+var __spreadArrays = (this && this.__spreadArrays) || function () {
+    for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+    for (var r = Array(s), k = 0, i = 0; i < il; i++)
+        for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+            r[k] = a[j];
+    return r;
+};
 // In a typed function call, argument expressions are contextually typed by their corresponding parameter types.
 function foo(_a) {
     var _b = _a.x, a = _b[0], b = _b[1], _c = _a.y, c = _c.c, d = _c.d, e = _c.e;
@@ -36,5 +43,5 @@ var tuple = ["string", 1, true];
 baz(tuple);
 baz(["string", 1, true]);
 baz(array); // Error
-baz(["string", 1, true].concat(array)); // Error
+baz(__spreadArrays(["string", 1, true], array)); // Error
 foo(o); // Error because x has an array type namely (string|number)[]
@@ -16,6 +16,13 @@ var spr2:[number, number, number] = [1, 2, 3, ...tup];  // Error
 
 
 //// [arrayLiteralExpressionContextualTyping.js]
+var __spreadArrays = (this && this.__spreadArrays) || function () {
+    for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+    for (var r = Array(s), k = 0, i = 0; i < il; i++)
+        for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+            r[k] = a[j];
+    return r;
+};
 // In a contextually typed array literal expression containing no spread elements, an element expression at index N is contextually typed by
 //      the type of the property with the numeric name N in the contextual type, if any, or otherwise
 //      the numeric index type of the contextual type, if any.
@@ -26,6 +33,6 @@ var tup1 = [1, 2, 3, "string"];
 var tup2 = [1, 2, 3, "string"]; // Error
 // In a contextually typed array literal expression containing one or more spread elements,
 // an element expression at index N is contextually typed by the numeric index type of the contextual type, if any.
-var spr = [1, 2, 3].concat(array);
-var spr1 = [1, 2, 3].concat(tup);
-var spr2 = [1, 2, 3].concat(tup); // Error
+var spr = __spreadArrays([1, 2, 3], array);
+var spr1 = __spreadArrays([1, 2, 3], tup);
+var spr2 = __spreadArrays([1, 2, 3], tup); // Error
@@ -24,20 +24,27 @@ function f2() {
 
 
 //// [arrayLiteralSpread.js]
+var __spreadArrays = (this && this.__spreadArrays) || function () {
+    for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+    for (var r = Array(s), k = 0, i = 0; i < il; i++)
+        for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+            r[k] = a[j];
+    return r;
+};
 function f0() {
     var a = [1, 2, 3];
-    var a1 = a.slice();
-    var a2 = [1].concat(a);
-    var a3 = [1, 2].concat(a);
-    var a4 = a.concat([1]);
-    var a5 = a.concat([1, 2]);
-    var a6 = [1, 2].concat(a, [1, 2]);
-    var a7 = [1].concat(a, [2], a);
-    var a8 = a.concat(a, a);
+    var a1 = __spreadArrays(a);
+    var a2 = __spreadArrays([1], a);
+    var a3 = __spreadArrays([1, 2], a);
+    var a4 = __spreadArrays(a, [1]);
+    var a5 = __spreadArrays(a, [1, 2]);
+    var a6 = __spreadArrays([1, 2], a, [1, 2]);
+    var a7 = __spreadArrays([1], a, [2], a);
+    var a8 = __spreadArrays(a, a, a);
 }
 function f1() {
     var a = [1, 2, 3];
-    var b = ["hello"].concat(a, [true]);
+    var b = __spreadArrays(["hello"], a, [true]);
     var b;
 }
 function f2() {
 
@@ -62,14 +62,21 @@ var d9 = [[...temp1], ...["hello"]];
 //      Elisionopt   SpreadElement
 //      ElementList, Elisionopt   AssignmentExpression
 //      ElementList, Elisionopt   SpreadElement
+var __spreadArrays = (this && this.__spreadArrays) || function () {
+    for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+    for (var r = Array(s), k = 0, i = 0; i < il; i++)
+        for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+            r[k] = a[j];
+    return r;
+};
 // SpreadElement:
 //      ...   AssignmentExpression
 var a0 = [, , 2, 3, 4];
 var a1 = ["hello", "world"];
-var a2 = [, , ].concat(a0, ["hello"]);
-var a3 = [, ].concat(a0);
+var a2 = __spreadArrays([, , ], a0, ["hello"]);
+var a3 = __spreadArrays([, ], a0);
 var a4 = [function () { return 1; },];
-var a5 = a0.concat([,]);
+var a5 = __spreadArrays(a0, [,]);
 // Each element expression in a non-empty array literal is processed as follows:
 //    - If the array literal contains no spread elements, and if the array literal is contextually typed (section 4.19)
 //      by a type T and T has a property with the numeric name N, where N is the index of the element expression in the array literal,
@@ -92,13 +99,13 @@ var temp1 = [1, 2, 3];
 var temp2 = [[1, 2, 3], ["hello", "string"]];
 var temp3 = [undefined, null, undefined];
 var temp4 = [];
-var d0 = [1, true].concat(temp); // has type (string|number|boolean)[]
-var d1 = temp.slice(); // has type string[]
-var d2 = temp1.slice();
-var d3 = temp1.slice();
-var d4 = temp.concat(temp1);
-var d5 = temp3.slice();
-var d6 = temp4.slice();
-var d7 = temp1.slice();
-var d8 = [temp1.slice()];
-var d9 = [temp1.slice()].concat(["hello"]);
+var d0 = __spreadArrays([1, true], temp); // has type (string|number|boolean)[]
+var d1 = __spreadArrays(temp); // has type string[]
+var d2 = __spreadArrays(temp1);
+var d3 = __spreadArrays(temp1);
+var d4 = __spreadArrays(temp, temp1);
+var d5 = __spreadArrays(temp3);
+var d6 = __spreadArrays(temp4);
+var d7 = __spreadArrays(temp1);
+var d8 = [__spreadArrays(temp1)];
+var d9 = __spreadArrays([__spreadArrays(temp1)], ["hello"]);
@@ -40,6 +40,13 @@ var c2: myArray = [...temp1, ...temp];            // Error cannot assign (number
 //    - If the array literal contains no spread elements, and if the array literal is contextually typed (section 4.19)
 //      by a type T and T has a property with the numeric name N, where N is the index of the element expression in the array literal,
 //      the element expression is contextually typed by the type of that property.
+var __spreadArrays = (this && this.__spreadArrays) || function () {
+    for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+    for (var r = Array(s), k = 0, i = 0; i < il; i++)
+        for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+            r[k] = a[j];
+    return r;
+};
 // The resulting type an array literal expression is determined as follows:
 //     - If the array literal contains no spread elements and is contextually typed by a tuple-like type,
 //       the resulting type is a tuple type constructed from the types of the element expressions.
@@ -55,6 +62,6 @@ var _a = [1, 2, "string", true], b1 = _a[0], b2 = _a[1];
 var temp = ["s", "t", "r"];
 var temp1 = [1, 2, 3];
 var temp2 = [[1, 2, 3], ["hello", "string"]];
-var c0 = temp2.slice(); // Error
-var c1 = temp1.slice(); // Error cannot assign number[] to [number, number, number]
-var c2 = temp1.concat(temp); // Error cannot assign (number|string)[] to number[]
+var c0 = __spreadArrays(temp2); // Error
+var c1 = __spreadArrays(temp1); // Error cannot assign number[] to [number, number, number]
+var c2 = __spreadArrays(temp1, temp); // Error cannot assign (number|string)[] to number[]
@@ -12,10 +12,17 @@ withRest();
 withRest(...n); 
 
 //// [callOverload.js]
+var __spreadArrays = (this && this.__spreadArrays) || function () {
+    for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
+    for (var r = Array(s), k = 0, i = 0; i < il; i++)
+        for (var a = arguments[i], j = 0, jl = a.length; j < jl; j++, k++)
+            r[k] = a[j];
+    return r;
+};
 var n;
 fn(1); // no error
 fn(1, 2, 3, 4);
 takeTwo(1, 2, 3, 4);
-withRest.apply(void 0, ['a'].concat(n)); // no error
+withRest.apply(void 0, __spreadArrays(['a'], n)); // no error
 withRest();
 withRest.apply(void 0, n);