diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 0f1a68de4c20d..707f749e36503 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -992,12 +992,12 @@ interface ReadonlyArray<T> {
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: ConcatArray<T>[]): T[];
     /**
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: (T | ConcatArray<T>)[]): T[];
     /**
       * Adds all the elements of an array separated by the specified separator string.
       * @param separator A string used to separate one element of an array from the next in the resulting String. If omitted, the array elements are separated with a comma.
@@ -1087,6 +1087,13 @@ interface ReadonlyArray<T> {
     readonly [n: number]: T;
 }
 
+interface ConcatArray<T> {
+    readonly length: number;
+    readonly [n: number]: T;
+    join(separator?: string): string;
+    slice(start?: number, end?: number): T[];
+}
+
 interface Array<T> {
     /**
       * Gets or sets the length of the array. This is a number one higher than the highest element defined in an array.
@@ -1113,12 +1120,12 @@ interface Array<T> {
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: ConcatArray<T>[]): T[];
     /**
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: (T | ConcatArray<T>)[]): T[];
     /**
       * Adds all the elements of an array separated by the specified separator string.
       * @param separator A string used to separate one element of an array from the next in the resulting String. If omitted, the array elements are separated with a comma.
diff --git a/tests/baselines/reference/arrayConcat2.types b/tests/baselines/reference/arrayConcat2.types
index 2e33d4cd023b3..f1ca2a5f10977 100644
--- a/tests/baselines/reference/arrayConcat2.types
+++ b/tests/baselines/reference/arrayConcat2.types
@@ -5,17 +5,17 @@ var a: string[] = [];
 
 a.concat("hello", 'world');
 >a.concat("hello", 'world') : string[]
->a.concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>a.concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >a : string[]
->concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >"hello" : "hello"
 >'world' : "world"
 
 a.concat('Hello');
 >a.concat('Hello') : string[]
->a.concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>a.concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >a : string[]
->concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >'Hello' : "Hello"
 
 var b = new Array<string>();
@@ -25,8 +25,8 @@ var b = new Array<string>();
 
 b.concat('hello');
 >b.concat('hello') : string[]
->b.concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>b.concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >b : string[]
->concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >'hello' : "hello"
 
diff --git a/tests/baselines/reference/arrayConcat3.types b/tests/baselines/reference/arrayConcat3.types
index 2bfbe09a1765d..33321c4321781 100644
--- a/tests/baselines/reference/arrayConcat3.types
+++ b/tests/baselines/reference/arrayConcat3.types
@@ -25,9 +25,9 @@ function doStuff<T extends object, T1 extends T>(a: Array<Fn<T>>, b: Array<Fn<T1
 
     b.concat(a);
 >b.concat(a) : Fn<T1>[]
->b.concat : { (...items: (Fn<T1>[] | ReadonlyArray<Fn<T1>>)[]): Fn<T1>[]; (...items: (Fn<T1> | Fn<T1>[] | ReadonlyArray<Fn<T1>>)[]): Fn<T1>[]; }
+>b.concat : { (...items: ConcatArray<Fn<T1>>[]): Fn<T1>[]; (...items: (Fn<T1> | ConcatArray<Fn<T1>>)[]): Fn<T1>[]; }
 >b : Fn<T1>[]
->concat : { (...items: (Fn<T1>[] | ReadonlyArray<Fn<T1>>)[]): Fn<T1>[]; (...items: (Fn<T1> | Fn<T1>[] | ReadonlyArray<Fn<T1>>)[]): Fn<T1>[]; }
+>concat : { (...items: ConcatArray<Fn<T1>>[]): Fn<T1>[]; (...items: (Fn<T1> | ConcatArray<Fn<T1>>)[]): Fn<T1>[]; }
 >a : Fn<T>[]
 }
 
diff --git a/tests/baselines/reference/arrayConcatMap.types b/tests/baselines/reference/arrayConcatMap.types
index cbb30b8304ad6..ffc49e548a350 100644
--- a/tests/baselines/reference/arrayConcatMap.types
+++ b/tests/baselines/reference/arrayConcatMap.types
@@ -4,9 +4,9 @@ var x = [].concat([{ a: 1 }], [{ a: 2 }])
 >[].concat([{ a: 1 }], [{ a: 2 }])          .map(b => b.a) : any[]
 >[].concat([{ a: 1 }], [{ a: 2 }])          .map : <U>(callbackfn: (value: any, index: number, array: any[]) => U, thisArg?: any) => U[]
 >[].concat([{ a: 1 }], [{ a: 2 }]) : any[]
->[].concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>[].concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >[] : undefined[]
->concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >[{ a: 1 }] : { a: number; }[]
 >{ a: 1 } : { a: number; }
 >a : number
diff --git a/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt b/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt
index 87c8b59ee1e61..7ae20699c85ff 100644
--- a/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt
+++ b/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt
@@ -1,12 +1,12 @@
 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(13,1): error TS2322: Type 'A[]' is not assignable to type 'ReadonlyArray<B>'.
   Types of property 'concat' are incompatible.
-    Type '{ (...items: (A[] | ReadonlyArray<A>)[]): A[]; (...items: (A | A[] | ReadonlyArray<A>)[]): A[]; }' is not assignable to type '{ (...items: (B[] | ReadonlyArray<B>)[]): B[]; (...items: (B | B[] | ReadonlyArray<B>)[]): B[]; }'.
+    Type '{ (...items: ConcatArray<A>[]): A[]; (...items: (A | ConcatArray<A>)[]): A[]; }' is not assignable to type '{ (...items: ConcatArray<B>[]): B[]; (...items: (B | ConcatArray<B>)[]): B[]; }'.
       Type 'A[]' is not assignable to type 'B[]'.
         Type 'A' is not assignable to type 'B'.
           Property 'b' is missing in type 'A'.
 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error TS2322: Type 'C<A>' is not assignable to type 'ReadonlyArray<B>'.
   Types of property 'concat' are incompatible.
-    Type '{ (...items: (A[] | ReadonlyArray<A>)[]): A[]; (...items: (A | A[] | ReadonlyArray<A>)[]): A[]; }' is not assignable to type '{ (...items: (B[] | ReadonlyArray<B>)[]): B[]; (...items: (B | B[] | ReadonlyArray<B>)[]): B[]; }'.
+    Type '{ (...items: ConcatArray<A>[]): A[]; (...items: (A | ConcatArray<A>)[]): A[]; }' is not assignable to type '{ (...items: ConcatArray<B>[]): B[]; (...items: (B | ConcatArray<B>)[]): B[]; }'.
       Type 'A[]' is not assignable to type 'B[]'.
 
 
@@ -27,7 +27,7 @@ tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error T
     ~~~
 !!! error TS2322: Type 'A[]' is not assignable to type 'ReadonlyArray<B>'.
 !!! error TS2322:   Types of property 'concat' are incompatible.
-!!! error TS2322:     Type '{ (...items: (A[] | ReadonlyArray<A>)[]): A[]; (...items: (A | A[] | ReadonlyArray<A>)[]): A[]; }' is not assignable to type '{ (...items: (B[] | ReadonlyArray<B>)[]): B[]; (...items: (B | B[] | ReadonlyArray<B>)[]): B[]; }'.
+!!! error TS2322:     Type '{ (...items: ConcatArray<A>[]): A[]; (...items: (A | ConcatArray<A>)[]): A[]; }' is not assignable to type '{ (...items: ConcatArray<B>[]): B[]; (...items: (B | ConcatArray<B>)[]): B[]; }'.
 !!! error TS2322:       Type 'A[]' is not assignable to type 'B[]'.
 !!! error TS2322:         Type 'A' is not assignable to type 'B'.
 !!! error TS2322:           Property 'b' is missing in type 'A'.
@@ -39,6 +39,6 @@ tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error T
     ~~~
 !!! error TS2322: Type 'C<A>' is not assignable to type 'ReadonlyArray<B>'.
 !!! error TS2322:   Types of property 'concat' are incompatible.
-!!! error TS2322:     Type '{ (...items: (A[] | ReadonlyArray<A>)[]): A[]; (...items: (A | A[] | ReadonlyArray<A>)[]): A[]; }' is not assignable to type '{ (...items: (B[] | ReadonlyArray<B>)[]): B[]; (...items: (B | B[] | ReadonlyArray<B>)[]): B[]; }'.
+!!! error TS2322:     Type '{ (...items: ConcatArray<A>[]): A[]; (...items: (A | ConcatArray<A>)[]): A[]; }' is not assignable to type '{ (...items: ConcatArray<B>[]): B[]; (...items: (B | ConcatArray<B>)[]): B[]; }'.
 !!! error TS2322:       Type 'A[]' is not assignable to type 'B[]'.
     
\ No newline at end of file
diff --git a/tests/baselines/reference/concatError.types b/tests/baselines/reference/concatError.types
index c7f07e4ea9605..52f413b20b6b4 100644
--- a/tests/baselines/reference/concatError.types
+++ b/tests/baselines/reference/concatError.types
@@ -15,9 +15,9 @@ fa = fa.concat([0]);
 >fa = fa.concat([0]) : number[]
 >fa : number[]
 >fa.concat([0]) : number[]
->fa.concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>fa.concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >fa : number[]
->concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >[0] : number[]
 >0 : 0
 
@@ -25,9 +25,9 @@ fa = fa.concat(0);
 >fa = fa.concat(0) : number[]
 >fa : number[]
 >fa.concat(0) : number[]
->fa.concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>fa.concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >fa : number[]
->concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >0 : 0
 
 
diff --git a/tests/baselines/reference/concatTuples.types b/tests/baselines/reference/concatTuples.types
index 46c06357b998b..d85ee92557041 100644
--- a/tests/baselines/reference/concatTuples.types
+++ b/tests/baselines/reference/concatTuples.types
@@ -10,9 +10,9 @@ ijs = ijs.concat([[3, 4], [5, 6]]);
 >ijs = ijs.concat([[3, 4], [5, 6]]) : [number, number][]
 >ijs : [number, number][]
 >ijs.concat([[3, 4], [5, 6]]) : [number, number][]
->ijs.concat : { (...items: ([number, number][] | ReadonlyArray<[number, number]>)[]): [number, number][]; (...items: ([number, number] | [number, number][] | ReadonlyArray<[number, number]>)[]): [number, number][]; }
+>ijs.concat : { (...items: ConcatArray<[number, number]>[]): [number, number][]; (...items: ([number, number] | ConcatArray<[number, number]>)[]): [number, number][]; }
 >ijs : [number, number][]
->concat : { (...items: ([number, number][] | ReadonlyArray<[number, number]>)[]): [number, number][]; (...items: ([number, number] | [number, number][] | ReadonlyArray<[number, number]>)[]): [number, number][]; }
+>concat : { (...items: ConcatArray<[number, number]>[]): [number, number][]; (...items: ([number, number] | ConcatArray<[number, number]>)[]): [number, number][]; }
 >[[3, 4], [5, 6]] : [number, number][]
 >[3, 4] : [number, number]
 >3 : 3
diff --git a/tests/baselines/reference/emitSkipsThisWithRestParameter.types b/tests/baselines/reference/emitSkipsThisWithRestParameter.types
index 9d560a9e92f19..9aa0e1fa55b66 100644
--- a/tests/baselines/reference/emitSkipsThisWithRestParameter.types
+++ b/tests/baselines/reference/emitSkipsThisWithRestParameter.types
@@ -18,10 +18,10 @@ function rebase(fn: (base: any, ...args: any[]) => any): (...args: any[]) => any
 >apply : (this: Function, thisArg: any, argArray?: any) => any
 >this : any
 >[ this ].concat(args) : any[]
->[ this ].concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>[ this ].concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >[ this ] : any[]
 >this : any
->concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >args : any[]
 
     };
diff --git a/tests/baselines/reference/intersectionTypeInference3.types b/tests/baselines/reference/intersectionTypeInference3.types
index e6f411a603c45..6239792f53d9c 100644
--- a/tests/baselines/reference/intersectionTypeInference3.types
+++ b/tests/baselines/reference/intersectionTypeInference3.types
@@ -32,13 +32,13 @@ declare const b: Set<A>;
 const c1 = Array.from(a).concat(Array.from(b));
 >c1 : Nominal<"A", string>[]
 >Array.from(a).concat(Array.from(b)) : Nominal<"A", string>[]
->Array.from(a).concat : { (...items: (Nominal<"A", string>[] | ReadonlyArray<Nominal<"A", string>>)[]): Nominal<"A", string>[]; (...items: (Nominal<"A", string> | Nominal<"A", string>[] | ReadonlyArray<Nominal<"A", string>>)[]): Nominal<"A", string>[]; }
+>Array.from(a).concat : { (...items: ConcatArray<Nominal<"A", string>>[]): Nominal<"A", string>[]; (...items: (Nominal<"A", string> | ConcatArray<Nominal<"A", string>>)[]): Nominal<"A", string>[]; }
 >Array.from(a) : Nominal<"A", string>[]
 >Array.from : { <T>(iterable: Iterable<T> | ArrayLike<T>): T[]; <T, U>(iterable: Iterable<T> | ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): U[]; <T>(arrayLike: ArrayLike<T>): T[]; <T, U>(arrayLike: ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): U[]; }
 >Array : ArrayConstructor
 >from : { <T>(iterable: Iterable<T> | ArrayLike<T>): T[]; <T, U>(iterable: Iterable<T> | ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): U[]; <T>(arrayLike: ArrayLike<T>): T[]; <T, U>(arrayLike: ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): U[]; }
 >a : Set<Nominal<"A", string>>
->concat : { (...items: (Nominal<"A", string>[] | ReadonlyArray<Nominal<"A", string>>)[]): Nominal<"A", string>[]; (...items: (Nominal<"A", string> | Nominal<"A", string>[] | ReadonlyArray<Nominal<"A", string>>)[]): Nominal<"A", string>[]; }
+>concat : { (...items: ConcatArray<Nominal<"A", string>>[]): Nominal<"A", string>[]; (...items: (Nominal<"A", string> | ConcatArray<Nominal<"A", string>>)[]): Nominal<"A", string>[]; }
 >Array.from(b) : Nominal<"A", string>[]
 >Array.from : { <T>(iterable: Iterable<T> | ArrayLike<T>): T[]; <T, U>(iterable: Iterable<T> | ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): U[]; <T>(arrayLike: ArrayLike<T>): T[]; <T, U>(arrayLike: ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): U[]; }
 >Array : ArrayConstructor
diff --git a/tests/baselines/reference/iteratorSpreadInArray6.errors.txt b/tests/baselines/reference/iteratorSpreadInArray6.errors.txt
index 17738ba06d58f..4f547c59ed045 100644
--- a/tests/baselines/reference/iteratorSpreadInArray6.errors.txt
+++ b/tests/baselines/reference/iteratorSpreadInArray6.errors.txt
@@ -1,17 +1,9 @@
-tests/cases/conformance/es6/spread/iteratorSpreadInArray6.ts(15,14): error TS2345: Argument of type 'symbol[]' is not assignable to parameter of type 'number | number[] | ReadonlyArray<number>'.
-  Type 'symbol[]' is not assignable to type 'ReadonlyArray<number>'.
-    Types of property 'concat' are incompatible.
-      Type '{ (...items: (symbol[] | ReadonlyArray<symbol>)[]): symbol[]; (...items: (symbol | symbol[] | ReadonlyArray<symbol>)[]): symbol[]; }' is not assignable to type '{ (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }'.
-        Types of parameters 'items' and 'items' are incompatible.
-          Type 'number[] | ReadonlyArray<number>' is not assignable to type 'symbol[] | ReadonlyArray<symbol>'.
-            Type 'number[]' is not assignable to type 'symbol[] | ReadonlyArray<symbol>'.
-              Type 'number[]' is not assignable to type 'ReadonlyArray<symbol>'.
-                Types of property 'concat' are incompatible.
-                  Type '{ (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }' is not assignable to type '{ (...items: (symbol[] | ReadonlyArray<symbol>)[]): symbol[]; (...items: (symbol | symbol[] | ReadonlyArray<symbol>)[]): symbol[]; }'.
-                    Types of parameters 'items' and 'items' are incompatible.
-                      Type 'symbol[] | ReadonlyArray<symbol>' is not assignable to type 'number[] | ReadonlyArray<number>'.
-                        Type 'symbol[]' is not assignable to type 'number[] | ReadonlyArray<number>'.
-                          Type 'symbol[]' is not assignable to type 'ReadonlyArray<number>'.
+tests/cases/conformance/es6/spread/iteratorSpreadInArray6.ts(15,14): error TS2345: Argument of type 'symbol[]' is not assignable to parameter of type 'number | ConcatArray<number>'.
+  Type 'symbol[]' is not assignable to type 'ConcatArray<number>'.
+    Types of property 'slice' are incompatible.
+      Type '(start?: number, end?: number) => symbol[]' is not assignable to type '(start?: number, end?: number) => number[]'.
+        Type 'symbol[]' is not assignable to type 'number[]'.
+          Type 'symbol' is not assignable to type 'number'.
 
 
 ==== tests/cases/conformance/es6/spread/iteratorSpreadInArray6.ts (1 errors) ====
@@ -31,17 +23,9 @@ tests/cases/conformance/es6/spread/iteratorSpreadInArray6.ts(15,14): error TS234
     var array: number[] = [0, 1];
     array.concat([...new SymbolIterator]);
                  ~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2345: Argument of type 'symbol[]' is not assignable to parameter of type 'number | number[] | ReadonlyArray<number>'.
-!!! error TS2345:   Type 'symbol[]' is not assignable to type 'ReadonlyArray<number>'.
-!!! error TS2345:     Types of property 'concat' are incompatible.
-!!! error TS2345:       Type '{ (...items: (symbol[] | ReadonlyArray<symbol>)[]): symbol[]; (...items: (symbol | symbol[] | ReadonlyArray<symbol>)[]): symbol[]; }' is not assignable to type '{ (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }'.
-!!! error TS2345:         Types of parameters 'items' and 'items' are incompatible.
-!!! error TS2345:           Type 'number[] | ReadonlyArray<number>' is not assignable to type 'symbol[] | ReadonlyArray<symbol>'.
-!!! error TS2345:             Type 'number[]' is not assignable to type 'symbol[] | ReadonlyArray<symbol>'.
-!!! error TS2345:               Type 'number[]' is not assignable to type 'ReadonlyArray<symbol>'.
-!!! error TS2345:                 Types of property 'concat' are incompatible.
-!!! error TS2345:                   Type '{ (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }' is not assignable to type '{ (...items: (symbol[] | ReadonlyArray<symbol>)[]): symbol[]; (...items: (symbol | symbol[] | ReadonlyArray<symbol>)[]): symbol[]; }'.
-!!! error TS2345:                     Types of parameters 'items' and 'items' are incompatible.
-!!! error TS2345:                       Type 'symbol[] | ReadonlyArray<symbol>' is not assignable to type 'number[] | ReadonlyArray<number>'.
-!!! error TS2345:                         Type 'symbol[]' is not assignable to type 'number[] | ReadonlyArray<number>'.
-!!! error TS2345:                           Type 'symbol[]' is not assignable to type 'ReadonlyArray<number>'.
\ No newline at end of file
+!!! error TS2345: Argument of type 'symbol[]' is not assignable to parameter of type 'number | ConcatArray<number>'.
+!!! error TS2345:   Type 'symbol[]' is not assignable to type 'ConcatArray<number>'.
+!!! error TS2345:     Types of property 'slice' are incompatible.
+!!! error TS2345:       Type '(start?: number, end?: number) => symbol[]' is not assignable to type '(start?: number, end?: number) => number[]'.
+!!! error TS2345:         Type 'symbol[]' is not assignable to type 'number[]'.
+!!! error TS2345:           Type 'symbol' is not assignable to type 'number'.
\ No newline at end of file
diff --git a/tests/baselines/reference/iteratorSpreadInArray6.types b/tests/baselines/reference/iteratorSpreadInArray6.types
index dbf94611fabf6..3fa1720de1460 100644
--- a/tests/baselines/reference/iteratorSpreadInArray6.types
+++ b/tests/baselines/reference/iteratorSpreadInArray6.types
@@ -38,9 +38,9 @@ var array: number[] = [0, 1];
 
 array.concat([...new SymbolIterator]);
 >array.concat([...new SymbolIterator]) : any
->array.concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>array.concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >array : number[]
->concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >[...new SymbolIterator] : symbol[]
 >...new SymbolIterator : symbol
 >new SymbolIterator : SymbolIterator
diff --git a/tests/baselines/reference/iteratorSpreadInArray7.types b/tests/baselines/reference/iteratorSpreadInArray7.types
index b0d9fbcc77cd5..65c34ae0a6e4a 100644
--- a/tests/baselines/reference/iteratorSpreadInArray7.types
+++ b/tests/baselines/reference/iteratorSpreadInArray7.types
@@ -35,9 +35,9 @@ var array: symbol[];
 
 array.concat([...new SymbolIterator]);
 >array.concat([...new SymbolIterator]) : symbol[]
->array.concat : { (...items: (symbol[] | ReadonlyArray<symbol>)[]): symbol[]; (...items: (symbol | symbol[] | ReadonlyArray<symbol>)[]): symbol[]; }
+>array.concat : { (...items: ConcatArray<symbol>[]): symbol[]; (...items: (symbol | ConcatArray<symbol>)[]): symbol[]; }
 >array : symbol[]
->concat : { (...items: (symbol[] | ReadonlyArray<symbol>)[]): symbol[]; (...items: (symbol | symbol[] | ReadonlyArray<symbol>)[]): symbol[]; }
+>concat : { (...items: ConcatArray<symbol>[]): symbol[]; (...items: (symbol | ConcatArray<symbol>)[]): symbol[]; }
 >[...new SymbolIterator] : symbol[]
 >...new SymbolIterator : symbol
 >new SymbolIterator : SymbolIterator
diff --git a/tests/baselines/reference/parserRealSource4.types b/tests/baselines/reference/parserRealSource4.types
index cc16886c3d549..175ea23db81ad 100644
--- a/tests/baselines/reference/parserRealSource4.types
+++ b/tests/baselines/reference/parserRealSource4.types
@@ -424,14 +424,14 @@ module TypeScript {
 
             return this.primaryTable.getAllKeys().concat(this.secondaryTable.getAllKeys());
 >this.primaryTable.getAllKeys().concat(this.secondaryTable.getAllKeys()) : string[]
->this.primaryTable.getAllKeys().concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>this.primaryTable.getAllKeys().concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >this.primaryTable.getAllKeys() : string[]
 >this.primaryTable.getAllKeys : () => string[]
 >this.primaryTable : IHashTable
 >this : this
 >primaryTable : IHashTable
 >getAllKeys : () => string[]
->concat : { (...items: (string[] | ReadonlyArray<string>)[]): string[]; (...items: (string | string[] | ReadonlyArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
 >this.secondaryTable.getAllKeys() : string[]
 >this.secondaryTable.getAllKeys : () => string[]
 >this.secondaryTable : IHashTable
diff --git a/tests/baselines/reference/parserharness.types b/tests/baselines/reference/parserharness.types
index 284eb887dd484..61a4a97fc7995 100644
--- a/tests/baselines/reference/parserharness.types
+++ b/tests/baselines/reference/parserharness.types
@@ -4868,9 +4868,9 @@ module Harness {
 >lines = lines.concat(v.file.lines) : any[]
 >lines : any[]
 >lines.concat(v.file.lines) : any[]
->lines.concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>lines.concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >lines : any[]
->concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >v.file.lines : string[]
 >v.file : WriterAggregator
 >v : { filename: string; file: WriterAggregator; }
diff --git a/tests/baselines/reference/restInvalidArgumentType.types b/tests/baselines/reference/restInvalidArgumentType.types
index 389e9a070aaf1..bad3f54c0e88a 100644
--- a/tests/baselines/reference/restInvalidArgumentType.types
+++ b/tests/baselines/reference/restInvalidArgumentType.types
@@ -87,7 +87,7 @@ function f<T extends { b: string }>(p1: T, p2: T[]) {
 >p1 : T
 
     var {...r2} = p2;   // OK
->r2 : { [n: number]: T; length: number; toString(): string; toLocaleString(): string; push(...items: T[]): number; pop(): T; concat(...items: (T[] | ReadonlyArray<T>)[]): T[]; concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[]; join(separator?: string): string; reverse(): T[]; shift(): T; slice(start?: number, end?: number): T[]; sort(compareFn?: (a: T, b: T) => number): T[]; splice(start: number, deleteCount?: number): T[]; splice(start: number, deleteCount: number, ...items: T[]): T[]; unshift(...items: T[]): number; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void; map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[]; filter<S extends T>(callbackfn: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[]; filter(callbackfn: (value: T, index: number, array: T[]) => any, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; }
+>r2 : { [n: number]: T; length: number; toString(): string; toLocaleString(): string; push(...items: T[]): number; pop(): T; concat(...items: ConcatArray<T>[]): T[]; concat(...items: (T | ConcatArray<T>)[]): T[]; join(separator?: string): string; reverse(): T[]; shift(): T; slice(start?: number, end?: number): T[]; sort(compareFn?: (a: T, b: T) => number): T[]; splice(start: number, deleteCount?: number): T[]; splice(start: number, deleteCount: number, ...items: T[]): T[]; unshift(...items: T[]): number; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void; map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[]; filter<S extends T>(callbackfn: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[]; filter(callbackfn: (value: T, index: number, array: T[]) => any, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; }
 >p2 : T[]
 
     var {...r3} = t;   // Error, generic type paramter
diff --git a/tests/baselines/reference/spreadInvalidArgumentType.types b/tests/baselines/reference/spreadInvalidArgumentType.types
index 49d5bc962611d..d85b974cc8b3a 100644
--- a/tests/baselines/reference/spreadInvalidArgumentType.types
+++ b/tests/baselines/reference/spreadInvalidArgumentType.types
@@ -89,8 +89,8 @@ function f<T extends { b: string }>(p1: T, p2: T[]) {
 >p1 : T
 
     var o2 = { ...p2 };   // OK
->o2 : { [x: number]: T; length: number; toString(): string; toLocaleString(): string; push(...items: T[]): number; pop(): T; concat(...items: (T[] | ReadonlyArray<T>)[]): T[]; concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[]; join(separator?: string): string; reverse(): T[]; shift(): T; slice(start?: number, end?: number): T[]; sort(compareFn?: (a: T, b: T) => number): T[]; splice(start: number, deleteCount?: number): T[]; splice(start: number, deleteCount: number, ...items: T[]): T[]; unshift(...items: T[]): number; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void; map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[]; filter<S extends T>(callbackfn: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[]; filter(callbackfn: (value: T, index: number, array: T[]) => any, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; }
->{ ...p2 } : { [n: number]: T; length: number; toString(): string; toLocaleString(): string; push(...items: T[]): number; pop(): T; concat(...items: (T[] | ReadonlyArray<T>)[]): T[]; concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[]; join(separator?: string): string; reverse(): T[]; shift(): T; slice(start?: number, end?: number): T[]; sort(compareFn?: (a: T, b: T) => number): T[]; splice(start: number, deleteCount?: number): T[]; splice(start: number, deleteCount: number, ...items: T[]): T[]; unshift(...items: T[]): number; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void; map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[]; filter<S extends T>(callbackfn: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[]; filter(callbackfn: (value: T, index: number, array: T[]) => any, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; }
+>o2 : { [x: number]: T; length: number; toString(): string; toLocaleString(): string; push(...items: T[]): number; pop(): T; concat(...items: ConcatArray<T>[]): T[]; concat(...items: (T | ConcatArray<T>)[]): T[]; join(separator?: string): string; reverse(): T[]; shift(): T; slice(start?: number, end?: number): T[]; sort(compareFn?: (a: T, b: T) => number): T[]; splice(start: number, deleteCount?: number): T[]; splice(start: number, deleteCount: number, ...items: T[]): T[]; unshift(...items: T[]): number; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void; map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[]; filter<S extends T>(callbackfn: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[]; filter(callbackfn: (value: T, index: number, array: T[]) => any, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; }
+>{ ...p2 } : { [n: number]: T; length: number; toString(): string; toLocaleString(): string; push(...items: T[]): number; pop(): T; concat(...items: ConcatArray<T>[]): T[]; concat(...items: (T | ConcatArray<T>)[]): T[]; join(separator?: string): string; reverse(): T[]; shift(): T; slice(start?: number, end?: number): T[]; sort(compareFn?: (a: T, b: T) => number): T[]; splice(start: number, deleteCount?: number): T[]; splice(start: number, deleteCount: number, ...items: T[]): T[]; unshift(...items: T[]): number; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number, array: T[]) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void; map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[]; filter<S extends T>(callbackfn: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[]; filter(callbackfn: (value: T, index: number, array: T[]) => any, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U; }
 >p2 : T[]
 
     var o3 = { ...t };   // Error, generic type paramter
diff --git a/tests/baselines/reference/staticAnonymousTypeNotReferencingTypeParameter.types b/tests/baselines/reference/staticAnonymousTypeNotReferencingTypeParameter.types
index e4dc5df9449f9..56cded213453d 100644
--- a/tests/baselines/reference/staticAnonymousTypeNotReferencingTypeParameter.types
+++ b/tests/baselines/reference/staticAnonymousTypeNotReferencingTypeParameter.types
@@ -348,9 +348,9 @@ class ListWrapper {
 >a : any[]
 >b : any[]
 >a.concat(b) : any[]
->a.concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>a.concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >a : any[]
->concat : { (...items: (any[] | ReadonlyArray<any>)[]): any[]; (...items: any[]): any[]; }
+>concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
 >b : any[]
 
   static insert<T>(dit: typeof ListWrapper, list: T[], index: number, value: T) { list.splice(index, 0, value); }
diff --git a/tests/baselines/reference/underscoreTest1.types b/tests/baselines/reference/underscoreTest1.types
index 8c6b2691e4ed6..2222de9e4120d 100644
--- a/tests/baselines/reference/underscoreTest1.types
+++ b/tests/baselines/reference/underscoreTest1.types
@@ -124,9 +124,9 @@ var flat = _.reduceRight(list, (a, b) => a.concat(b), []);
 >a : number[]
 >b : number[]
 >a.concat(b) : number[]
->a.concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>a.concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >a : number[]
->concat : { (...items: (number[] | ReadonlyArray<number>)[]): number[]; (...items: (number | number[] | ReadonlyArray<number>)[]): number[]; }
+>concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
 >b : number[]
 >[] : undefined[]
 
diff --git a/tests/cases/compiler/arrayConcat3.ts b/tests/cases/compiler/arrayConcat3.ts
index d66d488a20d64..b0e4c4581ddad 100644
--- a/tests/cases/compiler/arrayConcat3.ts
+++ b/tests/cases/compiler/arrayConcat3.ts
@@ -1,3 +1,4 @@
+// @strictFunctionTypes: true
 // TODO: remove lib hack when https://github.com/Microsoft/TypeScript/issues/20454 is fixed
 type Fn<T extends object> = <U extends T>(subj: U) => U
 function doStuff<T extends object, T1 extends T>(a: Array<Fn<T>>, b: Array<Fn<T1>>) {