Fix unsound Array.concat() typings

Author: jablko

src/harness/fourslashInterface.ts

@@ -937,6 +937,7 @@ namespace FourSlashInterface {
             varEntry("JSON"),
             interfaceEntry("ReadonlyArray"),
             interfaceEntry("ConcatArray"),
+            typeEntry("Flatten"),
             varEntry("Array"),
             interfaceEntry("ArrayConstructor"),
             interfaceEntry("TypedPropertyDescriptor"),
@@ -1602,4 +1603,4 @@ namespace FourSlashInterface {
         readonly providePrefixAndSuffixTextForRename?: boolean;
     };
     export type RenameLocationOptions = FourSlash.Range | { readonly range: FourSlash.Range, readonly prefixText?: string, readonly suffixText?: string };
-}
+}

src/lib/es5.d.ts

@@ -1085,7 +1085,7 @@ interface ReadonlyArray<T> {
      * Combines two or more arrays.
      * @param items Additional items to add to the end of array1.
      */
-    concat(...items: (T | ConcatArray<T>)[]): T[];
+    concat<U extends (T | ConcatArray<T>)[]>(...items: U): (T | Flatten<U[number]>)[];
     /**
      * 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.
@@ -1187,6 +1187,7 @@ interface ConcatArray<T> {
     join(separator?: string): string;
     slice(start?: number, end?: number): T[];
 }
+type Flatten<T> = T extends ConcatArray<infer U> ? U : T;
 
 interface Array<T> {
     /**
@@ -1219,7 +1220,7 @@ interface Array<T> {
      * Combines two or more arrays.
      * @param items Additional items to add to the end of array1.
      */
-    concat(...items: (T | ConcatArray<T>)[]): T[];
+    concat<U extends (T | ConcatArray<T>)[]>(...items: U): (T | Flatten<U[number]>)[];
     /**
      * 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.

tests/baselines/reference/arrayConcat2.types

@@ -5,17 +5,17 @@ var a: string[] = [];
 
 a.concat("hello", 'world');
 >a.concat("hello", 'world') : string[]
->a.concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
+>a.concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
 >a : string[]
->concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
 >"hello" : "hello"
 >'world' : "world"
 
 a.concat('Hello');
 >a.concat('Hello') : string[]
->a.concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
+>a.concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
 >a : string[]
->concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
 >'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: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
+>b.concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
 >b : string[]
->concat : { (...items: ConcatArray<string>[]): string[]; (...items: (string | ConcatArray<string>)[]): string[]; }
+>concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
 >'hello' : "hello"
 

tests/baselines/reference/arrayConcat3.types

@@ -11,9 +11,9 @@ 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: ConcatArray<Fn<T1>>[]): Fn<T1>[]; (...items: (Fn<T1> | ConcatArray<Fn<T1>>)[]): Fn<T1>[]; }
+>b.concat : { (...items: ConcatArray<Fn<T1>>[]): Fn<T1>[]; <U extends (Fn<T1> | ConcatArray<Fn<T1>>)[]>(...items: U): (Fn<T1> | Flatten<U[number]>)[]; }
 >b : Fn<T1>[]
->concat : { (...items: ConcatArray<Fn<T1>>[]): Fn<T1>[]; (...items: (Fn<T1> | ConcatArray<Fn<T1>>)[]): Fn<T1>[]; }
+>concat : { (...items: ConcatArray<Fn<T1>>[]): Fn<T1>[]; <U extends (Fn<T1> | ConcatArray<Fn<T1>>)[]>(...items: U): (Fn<T1> | Flatten<U[number]>)[]; }
 >a : Fn<T>[]
 }
 

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: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
+>[].concat : { (...items: ConcatArray<any>[]): any[]; <U extends any[]>(...items: U): any[]; }
 >[] : undefined[]
->concat : { (...items: ConcatArray<any>[]): any[]; (...items: any[]): any[]; }
+>concat : { (...items: ConcatArray<any>[]): any[]; <U extends any[]>(...items: U): any[]; }
 >[{ a: 1 }] : { a: number; }[]
 >{ a: 1 } : { a: number; }
 >a : number

tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt

@@ -1,7 +1,7 @@
 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(13,1): error TS2322: Type 'A[]' is not assignable to type 'readonly B[]'.
   Property 'b' is missing in type 'A' but required in type 'B'.
 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error TS2322: Type 'C<A>' is not assignable to type 'readonly B[]'.
-  The types returned by 'concat(...)' are incompatible between these types.
+  The types returned by 'slice(...)' are incompatible between these types.
     Type 'A[]' is not assignable to type 'B[]'.
       Type 'A' is not assignable to type 'B'.
 
@@ -31,7 +31,7 @@ tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error T
     rrb = cra; // error: 'A' is not assignable to 'B'
     ~~~
 !!! error TS2322: Type 'C<A>' is not assignable to type 'readonly B[]'.
-!!! error TS2322:   The types returned by 'concat(...)' are incompatible between these types.
+!!! error TS2322:   The types returned by 'slice(...)' are incompatible between these types.
 !!! error TS2322:     Type 'A[]' is not assignable to type 'B[]'.
 !!! error TS2322:       Type 'A' is not assignable to type 'B'.
 

tests/baselines/reference/concatError.types

@@ -15,19 +15,19 @@ fa = fa.concat([0]);
 >fa = fa.concat([0]) : number[]
 >fa : number[]
 >fa.concat([0]) : number[]
->fa.concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
+>fa.concat : { (...items: ConcatArray<number>[]): number[]; <U extends (number | ConcatArray<number>)[]>(...items: U): (number | Flatten<U[number]>)[]; }
 >fa : number[]
->concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
+>concat : { (...items: ConcatArray<number>[]): number[]; <U extends (number | ConcatArray<number>)[]>(...items: U): (number | Flatten<U[number]>)[]; }
 >[0] : number[]
 >0 : 0
 
 fa = fa.concat(0);
 >fa = fa.concat(0) : number[]
 >fa : number[]
 >fa.concat(0) : number[]
->fa.concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
+>fa.concat : { (...items: ConcatArray<number>[]): number[]; <U extends (number | ConcatArray<number>)[]>(...items: U): (number | Flatten<U[number]>)[]; }
 >fa : number[]
->concat : { (...items: ConcatArray<number>[]): number[]; (...items: (number | ConcatArray<number>)[]): number[]; }
+>concat : { (...items: ConcatArray<number>[]): number[]; <U extends (number | ConcatArray<number>)[]>(...items: U): (number | Flatten<U[number]>)[]; }
 >0 : 0
 
 

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: ConcatArray<[number, number]>[]): [number, number][]; (...items: ([number, number] | ConcatArray<[number, number]>)[]): [number, number][]; }
+>ijs.concat : { (...items: ConcatArray<[number, number]>[]): [number, number][]; <U extends ([number, number] | ConcatArray<[number, number]>)[]>(...items: U): ([number, number] | Flatten<U[number]>)[]; }
 >ijs : [number, number][]
->concat : { (...items: ConcatArray<[number, number]>[]): [number, number][]; (...items: ([number, number] | ConcatArray<[number, number]>)[]): [number, number][]; }
+>concat : { (...items: ConcatArray<[number, number]>[]): [number, number][]; <U extends ([number, number] | ConcatArray<[number, number]>)[]>(...items: U): ([number, number] | Flatten<U[number]>)[]; }
 >[[3, 4], [5, 6]] : [number, number][]
 >[3, 4] : [number, number]
 >3 : 3

tests/baselines/reference/contextualExpressionTypecheckingDoesntBlowStack.types

@@ -69,15 +69,15 @@ export default class Operation {
 
                 // Commenting out this line will fix the problem.
                 result = (result || []).concat(innerResult);
->result = (result || []).concat(innerResult) : IValidationError[]
+>result = (result || []).concat(innerResult) : any[]
 >result : IValidationError[] | null
->(result || []).concat(innerResult) : IValidationError[]
->(result || []).concat : { (...items: ConcatArray<IValidationError>[]): IValidationError[]; (...items: (IValidationError | ConcatArray<IValidationError>)[]): IValidationError[]; }
+>(result || []).concat(innerResult) : any[]
+>(result || []).concat : { (...items: ConcatArray<IValidationError>[]): IValidationError[]; <U extends (IValidationError | ConcatArray<IValidationError>)[]>(...items: U): (IValidationError | Flatten<U[number]>)[]; }
 >(result || []) : IValidationError[]
 >result || [] : IValidationError[]
 >result : IValidationError[] | null
 >[] : never[]
->concat : { (...items: ConcatArray<IValidationError>[]): IValidationError[]; (...items: (IValidationError | ConcatArray<IValidationError>)[]): IValidationError[]; }
+>concat : { (...items: ConcatArray<IValidationError>[]): IValidationError[]; <U extends (IValidationError | ConcatArray<IValidationError>)[]>(...items: U): (IValidationError | Flatten<U[number]>)[]; }
 >innerResult : any
             }
         }

tests/baselines/reference/destructuringParameterDeclaration4.errors.txt

@@ -41,7 +41,7 @@ tests/cases/conformance/es6/destructuring/destructuringParameterDeclaration4.ts(
     a1(...array2);            // Error parameter type is (number|string)[]
           ~~~~~~
 !!! error TS2552: Cannot find name 'array2'. Did you mean 'Array'?
-!!! related TS2728 /.ts/lib.es5.d.ts:1385:13: 'Array' is declared here.
+!!! related TS2728 /.ts/lib.es5.d.ts:1386:13: 'Array' is declared here.
     a5([1, 2, "string", false, true]);       // Error, parameter type is [any, any, [[any]]]
               ~~~~~~~~
 !!! error TS2322: Type 'string' is not assignable to type '[[any]]'.