Allow this when it appears in this is T positions

src/compiler/checker.ts

@@ -19673,7 +19673,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         if (isJSConstructor(container) && isNodeDescendantOf(node, container.body)) {
             return getDeclaredTypeOfClassOrInterface(getSymbolOfDeclaration(container)).thisType!;
         }
-        error(node, Diagnostics.A_this_type_is_available_only_in_a_non_static_member_of_a_class_or_interface);
+        if (!(node.parent.kind === SyntaxKind.TypePredicate && node === (node.parent as TypePredicateNode).parameterName)) {
+            error(node, Diagnostics.A_this_type_is_available_only_in_a_non_static_member_of_a_class_or_interface);
+        }
         return errorType;
     }
 

tests/baselines/reference/declarationEmitThisPredicates02.types

@@ -33,7 +33,6 @@ export const obj = {
 >this as {} : {}
 >           : ^^
 >this : any
->     : ^^^
 
         return dis.a != null && dis.b != null && dis.c != null;
 >dis.a != null && dis.b != null && dis.c != null : boolean

tests/baselines/reference/declarationEmitThisPredicatesWithPrivateName02.types

@@ -33,7 +33,6 @@ export const obj = {
 >this as {} : {}
 >           : ^^
 >this : any
->     : ^^^
 
         return dis.a != null && dis.b != null && dis.c != null;
 >dis.a != null && dis.b != null && dis.c != null : boolean

tests/baselines/reference/mappedTypeWithAsClauseAndLateBoundProperty2.js

@@ -107,113 +107,3 @@ export declare const thing: {
         readonly [Symbol.unscopables]?: boolean;
     };
 };
-
-
-//// [DtsFileErrors]
-
-
-mappedTypeWithAsClauseAndLateBoundProperty2.d.ts(27,118): error TS2526: A 'this' type is available only in a non-static member of a class or interface.
-
-
-==== mappedTypeWithAsClauseAndLateBoundProperty2.d.ts (1 errors) ====
-    export declare const thing: {
-        [x: number]: number;
-        toString: () => string;
-        toLocaleString: {
-            (): string;
-            (locales: string | string[], options?: Intl.NumberFormatOptions & Intl.DateTimeFormatOptions): string;
-        };
-        pop: () => number;
-        push: (...items: number[]) => number;
-        concat: {
-            (...items: ConcatArray<number>[]): number[];
-            (...items: (number | ConcatArray<number>)[]): number[];
-        };
-        join: (separator?: string) => string;
-        reverse: () => number[];
-        shift: () => number;
-        slice: (start?: number, end?: number) => number[];
-        sort: (compareFn?: (a: number, b: number) => number) => number[];
-        splice: {
-            (start: number, deleteCount?: number): number[];
-            (start: number, deleteCount: number, ...items: number[]): number[];
-        };
-        unshift: (...items: number[]) => number;
-        indexOf: (searchElement: number, fromIndex?: number) => number;
-        lastIndexOf: (searchElement: number, fromIndex?: number) => number;
-        every: {
-            <S extends number>(predicate: (value: number, index: number, array: number[]) => value is S, thisArg?: any): this is S[];
-                                                                                                                         ~~~~
-!!! error TS2526: A 'this' type is available only in a non-static member of a class or interface.
-            (predicate: (value: number, index: number, array: number[]) => unknown, thisArg?: any): boolean;
-        };
-        some: (predicate: (value: number, index: number, array: number[]) => unknown, thisArg?: any) => boolean;
-        forEach: (callbackfn: (value: number, index: number, array: number[]) => void, thisArg?: any) => void;
-        map: <U>(callbackfn: (value: number, index: number, array: number[]) => U, thisArg?: any) => U[];
-        filter: {
-            <S extends number>(predicate: (value: number, index: number, array: number[]) => value is S, thisArg?: any): S[];
-            (predicate: (value: number, index: number, array: number[]) => unknown, thisArg?: any): number[];
-        };
-        reduce: {
-            (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number;
-            (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number;
-            <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U;
-        };
-        reduceRight: {
-            (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number;
-            (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number;
-            <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U;
-        };
-        find: {
-            <S extends number>(predicate: (value: number, index: number, obj: number[]) => value is S, thisArg?: any): S;
-            (predicate: (value: number, index: number, obj: number[]) => unknown, thisArg?: any): number;
-        };
-        findIndex: (predicate: (value: number, index: number, obj: number[]) => unknown, thisArg?: any) => number;
-        fill: (value: number, start?: number, end?: number) => number[];
-        copyWithin: (target: number, start: number, end?: number) => number[];
-        entries: () => IterableIterator<[number, number]>;
-        keys: () => IterableIterator<number>;
-        values: () => IterableIterator<number>;
-        includes: (searchElement: number, fromIndex?: number) => boolean;
-        flatMap: <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[];
-        flat: <A, D extends number = 1>(this: A, depth?: D) => FlatArray<A, D>[];
-        [Symbol.iterator]: () => IterableIterator<number>;
-        readonly [Symbol.unscopables]: {
-            [x: number]: boolean;
-            length?: boolean;
-            toString?: boolean;
-            toLocaleString?: boolean;
-            pop?: boolean;
-            push?: boolean;
-            concat?: boolean;
-            join?: boolean;
-            reverse?: boolean;
-            shift?: boolean;
-            slice?: boolean;
-            sort?: boolean;
-            splice?: boolean;
-            unshift?: boolean;
-            indexOf?: boolean;
-            lastIndexOf?: boolean;
-            every?: boolean;
-            some?: boolean;
-            forEach?: boolean;
-            map?: boolean;
-            filter?: boolean;
-            reduce?: boolean;
-            reduceRight?: boolean;
-            find?: boolean;
-            findIndex?: boolean;
-            fill?: boolean;
-            copyWithin?: boolean;
-            entries?: boolean;
-            keys?: boolean;
-            values?: boolean;
-            includes?: boolean;
-            flatMap?: boolean;
-            flat?: boolean;
-            [Symbol.iterator]?: boolean;
-            readonly [Symbol.unscopables]?: boolean;
-        };
-    };
-

tests/baselines/reference/thisPredicateInObjectLiteral.errors.txt

@@ -0,0 +1,20 @@
+thisPredicateInObjectLiteral.ts(10,28): error TS2526: A 'this' type is available only in a non-static member of a class or interface.
+
+
+==== thisPredicateInObjectLiteral.ts (1 errors) ====
+    // Should be OK
+    const foo2 = {
+        isNumber(): this is { b: string } {
+            return true;
+        },
+    };
+
+    // Still an error
+    const foo3 = {
+        isNumber(x: any): x is this {
+                               ~~~~
+!!! error TS2526: A 'this' type is available only in a non-static member of a class or interface.
+            return true;
+        },
+    };
+

tests/baselines/reference/thisPredicateInObjectLiteral.js

@@ -0,0 +1,32 @@
+//// [tests/cases/compiler/thisPredicateInObjectLiteral.ts] ////
+
+//// [thisPredicateInObjectLiteral.ts]
+// Should be OK
+const foo2 = {
+    isNumber(): this is { b: string } {
+        return true;
+    },
+};
+
+// Still an error
+const foo3 = {
+    isNumber(x: any): x is this {
+        return true;
+    },
+};
+
+
+//// [thisPredicateInObjectLiteral.js]
+"use strict";
+// Should be OK
+var foo2 = {
+    isNumber: function () {
+        return true;
+    },
+};
+// Still an error
+var foo3 = {
+    isNumber: function (x) {
+        return true;
+    },
+};

tests/baselines/reference/thisPredicateInObjectLiteral.symbols

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/thisPredicateInObjectLiteral.ts] ////
+
+=== thisPredicateInObjectLiteral.ts ===
+// Should be OK
+const foo2 = {
+>foo2 : Symbol(foo2, Decl(thisPredicateInObjectLiteral.ts, 1, 5))
+
+    isNumber(): this is { b: string } {
+>isNumber : Symbol(isNumber, Decl(thisPredicateInObjectLiteral.ts, 1, 14))
+>b : Symbol(b, Decl(thisPredicateInObjectLiteral.ts, 2, 25))
+
+        return true;
+    },
+};
+
+// Still an error
+const foo3 = {
+>foo3 : Symbol(foo3, Decl(thisPredicateInObjectLiteral.ts, 8, 5))
+
+    isNumber(x: any): x is this {
+>isNumber : Symbol(isNumber, Decl(thisPredicateInObjectLiteral.ts, 8, 14))
+>x : Symbol(x, Decl(thisPredicateInObjectLiteral.ts, 9, 13))
+>x : Symbol(x, Decl(thisPredicateInObjectLiteral.ts, 9, 13))
+
+        return true;
+    },
+};
+

tests/baselines/reference/thisPredicateInObjectLiteral.types

@@ -0,0 +1,43 @@
+//// [tests/cases/compiler/thisPredicateInObjectLiteral.ts] ////
+
+=== thisPredicateInObjectLiteral.ts ===
+// Should be OK
+const foo2 = {
+>foo2 : { isNumber(): this is { b: string; }; }
+>     : ^^^^^^^^^^^^^^                      ^^^
+>{    isNumber(): this is { b: string } {        return true;    },} : { isNumber(): this is { b: string; }; }
+>                                                                    : ^^^^^^^^^^^^^^                      ^^^
+
+    isNumber(): this is { b: string } {
+>isNumber : () => this is { b: string; }
+>         : ^^^^^^                      
+>b : string
+>  : ^^^^^^
+
+        return true;
+>true : true
+>     : ^^^^
+
+    },
+};
+
+// Still an error
+const foo3 = {
+>foo3 : { isNumber(x: any): x is this; }
+>     : ^^^^^^^^^^^ ^^   ^^^         ^^^
+>{    isNumber(x: any): x is this {        return true;    },} : { isNumber(x: any): x is this; }
+>                                                              : ^^^^^^^^^^^ ^^   ^^^         ^^^
+
+    isNumber(x: any): x is this {
+>isNumber : (x: any) => x is this
+>         : ^ ^^   ^^^^^         
+>x : any
+>  : ^^^
+
+        return true;
+>true : true
+>     : ^^^^
+
+    },
+};
+

tests/baselines/reference/typeGuardFunctionOfFormThisErrors.errors.txt

@@ -10,12 +10,11 @@ typeGuardFunctionOfFormThisErrors.ts(26,1): error TS2322: Type '() => this is Le
 typeGuardFunctionOfFormThisErrors.ts(27,1): error TS2322: Type '() => this is FollowerGuard' is not assignable to type '() => this is LeadGuard'.
   Type predicate 'this is FollowerGuard' is not assignable to 'this is LeadGuard'.
     Property 'lead' is missing in type 'FollowerGuard' but required in type 'LeadGuard'.
-typeGuardFunctionOfFormThisErrors.ts(29,32): error TS2526: A 'this' type is available only in a non-static member of a class or interface.
 typeGuardFunctionOfFormThisErrors.ts(55,7): error TS2339: Property 'follow' does not exist on type 'RoyalGuard'.
 typeGuardFunctionOfFormThisErrors.ts(58,7): error TS2339: Property 'lead' does not exist on type 'RoyalGuard'.
 
 
-==== typeGuardFunctionOfFormThisErrors.ts (7 errors) ====
+==== typeGuardFunctionOfFormThisErrors.ts (6 errors) ====
     class RoyalGuard {
         isLeader(): this is LeadGuard {
             return this instanceof LeadGuard;
@@ -65,8 +64,6 @@ typeGuardFunctionOfFormThisErrors.ts(58,7): error TS2339: Property 'lead' does n
 !!! related TS2728 typeGuardFunctionOfFormThisErrors.ts:11:5: 'lead' is declared here.
 
     function invalidGuard(c: any): this is number {
-                                   ~~~~
-!!! error TS2526: A 'this' type is available only in a non-static member of a class or interface.
         return false;
     }
 

tests/cases/compiler/thisPredicateInObjectLiteral.ts

@@ -0,0 +1,15 @@
+// @strict: true
+
+// Should be OK
+const foo2 = {
+    isNumber(): this is { b: string } {
+        return true;
+    },
+};
+
+// Still an error
+const foo3 = {
+    isNumber(x: any): x is this {
+        return true;
+    },
+};