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diff --git a/‎src/compiler/checker.ts
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+82-7Lines changed: 82 additions & 7 deletions b/‎src/compiler/checker.ts
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+82-7Lines changed: 82 additions & 7 deletions
diff --git a/‎src/compiler/diagnosticMessages.json
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+5-1Lines changed: 5 additions & 1 deletion b/‎src/compiler/diagnosticMessages.json
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+5-1Lines changed: 5 additions & 1 deletion
diff --git a/‎src/compiler/emitter.ts
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+2Lines changed: 2 additions & 0 deletions b/‎src/compiler/emitter.ts
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+2Lines changed: 2 additions & 0 deletions
diff --git a/‎src/compiler/factory/emitHelpers.ts
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+30Lines changed: 30 additions & 0 deletions b/‎src/compiler/factory/emitHelpers.ts
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+30Lines changed: 30 additions & 0 deletions
diff --git a/‎src/compiler/factory/parenthesizerRules.ts
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+1-1Lines changed: 1 addition & 1 deletion b/‎src/compiler/factory/parenthesizerRules.ts
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+1-1Lines changed: 1 addition & 1 deletion
diff --git a/‎src/compiler/parser.ts
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+2Lines changed: 2 additions & 0 deletions b/‎src/compiler/parser.ts
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+2Lines changed: 2 additions & 0 deletions
diff --git a/‎src/compiler/transformers/classFields.ts
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+33-1Lines changed: 33 additions & 1 deletion b/‎src/compiler/transformers/classFields.ts
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+33-1Lines changed: 33 additions & 1 deletion
diff --git a/‎src/compiler/types.ts
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+4-2Lines changed: 4 additions & 2 deletions b/‎src/compiler/types.ts
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+4-2Lines changed: 4 additions & 2 deletions
diff --git a/‎src/compiler/utilities.ts
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+3Lines changed: 3 additions & 0 deletions b/‎src/compiler/utilities.ts
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diff --git a/‎src/compiler/utilitiesPublic.ts
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+1Lines changed: 1 addition & 0 deletions b/‎src/compiler/utilitiesPublic.ts
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@@ -23917,6 +23917,9 @@ namespace ts {
                     case SyntaxKind.InstanceOfKeyword:
                         return narrowTypeByInstanceof(type, expr, assumeTrue);
                     case SyntaxKind.InKeyword:
+                        if (isPrivateIdentifier(expr.left)) {
+                            return narrowTypeByPrivateIdentifierInInExpression(type, expr, assumeTrue);
+                        }
                         const target = getReferenceCandidate(expr.right);
                         const leftType = getTypeOfNode(expr.left);
                         if (leftType.flags & TypeFlags.StringLiteral) {
@@ -23947,6 +23950,24 @@ namespace ts {
                 return type;
             }
 
+            function narrowTypeByPrivateIdentifierInInExpression(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
+                const target = getReferenceCandidate(expr.right);
+                if (!isMatchingReference(reference, target)) {
+                    return type;
+                }
+
+                Debug.assertNode(expr.left, isPrivateIdentifier);
+                const symbol = getSymbolForPrivateIdentifierExpression(expr.left);
+                if (symbol === undefined) {
+                    return type;
+                }
+                const classSymbol = symbol.parent!;
+                const targetType = hasStaticModifier(Debug.checkDefined(symbol.valueDeclaration, "should always have a declaration"))
+                    ? getTypeOfSymbol(classSymbol) as InterfaceType
+                    : getDeclaredTypeOfSymbol(classSymbol);
+                return getNarrowedType(type, targetType, assumeTrue, isTypeDerivedFrom);
+            }
+
             function narrowTypeByOptionalChainContainment(type: Type, operator: SyntaxKind, value: Expression, assumeTrue: boolean): Type {
                 // We are in a branch of obj?.foo === value (or any one of the other equality operators). We narrow obj as follows:
                 // When operator is === and type of value excludes undefined, null and undefined is removed from type of obj in true branch.
@@ -27790,6 +27811,40 @@ namespace ts {
             }
         }
 
+        function checkGrammarPrivateIdentifierExpression(privId: PrivateIdentifier): boolean {
+            if (!getContainingClass(privId)) {
+                return grammarErrorOnNode(privId, Diagnostics.Private_identifiers_are_not_allowed_outside_class_bodies);
+            }
+            if (!isExpressionNode(privId)) {
+                return grammarErrorOnNode(privId, Diagnostics.Private_identifiers_are_only_allowed_in_class_bodies_and_may_only_be_used_as_part_of_a_class_member_declaration_property_access_or_on_the_left_hand_side_of_an_in_expression);
+            }
+            if (!getSymbolForPrivateIdentifierExpression(privId)) {
+                return grammarErrorOnNode(privId, Diagnostics.Cannot_find_name_0, idText(privId));
+            }
+            return false;
+        }
+
+        function checkPrivateIdentifierExpression(privId: PrivateIdentifier): Type {
+            checkGrammarPrivateIdentifierExpression(privId);
+            const symbol = getSymbolForPrivateIdentifierExpression(privId);
+            if (symbol) {
+                markPropertyAsReferenced(symbol, /* nodeForCheckWriteOnly: */ undefined, /* isThisAccess: */ false);
+            }
+            return anyType;
+        }
+
+        function getSymbolForPrivateIdentifierExpression(privId: PrivateIdentifier): Symbol | undefined {
+            if (!isExpressionNode(privId)) {
+                return undefined;
+            }
+
+            const links = getNodeLinks(privId);
+            if (links.resolvedSymbol === undefined) {
+                links.resolvedSymbol = lookupSymbolForPrivateIdentifierDeclaration(privId.escapedText, privId);
+            }
+            return links.resolvedSymbol;
+        }
+
         function getPrivateIdentifierPropertyOfType(leftType: Type, lexicallyScopedIdentifier: Symbol): Symbol | undefined {
             return getPropertyOfType(leftType, lexicallyScopedIdentifier.escapedName);
         }
@@ -32110,11 +32165,29 @@ namespace ts {
             if (leftType === silentNeverType || rightType === silentNeverType) {
                 return silentNeverType;
             }
-            leftType = checkNonNullType(leftType, left);
+            if (isPrivateIdentifier(left)) {
+                if (languageVersion < ScriptTarget.ESNext) {
+                    checkExternalEmitHelpers(left, ExternalEmitHelpers.ClassPrivateFieldIn);
+                }
+                // Unlike in 'checkPrivateIdentifierExpression' we now have access to the RHS type
+                // which provides us with the opportunity to emit more detailed errors
+                if (!getNodeLinks(left).resolvedSymbol && getContainingClass(left)) {
+                    const isUncheckedJS = isUncheckedJSSuggestion(left, rightType.symbol, /*excludeClasses*/ true);
+                    reportNonexistentProperty(left, rightType, isUncheckedJS);
+                }
+            }
+            else {
+                leftType = checkNonNullType(leftType, left);
+                // TypeScript 1.0 spec (April 2014): 4.15.5
+                // Require the left operand to be of type Any, the String primitive type, or the Number primitive type.
+                if (!(allTypesAssignableToKind(leftType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbolLike) ||
+                    isTypeAssignableToKind(leftType, TypeFlags.Index | TypeFlags.TemplateLiteral | TypeFlags.StringMapping | TypeFlags.TypeParameter))) {
+                    error(left, Diagnostics.The_left_hand_side_of_an_in_expression_must_be_a_private_identifier_or_of_type_any_string_number_or_symbol);
+                }
+            }
             rightType = checkNonNullType(rightType, right);
             // TypeScript 1.0 spec (April 2014): 4.15.5
-            // The in operator requires the left operand to be of type Any, the String primitive type, or the Number primitive type,
-            // and the right operand to be
+            // The in operator requires the right operand to be
             //
             //   1. assignable to the non-primitive type,
             //   2. an unconstrained type parameter,
@@ -32132,10 +32205,6 @@ namespace ts {
             // unless *all* instantiations would result in an error.
             //
             // The result is always of the Boolean primitive type.
-            if (!(allTypesAssignableToKind(leftType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbolLike) ||
-                  isTypeAssignableToKind(leftType, TypeFlags.Index | TypeFlags.TemplateLiteral | TypeFlags.StringMapping | TypeFlags.TypeParameter))) {
-                error(left, Diagnostics.The_left_hand_side_of_an_in_expression_must_be_of_type_any_string_number_or_symbol);
-            }
             const rightTypeConstraint = getConstraintOfType(rightType);
             if (!allTypesAssignableToKind(rightType, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive) ||
                 rightTypeConstraint && (
@@ -33517,6 +33586,8 @@ namespace ts {
             switch (kind) {
                 case SyntaxKind.Identifier:
                     return checkIdentifier(node as Identifier, checkMode);
+                case SyntaxKind.PrivateIdentifier:
+                    return checkPrivateIdentifierExpression(node as PrivateIdentifier);
                 case SyntaxKind.ThisKeyword:
                     return checkThisExpression(node);
                 case SyntaxKind.SuperKeyword:
@@ -40296,6 +40367,9 @@ namespace ts {
                     }
                     return result;
                 }
+                else if (isPrivateIdentifier(name)) {
+                    return getSymbolForPrivateIdentifierExpression(name);
+                }
                 else if (name.kind === SyntaxKind.PropertyAccessExpression || name.kind === SyntaxKind.QualifiedName) {
                     const links = getNodeLinks(name);
                     if (links.resolvedSymbol) {
@@ -41712,6 +41786,7 @@ namespace ts {
                 case ExternalEmitHelpers.MakeTemplateObject: return "__makeTemplateObject";
                 case ExternalEmitHelpers.ClassPrivateFieldGet: return "__classPrivateFieldGet";
                 case ExternalEmitHelpers.ClassPrivateFieldSet: return "__classPrivateFieldSet";
+                case ExternalEmitHelpers.ClassPrivateFieldIn: return "__classPrivateFieldIn";
                 case ExternalEmitHelpers.CreateBinding: return "__createBinding";
                 default: return Debug.fail("Unrecognized helper");
             }
 
@@ -1392,6 +1392,10 @@
         "category": "Message",
         "code": 1450
     },
+    "Private identifiers are only allowed in class bodies and may only be used as part of a class member declaration, property access, or on the left-hand-side of an 'in' expression": {
+        "category": "Error",
+        "code": 1451
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",
@@ -1654,7 +1658,7 @@
         "category": "Error",
         "code": 2359
     },
-    "The left-hand side of an 'in' expression must be of type 'any', 'string', 'number', or 'symbol'.": {
+    "The left-hand side of an 'in' expression must be a private identifier or of type 'any', 'string', 'number', or 'symbol'.": {
         "category": "Error",
         "code": 2360
     },
 
@@ -1684,6 +1684,8 @@ namespace ts {
                     // Identifiers
                     case SyntaxKind.Identifier:
                         return emitIdentifier(node as Identifier);
+                    case SyntaxKind.PrivateIdentifier:
+                        return emitPrivateIdentifier(node as PrivateIdentifier);
 
                     // Expressions
                     case SyntaxKind.ArrayLiteralExpression:
 
@@ -34,6 +34,7 @@ namespace ts {
         // Class Fields Helpers
         createClassPrivateFieldGetHelper(receiver: Expression, state: Identifier, kind: PrivateIdentifierKind, f: Identifier | undefined): Expression;
         createClassPrivateFieldSetHelper(receiver: Expression, state: Identifier, value: Expression, kind: PrivateIdentifierKind, f: Identifier | undefined): Expression;
+        createClassPrivateFieldInHelper(state: Identifier, receiver: Expression): Expression;
     }
 
     export function createEmitHelperFactory(context: TransformationContext): EmitHelperFactory {
@@ -75,6 +76,7 @@ namespace ts {
             // Class Fields Helpers
             createClassPrivateFieldGetHelper,
             createClassPrivateFieldSetHelper,
+            createClassPrivateFieldInHelper
         };
 
         /**
@@ -395,6 +397,10 @@ namespace ts {
             return factory.createCallExpression(getUnscopedHelperName("__classPrivateFieldSet"), /*typeArguments*/ undefined, args);
         }
 
+        function createClassPrivateFieldInHelper(state: Identifier, receiver: Expression) {
+            context.requestEmitHelper(classPrivateFieldInHelper);
+            return factory.createCallExpression(getUnscopedHelperName("__classPrivateFieldIn"), /* typeArguments*/ undefined, [state, receiver]);
+        }
     }
 
     /* @internal */
@@ -961,6 +967,29 @@ namespace ts {
             };`
     };
 
+    /**
+     * Parameters:
+     *  @param state — One of the following:
+     *      - A WeakMap when the member is a private instance field.
+     *      - A WeakSet when the member is a private instance method or accessor.
+     *      - A function value that should be the undecorated class constructor when the member is a private static field, method, or accessor.
+     *  @param receiver — The object being checked if it has the private member.
+     *
+     * Usage:
+     * This helper is used to transform `#field in expression` to
+     *      `__classPrivateFieldIn(<weakMap/weakSet/constructor>, expression)`
+     */
+    export const classPrivateFieldInHelper: UnscopedEmitHelper = {
+        name: "typescript:classPrivateFieldIn",
+        importName: "__classPrivateFieldIn",
+        scoped: false,
+        text: `
+            var __classPrivateFieldIn = (this && this.__classPrivateFieldIn) || function(state, receiver) {
+                if (receiver === null || (typeof receiver !== "object" && typeof receiver !== "function")) throw new TypeError("Cannot use 'in' operator on non-object");
+                return typeof state === "function" ? receiver === state : state.has(receiver);
+            };`
+    };
+
     let allUnscopedEmitHelpers: ReadonlyESMap<string, UnscopedEmitHelper> | undefined;
 
     export function getAllUnscopedEmitHelpers() {
@@ -986,6 +1015,7 @@ namespace ts {
             exportStarHelper,
             classPrivateFieldGetHelper,
             classPrivateFieldSetHelper,
+            classPrivateFieldInHelper,
             createBindingHelper,
             setModuleDefaultHelper
         ], helper => helper.name));
 
@@ -452,4 +452,4 @@ namespace ts {
         parenthesizeConstituentTypesOfUnionOrIntersectionType: nodes => cast(nodes, isNodeArray),
         parenthesizeTypeArguments: nodes => nodes && cast(nodes, isNodeArray),
     };
-}
+}
@@ -5617,6 +5617,8 @@ namespace ts {
                     break;
                 case SyntaxKind.TemplateHead:
                     return parseTemplateExpression(/* isTaggedTemplate */ false);
+                case SyntaxKind.PrivateIdentifier:
+                    return parsePrivateIdentifier();
             }
 
             return parseIdentifier(Diagnostics.Expression_expected);
 
@@ -266,15 +266,44 @@ namespace ts {
 
         /**
          * If we visit a private name, this means it is an undeclared private name.
-         * Replace it with an empty identifier to indicate a problem with the code.
+         * Replace it with an empty identifier to indicate a problem with the code,
+         * unless we are in a statement position - otherwise this will not trigger
+         * a SyntaxError.
          */
         function visitPrivateIdentifier(node: PrivateIdentifier) {
             if (!shouldTransformPrivateElementsOrClassStaticBlocks) {
                 return node;
             }
+            if (isStatement(node.parent)) {
+                return node;
+            }
             return setOriginalNode(factory.createIdentifier(""), node);
         }
 
+        /**
+         * Visits `#id in expr`
+         */
+        function visitPrivateIdentifierInInExpression(node: BinaryExpression) {
+            if (!shouldTransformPrivateElementsOrClassStaticBlocks) {
+                return node;
+            }
+            const privId = node.left;
+            Debug.assertNode(privId, isPrivateIdentifier);
+            Debug.assert(node.operatorToken.kind === SyntaxKind.InKeyword);
+            const info = accessPrivateIdentifier(privId);
+            if (info) {
+                const receiver = visitNode(node.right, visitor, isExpression);
+
+                return setOriginalNode(
+                    context.getEmitHelperFactory().createClassPrivateFieldInHelper(info.brandCheckIdentifier, receiver),
+                    node
+                );
+            }
+
+            // Private name has not been declared. Subsequent transformers will handle this error
+            return visitEachChild(node, visitor, context);
+        }
+
         /**
          * Visits the members of a class that has fields.
          *
@@ -827,6 +856,9 @@ namespace ts {
                     }
                 }
             }
+            if (node.operatorToken.kind === SyntaxKind.InKeyword && isPrivateIdentifier(node.left)) {
+                return visitPrivateIdentifierInInExpression(node);
+            }
             return visitEachChild(node, visitor, context);
         }
 
 
@@ -1214,7 +1214,8 @@ namespace ts {
         readonly expression: Expression;
     }
 
-    export interface PrivateIdentifier extends Node {
+    // Typed as a PrimaryExpression due to its presence in BinaryExpressions (#field in expr)
+    export interface PrivateIdentifier extends PrimaryExpression {
         readonly kind: SyntaxKind.PrivateIdentifier;
         // escaping not strictly necessary
         // avoids gotchas in transforms and utils
@@ -6854,7 +6855,8 @@ namespace ts {
         MakeTemplateObject = 1 << 18,   // __makeTemplateObject (used for constructing template string array objects)
         ClassPrivateFieldGet = 1 << 19, // __classPrivateFieldGet (used by the class private field transformation)
         ClassPrivateFieldSet = 1 << 20, // __classPrivateFieldSet (used by the class private field transformation)
-        CreateBinding = 1 << 21,        // __createBinding (use by the module transform for (re)exports and namespace imports)
+        ClassPrivateFieldIn = 1 << 21,  // __classPrivateFieldIn (used by the class private field transformation)
+        CreateBinding = 1 << 22,        // __createBinding (use by the module transform for (re)exports and namespace imports)
         FirstEmitHelper = Extends,
         LastEmitHelper = CreateBinding,
 
 
@@ -1949,6 +1949,8 @@ namespace ts {
                     node = node.parent;
                 }
                 return node.parent.kind === SyntaxKind.TypeQuery || isJSDocLinkLike(node.parent) || isJSDocNameReference(node.parent) || isJSDocMemberName(node.parent) || isJSXTagName(node);
+            case SyntaxKind.PrivateIdentifier:
+                return isBinaryExpression(node.parent) && node.parent.left === node && node.parent.operatorToken.kind === SyntaxKind.InKeyword;
             case SyntaxKind.Identifier:
                 if (node.parent.kind === SyntaxKind.TypeQuery || isJSDocLinkLike(node.parent) || isJSDocNameReference(node.parent) || isJSDocMemberName(node.parent) || isJSXTagName(node)) {
                     return true;
@@ -3706,6 +3708,7 @@ namespace ts {
             case SyntaxKind.ThisKeyword:
             case SyntaxKind.SuperKeyword:
             case SyntaxKind.Identifier:
+            case SyntaxKind.PrivateIdentifier:
             case SyntaxKind.NullKeyword:
             case SyntaxKind.TrueKeyword:
             case SyntaxKind.FalseKeyword:
 
@@ -1516,6 +1516,7 @@ namespace ts {
             case SyntaxKind.ClassExpression:
             case SyntaxKind.FunctionExpression:
             case SyntaxKind.Identifier:
+            case SyntaxKind.PrivateIdentifier: // technically this is only an Expression if it's in a `#field in expr` BinaryExpression
             case SyntaxKind.RegularExpressionLiteral:
             case SyntaxKind.NumericLiteral:
             case SyntaxKind.BigIntLiteral:
Original file line number	Diff line number	Diff line change
`@@ -5617,6 +5617,8 @@ namespace ts {`
`5617`	`5617`	`break;`
`5618`	`5618`	`case SyntaxKind.TemplateHead:`
`5619`	`5619`	`return parseTemplateExpression(/* isTaggedTemplate */ false);`
	`5620`	`+ case SyntaxKind.PrivateIdentifier:`
	`5621`	`+ return parsePrivateIdentifier();`
`5620`	`5622`	`}`
`5621`	`5623`
`5622`	`5624`	`return parseIdentifier(Diagnostics.Expression_expected);`