microsoft
diff --git a/‎src/compiler/checker.ts
+143-76 b/‎src/compiler/checker.ts
+143-76
diff --git a/‎src/compiler/diagnosticMessages.json
+24-8 b/‎src/compiler/diagnosticMessages.json
+24-8
diff --git a/‎src/compiler/factory/emitHelpers.ts
+130-20 b/‎src/compiler/factory/emitHelpers.ts
+130-20
@@ -3288,6 +3288,26 @@
         "category": "Error",
         "code": 2802
     },
+    "Cannot assign to private method '{0}'. Private methods are not writable.": {
+        "category": "Error",
+        "code": 2803
+    },
+    "Duplicate identifier '{0}'. Static and instance elements cannot share the same private name.": {
+        "category": "Error",
+        "code": 2804
+    },
+    "Static fields with private names can't have initializers when the '--useDefineForClassFields' flag is not specified with a '--target' of 'esnext'. Consider adding the '--useDefineForClassFields' flag.": {
+        "category": "Error",
+        "code": 2805
+    },
+    "Private accessor was defined without a getter.": {
+        "category": "Error",
+        "code": 2806
+    },
+    "This syntax requires an imported helper named '{1}' with {2} parameters, which is not compatible with the one in '{0}'. Consider upgrading your version of '{0}'.": {
+        "category": "Error",
+        "code": 2807
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
@@ -6338,14 +6358,6 @@
         "category": "Error",
         "code": 18019
     },
-    "A method cannot be named with a private identifier.": {
-        "category": "Error",
-        "code": 18022
-    },
-    "An accessor cannot be named with a private identifier.": {
-        "category": "Error",
-        "code": 18023
-    },
     "An enum member cannot be named with a private identifier.": {
         "category": "Error",
         "code": 18024
@@ -6389,5 +6401,9 @@
     "Invalid value for 'jsxFragmentFactory'. '{0}' is not a valid identifier or qualified-name.": {
         "category": "Error",
         "code": 18035
+    },
+    "Class decorators can't be used with static private identifier. Consider removing the experimental decorator.": {
+        "category": "Error",
+        "code": 18036
     }
 }
@@ -32,8 +32,8 @@ namespace ts {
         createImportDefaultHelper(expression: Expression): Expression;
         createExportStarHelper(moduleExpression: Expression, exportsExpression?: Expression): Expression;
         // Class Fields Helpers
-        createClassPrivateFieldGetHelper(receiver: Expression, privateField: Identifier): Expression;
-        createClassPrivateFieldSetHelper(receiver: Expression, privateField: Identifier, value: Expression): Expression;
+        createClassPrivateFieldGetHelper(receiver: Expression, state: Identifier, kind: PrivateIdentifierKind, f: Identifier | undefined): Expression;
+        createClassPrivateFieldSetHelper(receiver: Expression, state: Identifier, value: Expression, kind: PrivateIdentifierKind, f: Identifier | undefined): Expression;
     }
 
     export function createEmitHelperFactory(context: TransformationContext): EmitHelperFactory {
@@ -368,15 +368,30 @@ namespace ts {
 
         // Class Fields Helpers
 
-        function createClassPrivateFieldGetHelper(receiver: Expression, privateField: Identifier) {
+        function createClassPrivateFieldGetHelper(receiver: Expression, state: Identifier, kind: PrivateIdentifierKind, f: Identifier | undefined) {
             context.requestEmitHelper(classPrivateFieldGetHelper);
-            return factory.createCallExpression(getUnscopedHelperName("__classPrivateFieldGet"), /*typeArguments*/ undefined, [receiver, privateField]);
+            let args;
+            if (!f) {
+                args = [receiver, state, factory.createStringLiteral(kind)];
+            }
+            else {
+                args = [receiver, state, factory.createStringLiteral(kind), f];
+            }
+            return factory.createCallExpression(getUnscopedHelperName("__classPrivateFieldGet"), /*typeArguments*/ undefined, args);
         }
 
-        function createClassPrivateFieldSetHelper(receiver: Expression, privateField: Identifier, value: Expression) {
+        function createClassPrivateFieldSetHelper(receiver: Expression, state: Identifier, value: Expression, kind: PrivateIdentifierKind, f: Identifier | undefined) {
             context.requestEmitHelper(classPrivateFieldSetHelper);
-            return factory.createCallExpression(getUnscopedHelperName("__classPrivateFieldSet"), /*typeArguments*/ undefined, [receiver, privateField, value]);
+            let args;
+            if (!f) {
+                args = [receiver, state, value, factory.createStringLiteral(kind)];
+            }
+            else {
+                args = [receiver, state, value, factory.createStringLiteral(kind), f];
+            }
+            return factory.createCallExpression(getUnscopedHelperName("__classPrivateFieldSet"), /*typeArguments*/ undefined, args);
         }
+
     }
 
     /* @internal */
@@ -803,7 +818,6 @@ namespace ts {
             };`
     };
 
-    // emit output for the __export helper function
     export const exportStarHelper: UnscopedEmitHelper = {
         name: "typescript:export-star",
         importName: "__exportStar",
@@ -816,31 +830,127 @@ namespace ts {
             };`
     };
 
-    // Class fields helpers
+    /**
+     * Parameters:
+     *  @param receiver — The object from which the private member will be read.
+     *  @param state — One of the following:
+     *      - A WeakMap used to read a private instance field.
+     *      - A WeakSet used as an instance brand for private instance methods and accessors.
+     *      - A function value that should be the undecorated class constructor used to brand check private static fields, methods, and accessors.
+     *  @param kind — (optional pre TS 4.3, required for TS 4.3+) One of the following values:
+     *      - undefined — Indicates a private instance field (pre TS 4.3).
+     *      - "f" — Indicates a private field (instance or static).
+     *      - "m" — Indicates a private method (instance or static).
+     *      - "a" — Indicates a private accessor (instance or static).
+     *  @param f — (optional pre TS 4.3) Depends on the arguments for state and kind:
+     *      - If kind is "m", this should be the function corresponding to the static or instance method.
+     *      - If kind is "a", this should be the function corresponding to the getter method, or undefined if the getter was not defined.
+     *      - If kind is "f" and state is a function, this should be an object holding the value of a static field, or undefined if the static field declaration has not yet been evaluated.
+     * Usage:
+     * This helper will only ever be used by the compiler in the following ways:
+     *
+     * Reading from a private instance field (pre TS 4.3):
+     *      __classPrivateFieldGet(<any>, <WeakMap>)
+     *
+     * Reading from a private instance field (TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <WeakMap>, "f")
+     *
+     * Reading from a private instance get accessor (when defined, TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <WeakSet>, "a", <function>)
+     *
+     * Reading from a private instance get accessor (when not defined, TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <WeakSet>, "a", void 0)
+     *      NOTE: This always results in a runtime error.
+     *
+     * Reading from a private instance method (TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <WeakSet>, "m", <function>)
+     *
+     * Reading from a private static field (TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <constructor>, "f", <{ value: any }>)
+     *
+     * Reading from a private static get accessor (when defined, TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <constructor>, "a", <function>)
+     *
+     * Reading from a private static get accessor (when not defined, TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <constructor>, "a", void 0)
+     *      NOTE: This always results in a runtime error.
+     *
+     * Reading from a private static method (TS 4.3+):
+     *      __classPrivateFieldGet(<any>, <constructor>, "m", <function>)
+     */
     export const classPrivateFieldGetHelper: UnscopedEmitHelper = {
         name: "typescript:classPrivateFieldGet",
         importName: "__classPrivateFieldGet",
         scoped: false,
         text: `
-            var __classPrivateFieldGet = (this && this.__classPrivateFieldGet) || function (receiver, privateMap) {
-                if (!privateMap.has(receiver)) {
-                    throw new TypeError("attempted to get private field on non-instance");
-                }
-                return privateMap.get(receiver);
+            var __classPrivateFieldGet = (this && this.__classPrivateFieldGet) || function (receiver, state, kind, f) {
+                if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a getter");
+                if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver)) throw new TypeError("Cannot read private member from an object whose class did not declare it");
+                return kind === "m" ? f : kind === "a" ? f.call(receiver) : f ? f.value : state.get(receiver);
             };`
     };
 
+    /**
+     * Parameters:
+     *  @param receiver — The object on which the private member will be set.
+     *  @param state — One of the following:
+     *      - A WeakMap used to store a private instance field.
+     *      - A WeakSet used as an instance brand for private instance methods and accessors.
+     *      - A function value that should be the undecorated class constructor used to brand check private static fields, methods, and accessors.
+     *  @param value — The value to set.
+     *  @param kind — (optional pre TS 4.3, required for TS 4.3+) One of the following values:
+     *       - undefined — Indicates a private instance field (pre TS 4.3).
+     *       - "f" — Indicates a private field (instance or static).
+     *       - "m" — Indicates a private method (instance or static).
+     *       - "a" — Indicates a private accessor (instance or static).
+     *   @param f — (optional pre TS 4.3) Depends on the arguments for state and kind:
+     *       - If kind is "m", this should be the function corresponding to the static or instance method.
+     *       - If kind is "a", this should be the function corresponding to the setter method, or undefined if the setter was not defined.
+     *       - If kind is "f" and state is a function, this should be an object holding the value of a static field, or undefined if the static field declaration has not yet been evaluated.
+     * Usage:
+     * This helper will only ever be used by the compiler in the following ways:
+     *
+     * Writing to a private instance field (pre TS 4.3):
+     *      __classPrivateFieldSet(<any>, <WeakMap>, <any>)
+     *
+     * Writing to a private instance field (TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <WeakMap>, <any>, "f")
+     *
+     * Writing to a private instance set accessor (when defined, TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <WeakSet>, <any>, "a", <function>)
+     *
+     * Writing to a private instance set accessor (when not defined, TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <WeakSet>, <any>, "a", void 0)
+     *      NOTE: This always results in a runtime error.
+     *
+     * Writing to a private instance method (TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <WeakSet>, <any>, "m", <function>)
+     *      NOTE: This always results in a runtime error.
+     *
+     * Writing to a private static field (TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <constructor>, <any>, "f", <{ value: any }>)
+     *
+     * Writing to a private static set accessor (when defined, TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <constructor>, <any>, "a", <function>)
+     *
+     * Writing to a private static set accessor (when not defined, TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <constructor>, <any>, "a", void 0)
+     *      NOTE: This always results in a runtime error.
+     *
+     * Writing to a private static method (TS 4.3+):
+     *      __classPrivateFieldSet(<any>, <constructor>, <any>, "m", <function>)
+     *      NOTE: This always results in a runtime error.
+     */
     export const classPrivateFieldSetHelper: UnscopedEmitHelper = {
         name: "typescript:classPrivateFieldSet",
         importName: "__classPrivateFieldSet",
         scoped: false,
         text: `
-            var __classPrivateFieldSet = (this && this.__classPrivateFieldSet) || function (receiver, privateMap, value) {
-                if (!privateMap.has(receiver)) {
-                    throw new TypeError("attempted to set private field on non-instance");
-                }
-                privateMap.set(receiver, value);
-                return value;
+            var __classPrivateFieldSet = (this && this.__classPrivateFieldSet) || function (receiver, state, value, kind, f) {
+                if (kind === "m") throw new TypeError("Private method is not writable");
+                if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a setter");
+                if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver)) throw new TypeError("Cannot write private member to an object whose class did not declare it");
+                return (kind === "a" ? f.call(receiver, value) : f ? f.value = value : state.set(receiver, value)), value;
             };`
     };
 
@@ -897,4 +1007,4 @@ namespace ts {
             && (getEmitFlags(firstSegment.expression) & EmitFlags.HelperName)
             && firstSegment.expression.escapedText === helperName;
     }
-}
+}