Discriminant of type never should never be matched (#50755)

* Discriminant of type 'never' should never be matched * Add tests
2026-02-05 07:45:18 -06:00 · 2022-09-13 10:29:50 -07:00 · 2022-09-13 10:29:50 -07:00 · 60963d7216
commit 60963d7216
parent e37ea53715
8 changed files with 1101 additions and 1 deletions
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@ -25130,7 +25130,7 @@ namespace ts {
                const narrowedPropType = narrowType(propType);
                return filterType(type, t => {
                    const discriminantType = getTypeOfPropertyOrIndexSignature(t, propName);
-                    return !(narrowedPropType.flags & TypeFlags.Never) && areTypesComparable(narrowedPropType, discriminantType);
+                    return !(discriminantType.flags & TypeFlags.Never) && !(narrowedPropType.flags & TypeFlags.Never) && areTypesComparable(narrowedPropType, discriminantType);
                });
            }

--- a/tests/baselines/reference/neverAsDiscriminantType(strict=false).js
+++ b/tests/baselines/reference/neverAsDiscriminantType(strict=false).js
@ -0,0 +1,143 @@
+//// [neverAsDiscriminantType.ts]
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+
+function f1(foo: Foo1) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+
+function f2(foo: Foo2) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+    op: O
+    d: D
+    t?: T
+    s?: number
+}
+
+export type GatewayPayload = {
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+    }[keyof GatewayEvents]
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+}[GatewayOpcode]
+
+export interface GatewayParams {
+    [GatewayOpcode.HELLO]: { b: 1 }
+}
+
+export enum GatewayOpcode {
+    DISPATCH = 0,
+    HEARTBEAT = 1,
+    IDENTIFY = 2,
+    PRESENCE_UPDATE = 3,
+    VOICE_STATE_UPDATE = 4,
+    RESUME = 6,
+    RECONNECT = 7,
+    REQUEST_GUILD_MEMBERS = 8,
+    INVALID_SESSION = 9,
+    HELLO = 10,
+    HEARTBEAT_ACK = 11,
+}
+
+export interface GatewayEvents {
+    MESSAGE_CREATE: { a: 1 }
+    MESSAGE_UPDATE: { a: 2 }
+    MESSAGE_DELETE: { a: 3 }
+}
+
+function assertMessage(event: { a: 1 }) { }
+
+export async function adaptSession(input: GatewayPayload) {
+    if (input.t === 'MESSAGE_CREATE') {
+        assertMessage(input.d)
+    }
+}
+
+
+//// [neverAsDiscriminantType.js]
+"use strict";
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }
+    return new (P || (P = Promise))(function (resolve, reject) {
+        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
+        function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }
+        step((generator = generator.apply(thisArg, _arguments || [])).next());
+    });
+};
+var __generator = (this && this.__generator) || function (thisArg, body) {
+    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
+    return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
+    function verb(n) { return function (v) { return step([n, v]); }; }
+    function step(op) {
+        if (f) throw new TypeError("Generator is already executing.");
+        while (_) try {
+            if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
+            if (y = 0, t) op = [op[0] & 2, t.value];
+            switch (op[0]) {
+                case 0: case 1: t = op; break;
+                case 4: _.label++; return { value: op[1], done: false };
+                case 5: _.label++; y = op[1]; op = [0]; continue;
+                case 7: op = _.ops.pop(); _.trys.pop(); continue;
+                default:
+                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
+                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
+                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
+                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
+                    if (t[2]) _.ops.pop();
+                    _.trys.pop(); continue;
+            }
+            op = body.call(thisArg, _);
+        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
+        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
+    }
+};
+exports.__esModule = true;
+exports.adaptSession = exports.GatewayOpcode = void 0;
+function f1(foo) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+function f2(foo) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+var GatewayOpcode;
+(function (GatewayOpcode) {
+    GatewayOpcode[GatewayOpcode["DISPATCH"] = 0] = "DISPATCH";
+    GatewayOpcode[GatewayOpcode["HEARTBEAT"] = 1] = "HEARTBEAT";
+    GatewayOpcode[GatewayOpcode["IDENTIFY"] = 2] = "IDENTIFY";
+    GatewayOpcode[GatewayOpcode["PRESENCE_UPDATE"] = 3] = "PRESENCE_UPDATE";
+    GatewayOpcode[GatewayOpcode["VOICE_STATE_UPDATE"] = 4] = "VOICE_STATE_UPDATE";
+    GatewayOpcode[GatewayOpcode["RESUME"] = 6] = "RESUME";
+    GatewayOpcode[GatewayOpcode["RECONNECT"] = 7] = "RECONNECT";
+    GatewayOpcode[GatewayOpcode["REQUEST_GUILD_MEMBERS"] = 8] = "REQUEST_GUILD_MEMBERS";
+    GatewayOpcode[GatewayOpcode["INVALID_SESSION"] = 9] = "INVALID_SESSION";
+    GatewayOpcode[GatewayOpcode["HELLO"] = 10] = "HELLO";
+    GatewayOpcode[GatewayOpcode["HEARTBEAT_ACK"] = 11] = "HEARTBEAT_ACK";
+})(GatewayOpcode = exports.GatewayOpcode || (exports.GatewayOpcode = {}));
+function assertMessage(event) { }
+function adaptSession(input) {
+    return __awaiter(this, void 0, void 0, function () {
+        return __generator(this, function (_a) {
+            if (input.t === 'MESSAGE_CREATE') {
+                assertMessage(input.d);
+            }
+            return [2 /*return*/];
+        });
+    });
+}
+exports.adaptSession = adaptSession;
--- a/tests/baselines/reference/neverAsDiscriminantType(strict=false).symbols
+++ b/tests/baselines/reference/neverAsDiscriminantType(strict=false).symbols
@ -0,0 +1,197 @@
+=== tests/cases/compiler/neverAsDiscriminantType.ts ===
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+>Foo1 : Symbol(Foo1, Decl(neverAsDiscriminantType.ts, 0, 0))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 13))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 0, 24))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 40))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 56))
+
+function f1(foo: Foo1) {
+>f1 : Symbol(f1, Decl(neverAsDiscriminantType.ts, 0, 71))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 2, 12))
+>Foo1 : Symbol(Foo1, Decl(neverAsDiscriminantType.ts, 0, 0))
+
+    if (foo.kind === 'a') {
+>foo.kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 13), Decl(neverAsDiscriminantType.ts, 0, 40), Decl(neverAsDiscriminantType.ts, 0, 56))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 2, 12))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 13), Decl(neverAsDiscriminantType.ts, 0, 40), Decl(neverAsDiscriminantType.ts, 0, 56))
+
+        foo.a;
+>foo.a : Symbol(a, Decl(neverAsDiscriminantType.ts, 0, 24))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 2, 12))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 0, 24))
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+>Foo2 : Symbol(Foo2, Decl(neverAsDiscriminantType.ts, 6, 1))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 13))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 8, 25))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 41))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 58))
+
+function f2(foo: Foo2) {
+>f2 : Symbol(f2, Decl(neverAsDiscriminantType.ts, 8, 74))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 10, 12))
+>Foo2 : Symbol(Foo2, Decl(neverAsDiscriminantType.ts, 6, 1))
+
+    if (foo.kind === 'a') {
+>foo.kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 13), Decl(neverAsDiscriminantType.ts, 8, 41), Decl(neverAsDiscriminantType.ts, 8, 58))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 10, 12))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 13), Decl(neverAsDiscriminantType.ts, 8, 41), Decl(neverAsDiscriminantType.ts, 8, 58))
+
+        foo.a;
+>foo.a : Symbol(a, Decl(neverAsDiscriminantType.ts, 8, 25))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 10, 12))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 8, 25))
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+>GatewayPayloadStructure : Symbol(GatewayPayloadStructure, Decl(neverAsDiscriminantType.ts, 14, 1))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 18, 41))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 18, 65))
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+>D : Symbol(D, Decl(neverAsDiscriminantType.ts, 18, 96))
+
+    op: O
+>op : Symbol(GatewayPayloadStructure.op, Decl(neverAsDiscriminantType.ts, 18, 101))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 18, 41))
+
+    d: D
+>d : Symbol(GatewayPayloadStructure.d, Decl(neverAsDiscriminantType.ts, 19, 9))
+>D : Symbol(D, Decl(neverAsDiscriminantType.ts, 18, 96))
+
+    t?: T
+>t : Symbol(GatewayPayloadStructure.t, Decl(neverAsDiscriminantType.ts, 20, 8))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 18, 65))
+
+    s?: number
+>s : Symbol(GatewayPayloadStructure.s, Decl(neverAsDiscriminantType.ts, 21, 9))
+}
+
+export type GatewayPayload = {
+>GatewayPayload : Symbol(GatewayPayload, Decl(neverAsDiscriminantType.ts, 23, 1))
+
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>DISPATCH : Symbol(GatewayOpcode.DISPATCH, Decl(neverAsDiscriminantType.ts, 37, 27))
+
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 28, 9))
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+>GatewayPayloadStructure : Symbol(GatewayPayloadStructure, Decl(neverAsDiscriminantType.ts, 14, 1))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>DISPATCH : Symbol(GatewayOpcode.DISPATCH, Decl(neverAsDiscriminantType.ts, 37, 27))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 28, 9))
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 28, 9))
+
+    }[keyof GatewayEvents]
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+>GatewayPayloadStructure : Symbol(GatewayPayloadStructure, Decl(neverAsDiscriminantType.ts, 14, 1))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>GatewayParams : Symbol(GatewayParams, Decl(neverAsDiscriminantType.ts, 31, 16))
+>GatewayParams : Symbol(GatewayParams, Decl(neverAsDiscriminantType.ts, 31, 16))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+
+}[GatewayOpcode]
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+
+export interface GatewayParams {
+>GatewayParams : Symbol(GatewayParams, Decl(neverAsDiscriminantType.ts, 31, 16))
+
+    [GatewayOpcode.HELLO]: { b: 1 }
+>[GatewayOpcode.HELLO] : Symbol(GatewayParams[GatewayOpcode.HELLO], Decl(neverAsDiscriminantType.ts, 33, 32))
+>GatewayOpcode.HELLO : Symbol(GatewayOpcode.HELLO, Decl(neverAsDiscriminantType.ts, 46, 24))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>HELLO : Symbol(GatewayOpcode.HELLO, Decl(neverAsDiscriminantType.ts, 46, 24))
+>b : Symbol(b, Decl(neverAsDiscriminantType.ts, 34, 28))
+}
+
+export enum GatewayOpcode {
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+
+    DISPATCH = 0,
+>DISPATCH : Symbol(GatewayOpcode.DISPATCH, Decl(neverAsDiscriminantType.ts, 37, 27))
+
+    HEARTBEAT = 1,
+>HEARTBEAT : Symbol(GatewayOpcode.HEARTBEAT, Decl(neverAsDiscriminantType.ts, 38, 17))
+
+    IDENTIFY = 2,
+>IDENTIFY : Symbol(GatewayOpcode.IDENTIFY, Decl(neverAsDiscriminantType.ts, 39, 18))
+
+    PRESENCE_UPDATE = 3,
+>PRESENCE_UPDATE : Symbol(GatewayOpcode.PRESENCE_UPDATE, Decl(neverAsDiscriminantType.ts, 40, 17))
+
+    VOICE_STATE_UPDATE = 4,
+>VOICE_STATE_UPDATE : Symbol(GatewayOpcode.VOICE_STATE_UPDATE, Decl(neverAsDiscriminantType.ts, 41, 24))
+
+    RESUME = 6,
+>RESUME : Symbol(GatewayOpcode.RESUME, Decl(neverAsDiscriminantType.ts, 42, 27))
+
+    RECONNECT = 7,
+>RECONNECT : Symbol(GatewayOpcode.RECONNECT, Decl(neverAsDiscriminantType.ts, 43, 15))
+
+    REQUEST_GUILD_MEMBERS = 8,
+>REQUEST_GUILD_MEMBERS : Symbol(GatewayOpcode.REQUEST_GUILD_MEMBERS, Decl(neverAsDiscriminantType.ts, 44, 18))
+
+    INVALID_SESSION = 9,
+>INVALID_SESSION : Symbol(GatewayOpcode.INVALID_SESSION, Decl(neverAsDiscriminantType.ts, 45, 30))
+
+    HELLO = 10,
+>HELLO : Symbol(GatewayOpcode.HELLO, Decl(neverAsDiscriminantType.ts, 46, 24))
+
+    HEARTBEAT_ACK = 11,
+>HEARTBEAT_ACK : Symbol(GatewayOpcode.HEARTBEAT_ACK, Decl(neverAsDiscriminantType.ts, 47, 15))
+}
+
+export interface GatewayEvents {
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+
+    MESSAGE_CREATE: { a: 1 }
+>MESSAGE_CREATE : Symbol(GatewayEvents.MESSAGE_CREATE, Decl(neverAsDiscriminantType.ts, 51, 32))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 52, 21))
+
+    MESSAGE_UPDATE: { a: 2 }
+>MESSAGE_UPDATE : Symbol(GatewayEvents.MESSAGE_UPDATE, Decl(neverAsDiscriminantType.ts, 52, 28))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 53, 21))
+
+    MESSAGE_DELETE: { a: 3 }
+>MESSAGE_DELETE : Symbol(GatewayEvents.MESSAGE_DELETE, Decl(neverAsDiscriminantType.ts, 53, 28))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 54, 21))
+}
+
+function assertMessage(event: { a: 1 }) { }
+>assertMessage : Symbol(assertMessage, Decl(neverAsDiscriminantType.ts, 55, 1))
+>event : Symbol(event, Decl(neverAsDiscriminantType.ts, 57, 23))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 57, 31))
+
+export async function adaptSession(input: GatewayPayload) {
+>adaptSession : Symbol(adaptSession, Decl(neverAsDiscriminantType.ts, 57, 43))
+>input : Symbol(input, Decl(neverAsDiscriminantType.ts, 59, 35))
+>GatewayPayload : Symbol(GatewayPayload, Decl(neverAsDiscriminantType.ts, 23, 1))
+
+    if (input.t === 'MESSAGE_CREATE') {
+>input.t : Symbol(GatewayPayloadStructure.t, Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8) ... and 8 more)
+>input : Symbol(input, Decl(neverAsDiscriminantType.ts, 59, 35))
+>t : Symbol(GatewayPayloadStructure.t, Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8) ... and 8 more)
+
+        assertMessage(input.d)
+>assertMessage : Symbol(assertMessage, Decl(neverAsDiscriminantType.ts, 55, 1))
+>input.d : Symbol(GatewayPayloadStructure.d, Decl(neverAsDiscriminantType.ts, 19, 9))
+>input : Symbol(input, Decl(neverAsDiscriminantType.ts, 59, 35))
+>d : Symbol(GatewayPayloadStructure.d, Decl(neverAsDiscriminantType.ts, 19, 9))
+    }
+}
+
--- a/tests/baselines/reference/neverAsDiscriminantType(strict=false).types
+++ b/tests/baselines/reference/neverAsDiscriminantType(strict=false).types
@ -0,0 +1,177 @@
+=== tests/cases/compiler/neverAsDiscriminantType.ts ===
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+>Foo1 : { kind: 'a'; a: number; } | { kind: 'b'; } | { kind: never; }
+>kind : "a"
+>a : number
+>kind : "b"
+>kind : never
+
+function f1(foo: Foo1) {
+>f1 : (foo: Foo1) => void
+>foo : Foo1
+
+    if (foo.kind === 'a') {
+>foo.kind === 'a' : boolean
+>foo.kind : "a" | "b"
+>foo : Foo1
+>kind : "a" | "b"
+>'a' : "a"
+
+        foo.a;
+>foo.a : number
+>foo : { kind: "a"; a: number; }
+>a : number
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+>Foo2 : { kind?: 'a'; a: number; } | { kind?: 'b'; } | { kind?: never; }
+>kind : "a"
+>a : number
+>kind : "b"
+>kind : never
+
+function f2(foo: Foo2) {
+>f2 : (foo: Foo2) => void
+>foo : Foo2
+
+    if (foo.kind === 'a') {
+>foo.kind === 'a' : boolean
+>foo.kind : "a" | "b"
+>foo : Foo2
+>kind : "a" | "b"
+>'a' : "a"
+
+        foo.a;
+>foo.a : number
+>foo : { kind?: "a"; a: number; }
+>a : number
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+    op: O
+>op : O
+
+    d: D
+>d : D
+
+    t?: T
+>t : T
+
+    s?: number
+>s : number
+}
+
+export type GatewayPayload = {
+>GatewayPayload : GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_CREATE", { a: 1; }> | GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_UPDATE", { a: 2; }> | GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_DELETE", { a: 3; }> | GatewayPayloadStructure<GatewayOpcode.HEARTBEAT, never, never> | GatewayPayloadStructure<GatewayOpcode.IDENTIFY, never, never> | GatewayPayloadStructure<GatewayOpcode.PRESENCE_UPDATE, never, never> | GatewayPayloadStructure<GatewayOpcode.VOICE_STATE_UPDATE, never, never> | GatewayPayloadStructure<GatewayOpcode.RESUME, never, never> | GatewayPayloadStructure<GatewayOpcode.RECONNECT, never, never> | GatewayPayloadStructure<GatewayOpcode.REQUEST_GUILD_MEMBERS, never, never> | GatewayPayloadStructure<GatewayOpcode.INVALID_SESSION, never, never> | GatewayPayloadStructure<GatewayOpcode.HELLO, never, { b: 1; }> | GatewayPayloadStructure<GatewayOpcode.HEARTBEAT_ACK, never, never>
+
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+>GatewayOpcode : any
+
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+>GatewayOpcode : any
+
+    }[keyof GatewayEvents]
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+}[GatewayOpcode]
+
+export interface GatewayParams {
+    [GatewayOpcode.HELLO]: { b: 1 }
+>[GatewayOpcode.HELLO] : { b: 1; }
+>GatewayOpcode.HELLO : GatewayOpcode.HELLO
+>GatewayOpcode : typeof GatewayOpcode
+>HELLO : GatewayOpcode.HELLO
+>b : 1
+}
+
+export enum GatewayOpcode {
+>GatewayOpcode : GatewayOpcode
+
+    DISPATCH = 0,
+>DISPATCH : GatewayOpcode.DISPATCH
+>0 : 0
+
+    HEARTBEAT = 1,
+>HEARTBEAT : GatewayOpcode.HEARTBEAT
+>1 : 1
+
+    IDENTIFY = 2,
+>IDENTIFY : GatewayOpcode.IDENTIFY
+>2 : 2
+
+    PRESENCE_UPDATE = 3,
+>PRESENCE_UPDATE : GatewayOpcode.PRESENCE_UPDATE
+>3 : 3
+
+    VOICE_STATE_UPDATE = 4,
+>VOICE_STATE_UPDATE : GatewayOpcode.VOICE_STATE_UPDATE
+>4 : 4
+
+    RESUME = 6,
+>RESUME : GatewayOpcode.RESUME
+>6 : 6
+
+    RECONNECT = 7,
+>RECONNECT : GatewayOpcode.RECONNECT
+>7 : 7
+
+    REQUEST_GUILD_MEMBERS = 8,
+>REQUEST_GUILD_MEMBERS : GatewayOpcode.REQUEST_GUILD_MEMBERS
+>8 : 8
+
+    INVALID_SESSION = 9,
+>INVALID_SESSION : GatewayOpcode.INVALID_SESSION
+>9 : 9
+
+    HELLO = 10,
+>HELLO : GatewayOpcode.HELLO
+>10 : 10
+
+    HEARTBEAT_ACK = 11,
+>HEARTBEAT_ACK : GatewayOpcode.HEARTBEAT_ACK
+>11 : 11
+}
+
+export interface GatewayEvents {
+    MESSAGE_CREATE: { a: 1 }
+>MESSAGE_CREATE : { a: 1; }
+>a : 1
+
+    MESSAGE_UPDATE: { a: 2 }
+>MESSAGE_UPDATE : { a: 2; }
+>a : 2
+
+    MESSAGE_DELETE: { a: 3 }
+>MESSAGE_DELETE : { a: 3; }
+>a : 3
+}
+
+function assertMessage(event: { a: 1 }) { }
+>assertMessage : (event: {    a: 1;}) => void
+>event : { a: 1; }
+>a : 1
+
+export async function adaptSession(input: GatewayPayload) {
+>adaptSession : (input: GatewayPayload) => Promise<void>
+>input : GatewayPayload
+
+    if (input.t === 'MESSAGE_CREATE') {
+>input.t === 'MESSAGE_CREATE' : boolean
+>input.t : "MESSAGE_CREATE" | "MESSAGE_UPDATE" | "MESSAGE_DELETE"
+>input : GatewayPayload
+>t : "MESSAGE_CREATE" | "MESSAGE_UPDATE" | "MESSAGE_DELETE"
+>'MESSAGE_CREATE' : "MESSAGE_CREATE"
+
+        assertMessage(input.d)
+>assertMessage(input.d) : void
+>assertMessage : (event: { a: 1; }) => void
+>input.d : { a: 1; }
+>input : GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_CREATE", { a: 1; }>
+>d : { a: 1; }
+    }
+}
+
--- a/tests/baselines/reference/neverAsDiscriminantType(strict=true).js
+++ b/tests/baselines/reference/neverAsDiscriminantType(strict=true).js
@ -0,0 +1,143 @@
+//// [neverAsDiscriminantType.ts]
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+
+function f1(foo: Foo1) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+
+function f2(foo: Foo2) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+    op: O
+    d: D
+    t?: T
+    s?: number
+}
+
+export type GatewayPayload = {
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+    }[keyof GatewayEvents]
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+}[GatewayOpcode]
+
+export interface GatewayParams {
+    [GatewayOpcode.HELLO]: { b: 1 }
+}
+
+export enum GatewayOpcode {
+    DISPATCH = 0,
+    HEARTBEAT = 1,
+    IDENTIFY = 2,
+    PRESENCE_UPDATE = 3,
+    VOICE_STATE_UPDATE = 4,
+    RESUME = 6,
+    RECONNECT = 7,
+    REQUEST_GUILD_MEMBERS = 8,
+    INVALID_SESSION = 9,
+    HELLO = 10,
+    HEARTBEAT_ACK = 11,
+}
+
+export interface GatewayEvents {
+    MESSAGE_CREATE: { a: 1 }
+    MESSAGE_UPDATE: { a: 2 }
+    MESSAGE_DELETE: { a: 3 }
+}
+
+function assertMessage(event: { a: 1 }) { }
+
+export async function adaptSession(input: GatewayPayload) {
+    if (input.t === 'MESSAGE_CREATE') {
+        assertMessage(input.d)
+    }
+}
+
+
+//// [neverAsDiscriminantType.js]
+"use strict";
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }
+    return new (P || (P = Promise))(function (resolve, reject) {
+        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
+        function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }
+        step((generator = generator.apply(thisArg, _arguments || [])).next());
+    });
+};
+var __generator = (this && this.__generator) || function (thisArg, body) {
+    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
+    return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
+    function verb(n) { return function (v) { return step([n, v]); }; }
+    function step(op) {
+        if (f) throw new TypeError("Generator is already executing.");
+        while (_) try {
+            if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
+            if (y = 0, t) op = [op[0] & 2, t.value];
+            switch (op[0]) {
+                case 0: case 1: t = op; break;
+                case 4: _.label++; return { value: op[1], done: false };
+                case 5: _.label++; y = op[1]; op = [0]; continue;
+                case 7: op = _.ops.pop(); _.trys.pop(); continue;
+                default:
+                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
+                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
+                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
+                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
+                    if (t[2]) _.ops.pop();
+                    _.trys.pop(); continue;
+            }
+            op = body.call(thisArg, _);
+        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
+        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
+    }
+};
+exports.__esModule = true;
+exports.adaptSession = exports.GatewayOpcode = void 0;
+function f1(foo) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+function f2(foo) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+var GatewayOpcode;
+(function (GatewayOpcode) {
+    GatewayOpcode[GatewayOpcode["DISPATCH"] = 0] = "DISPATCH";
+    GatewayOpcode[GatewayOpcode["HEARTBEAT"] = 1] = "HEARTBEAT";
+    GatewayOpcode[GatewayOpcode["IDENTIFY"] = 2] = "IDENTIFY";
+    GatewayOpcode[GatewayOpcode["PRESENCE_UPDATE"] = 3] = "PRESENCE_UPDATE";
+    GatewayOpcode[GatewayOpcode["VOICE_STATE_UPDATE"] = 4] = "VOICE_STATE_UPDATE";
+    GatewayOpcode[GatewayOpcode["RESUME"] = 6] = "RESUME";
+    GatewayOpcode[GatewayOpcode["RECONNECT"] = 7] = "RECONNECT";
+    GatewayOpcode[GatewayOpcode["REQUEST_GUILD_MEMBERS"] = 8] = "REQUEST_GUILD_MEMBERS";
+    GatewayOpcode[GatewayOpcode["INVALID_SESSION"] = 9] = "INVALID_SESSION";
+    GatewayOpcode[GatewayOpcode["HELLO"] = 10] = "HELLO";
+    GatewayOpcode[GatewayOpcode["HEARTBEAT_ACK"] = 11] = "HEARTBEAT_ACK";
+})(GatewayOpcode = exports.GatewayOpcode || (exports.GatewayOpcode = {}));
+function assertMessage(event) { }
+function adaptSession(input) {
+    return __awaiter(this, void 0, void 0, function () {
+        return __generator(this, function (_a) {
+            if (input.t === 'MESSAGE_CREATE') {
+                assertMessage(input.d);
+            }
+            return [2 /*return*/];
+        });
+    });
+}
+exports.adaptSession = adaptSession;
--- a/tests/baselines/reference/neverAsDiscriminantType(strict=true).symbols
+++ b/tests/baselines/reference/neverAsDiscriminantType(strict=true).symbols
@ -0,0 +1,197 @@
+=== tests/cases/compiler/neverAsDiscriminantType.ts ===
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+>Foo1 : Symbol(Foo1, Decl(neverAsDiscriminantType.ts, 0, 0))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 13))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 0, 24))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 40))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 56))
+
+function f1(foo: Foo1) {
+>f1 : Symbol(f1, Decl(neverAsDiscriminantType.ts, 0, 71))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 2, 12))
+>Foo1 : Symbol(Foo1, Decl(neverAsDiscriminantType.ts, 0, 0))
+
+    if (foo.kind === 'a') {
+>foo.kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 13), Decl(neverAsDiscriminantType.ts, 0, 40), Decl(neverAsDiscriminantType.ts, 0, 56))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 2, 12))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 0, 13), Decl(neverAsDiscriminantType.ts, 0, 40), Decl(neverAsDiscriminantType.ts, 0, 56))
+
+        foo.a;
+>foo.a : Symbol(a, Decl(neverAsDiscriminantType.ts, 0, 24))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 2, 12))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 0, 24))
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+>Foo2 : Symbol(Foo2, Decl(neverAsDiscriminantType.ts, 6, 1))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 13))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 8, 25))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 41))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 58))
+
+function f2(foo: Foo2) {
+>f2 : Symbol(f2, Decl(neverAsDiscriminantType.ts, 8, 74))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 10, 12))
+>Foo2 : Symbol(Foo2, Decl(neverAsDiscriminantType.ts, 6, 1))
+
+    if (foo.kind === 'a') {
+>foo.kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 13), Decl(neverAsDiscriminantType.ts, 8, 41), Decl(neverAsDiscriminantType.ts, 8, 58))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 10, 12))
+>kind : Symbol(kind, Decl(neverAsDiscriminantType.ts, 8, 13), Decl(neverAsDiscriminantType.ts, 8, 41), Decl(neverAsDiscriminantType.ts, 8, 58))
+
+        foo.a;
+>foo.a : Symbol(a, Decl(neverAsDiscriminantType.ts, 8, 25))
+>foo : Symbol(foo, Decl(neverAsDiscriminantType.ts, 10, 12))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 8, 25))
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+>GatewayPayloadStructure : Symbol(GatewayPayloadStructure, Decl(neverAsDiscriminantType.ts, 14, 1))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 18, 41))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 18, 65))
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+>D : Symbol(D, Decl(neverAsDiscriminantType.ts, 18, 96))
+
+    op: O
+>op : Symbol(GatewayPayloadStructure.op, Decl(neverAsDiscriminantType.ts, 18, 101))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 18, 41))
+
+    d: D
+>d : Symbol(GatewayPayloadStructure.d, Decl(neverAsDiscriminantType.ts, 19, 9))
+>D : Symbol(D, Decl(neverAsDiscriminantType.ts, 18, 96))
+
+    t?: T
+>t : Symbol(GatewayPayloadStructure.t, Decl(neverAsDiscriminantType.ts, 20, 8))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 18, 65))
+
+    s?: number
+>s : Symbol(GatewayPayloadStructure.s, Decl(neverAsDiscriminantType.ts, 21, 9))
+}
+
+export type GatewayPayload = {
+>GatewayPayload : Symbol(GatewayPayload, Decl(neverAsDiscriminantType.ts, 23, 1))
+
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>DISPATCH : Symbol(GatewayOpcode.DISPATCH, Decl(neverAsDiscriminantType.ts, 37, 27))
+
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 28, 9))
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+>GatewayPayloadStructure : Symbol(GatewayPayloadStructure, Decl(neverAsDiscriminantType.ts, 14, 1))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>DISPATCH : Symbol(GatewayOpcode.DISPATCH, Decl(neverAsDiscriminantType.ts, 37, 27))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 28, 9))
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+>T : Symbol(T, Decl(neverAsDiscriminantType.ts, 28, 9))
+
+    }[keyof GatewayEvents]
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+>GatewayPayloadStructure : Symbol(GatewayPayloadStructure, Decl(neverAsDiscriminantType.ts, 14, 1))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+>GatewayParams : Symbol(GatewayParams, Decl(neverAsDiscriminantType.ts, 31, 16))
+>GatewayParams : Symbol(GatewayParams, Decl(neverAsDiscriminantType.ts, 31, 16))
+>O : Symbol(O, Decl(neverAsDiscriminantType.ts, 26, 5))
+
+}[GatewayOpcode]
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+
+export interface GatewayParams {
+>GatewayParams : Symbol(GatewayParams, Decl(neverAsDiscriminantType.ts, 31, 16))
+
+    [GatewayOpcode.HELLO]: { b: 1 }
+>[GatewayOpcode.HELLO] : Symbol(GatewayParams[GatewayOpcode.HELLO], Decl(neverAsDiscriminantType.ts, 33, 32))
+>GatewayOpcode.HELLO : Symbol(GatewayOpcode.HELLO, Decl(neverAsDiscriminantType.ts, 46, 24))
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+>HELLO : Symbol(GatewayOpcode.HELLO, Decl(neverAsDiscriminantType.ts, 46, 24))
+>b : Symbol(b, Decl(neverAsDiscriminantType.ts, 34, 28))
+}
+
+export enum GatewayOpcode {
+>GatewayOpcode : Symbol(GatewayOpcode, Decl(neverAsDiscriminantType.ts, 35, 1))
+
+    DISPATCH = 0,
+>DISPATCH : Symbol(GatewayOpcode.DISPATCH, Decl(neverAsDiscriminantType.ts, 37, 27))
+
+    HEARTBEAT = 1,
+>HEARTBEAT : Symbol(GatewayOpcode.HEARTBEAT, Decl(neverAsDiscriminantType.ts, 38, 17))
+
+    IDENTIFY = 2,
+>IDENTIFY : Symbol(GatewayOpcode.IDENTIFY, Decl(neverAsDiscriminantType.ts, 39, 18))
+
+    PRESENCE_UPDATE = 3,
+>PRESENCE_UPDATE : Symbol(GatewayOpcode.PRESENCE_UPDATE, Decl(neverAsDiscriminantType.ts, 40, 17))
+
+    VOICE_STATE_UPDATE = 4,
+>VOICE_STATE_UPDATE : Symbol(GatewayOpcode.VOICE_STATE_UPDATE, Decl(neverAsDiscriminantType.ts, 41, 24))
+
+    RESUME = 6,
+>RESUME : Symbol(GatewayOpcode.RESUME, Decl(neverAsDiscriminantType.ts, 42, 27))
+
+    RECONNECT = 7,
+>RECONNECT : Symbol(GatewayOpcode.RECONNECT, Decl(neverAsDiscriminantType.ts, 43, 15))
+
+    REQUEST_GUILD_MEMBERS = 8,
+>REQUEST_GUILD_MEMBERS : Symbol(GatewayOpcode.REQUEST_GUILD_MEMBERS, Decl(neverAsDiscriminantType.ts, 44, 18))
+
+    INVALID_SESSION = 9,
+>INVALID_SESSION : Symbol(GatewayOpcode.INVALID_SESSION, Decl(neverAsDiscriminantType.ts, 45, 30))
+
+    HELLO = 10,
+>HELLO : Symbol(GatewayOpcode.HELLO, Decl(neverAsDiscriminantType.ts, 46, 24))
+
+    HEARTBEAT_ACK = 11,
+>HEARTBEAT_ACK : Symbol(GatewayOpcode.HEARTBEAT_ACK, Decl(neverAsDiscriminantType.ts, 47, 15))
+}
+
+export interface GatewayEvents {
+>GatewayEvents : Symbol(GatewayEvents, Decl(neverAsDiscriminantType.ts, 49, 1))
+
+    MESSAGE_CREATE: { a: 1 }
+>MESSAGE_CREATE : Symbol(GatewayEvents.MESSAGE_CREATE, Decl(neverAsDiscriminantType.ts, 51, 32))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 52, 21))
+
+    MESSAGE_UPDATE: { a: 2 }
+>MESSAGE_UPDATE : Symbol(GatewayEvents.MESSAGE_UPDATE, Decl(neverAsDiscriminantType.ts, 52, 28))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 53, 21))
+
+    MESSAGE_DELETE: { a: 3 }
+>MESSAGE_DELETE : Symbol(GatewayEvents.MESSAGE_DELETE, Decl(neverAsDiscriminantType.ts, 53, 28))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 54, 21))
+}
+
+function assertMessage(event: { a: 1 }) { }
+>assertMessage : Symbol(assertMessage, Decl(neverAsDiscriminantType.ts, 55, 1))
+>event : Symbol(event, Decl(neverAsDiscriminantType.ts, 57, 23))
+>a : Symbol(a, Decl(neverAsDiscriminantType.ts, 57, 31))
+
+export async function adaptSession(input: GatewayPayload) {
+>adaptSession : Symbol(adaptSession, Decl(neverAsDiscriminantType.ts, 57, 43))
+>input : Symbol(input, Decl(neverAsDiscriminantType.ts, 59, 35))
+>GatewayPayload : Symbol(GatewayPayload, Decl(neverAsDiscriminantType.ts, 23, 1))
+
+    if (input.t === 'MESSAGE_CREATE') {
+>input.t : Symbol(GatewayPayloadStructure.t, Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8) ... and 8 more)
+>input : Symbol(input, Decl(neverAsDiscriminantType.ts, 59, 35))
+>t : Symbol(GatewayPayloadStructure.t, Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8), Decl(neverAsDiscriminantType.ts, 20, 8) ... and 8 more)
+
+        assertMessage(input.d)
+>assertMessage : Symbol(assertMessage, Decl(neverAsDiscriminantType.ts, 55, 1))
+>input.d : Symbol(GatewayPayloadStructure.d, Decl(neverAsDiscriminantType.ts, 19, 9))
+>input : Symbol(input, Decl(neverAsDiscriminantType.ts, 59, 35))
+>d : Symbol(GatewayPayloadStructure.d, Decl(neverAsDiscriminantType.ts, 19, 9))
+    }
+}
+
--- a/tests/baselines/reference/neverAsDiscriminantType(strict=true).types
+++ b/tests/baselines/reference/neverAsDiscriminantType(strict=true).types
@ -0,0 +1,177 @@
+=== tests/cases/compiler/neverAsDiscriminantType.ts ===
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+>Foo1 : { kind: 'a'; a: number; } | { kind: 'b'; } | { kind: never; }
+>kind : "a"
+>a : number
+>kind : "b"
+>kind : never
+
+function f1(foo: Foo1) {
+>f1 : (foo: Foo1) => void
+>foo : Foo1
+
+    if (foo.kind === 'a') {
+>foo.kind === 'a' : boolean
+>foo.kind : "a" | "b"
+>foo : Foo1
+>kind : "a" | "b"
+>'a' : "a"
+
+        foo.a;
+>foo.a : number
+>foo : { kind: "a"; a: number; }
+>a : number
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+>Foo2 : { kind?: "a" | undefined; a: number; } | { kind?: "b" | undefined; } | { kind?: undefined; }
+>kind : "a" | undefined
+>a : number
+>kind : "b" | undefined
+>kind : undefined
+
+function f2(foo: Foo2) {
+>f2 : (foo: Foo2) => void
+>foo : Foo2
+
+    if (foo.kind === 'a') {
+>foo.kind === 'a' : boolean
+>foo.kind : "a" | "b" | undefined
+>foo : Foo2
+>kind : "a" | "b" | undefined
+>'a' : "a"
+
+        foo.a;
+>foo.a : number
+>foo : { kind?: "a" | undefined; a: number; }
+>a : number
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+    op: O
+>op : O
+
+    d: D
+>d : D
+
+    t?: T
+>t : T | undefined
+
+    s?: number
+>s : number | undefined
+}
+
+export type GatewayPayload = {
+>GatewayPayload : GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_CREATE", { a: 1; }> | GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_UPDATE", { a: 2; }> | GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_DELETE", { a: 3; }> | GatewayPayloadStructure<GatewayOpcode.HEARTBEAT, never, never> | GatewayPayloadStructure<GatewayOpcode.IDENTIFY, never, never> | GatewayPayloadStructure<GatewayOpcode.PRESENCE_UPDATE, never, never> | GatewayPayloadStructure<GatewayOpcode.VOICE_STATE_UPDATE, never, never> | GatewayPayloadStructure<GatewayOpcode.RESUME, never, never> | GatewayPayloadStructure<GatewayOpcode.RECONNECT, never, never> | GatewayPayloadStructure<GatewayOpcode.REQUEST_GUILD_MEMBERS, never, never> | GatewayPayloadStructure<GatewayOpcode.INVALID_SESSION, never, never> | GatewayPayloadStructure<GatewayOpcode.HELLO, never, { b: 1; }> | GatewayPayloadStructure<GatewayOpcode.HEARTBEAT_ACK, never, never>
+
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+>GatewayOpcode : any
+
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+>GatewayOpcode : any
+
+    }[keyof GatewayEvents]
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+}[GatewayOpcode]
+
+export interface GatewayParams {
+    [GatewayOpcode.HELLO]: { b: 1 }
+>[GatewayOpcode.HELLO] : { b: 1; }
+>GatewayOpcode.HELLO : GatewayOpcode.HELLO
+>GatewayOpcode : typeof GatewayOpcode
+>HELLO : GatewayOpcode.HELLO
+>b : 1
+}
+
+export enum GatewayOpcode {
+>GatewayOpcode : GatewayOpcode
+
+    DISPATCH = 0,
+>DISPATCH : GatewayOpcode.DISPATCH
+>0 : 0
+
+    HEARTBEAT = 1,
+>HEARTBEAT : GatewayOpcode.HEARTBEAT
+>1 : 1
+
+    IDENTIFY = 2,
+>IDENTIFY : GatewayOpcode.IDENTIFY
+>2 : 2
+
+    PRESENCE_UPDATE = 3,
+>PRESENCE_UPDATE : GatewayOpcode.PRESENCE_UPDATE
+>3 : 3
+
+    VOICE_STATE_UPDATE = 4,
+>VOICE_STATE_UPDATE : GatewayOpcode.VOICE_STATE_UPDATE
+>4 : 4
+
+    RESUME = 6,
+>RESUME : GatewayOpcode.RESUME
+>6 : 6
+
+    RECONNECT = 7,
+>RECONNECT : GatewayOpcode.RECONNECT
+>7 : 7
+
+    REQUEST_GUILD_MEMBERS = 8,
+>REQUEST_GUILD_MEMBERS : GatewayOpcode.REQUEST_GUILD_MEMBERS
+>8 : 8
+
+    INVALID_SESSION = 9,
+>INVALID_SESSION : GatewayOpcode.INVALID_SESSION
+>9 : 9
+
+    HELLO = 10,
+>HELLO : GatewayOpcode.HELLO
+>10 : 10
+
+    HEARTBEAT_ACK = 11,
+>HEARTBEAT_ACK : GatewayOpcode.HEARTBEAT_ACK
+>11 : 11
+}
+
+export interface GatewayEvents {
+    MESSAGE_CREATE: { a: 1 }
+>MESSAGE_CREATE : { a: 1; }
+>a : 1
+
+    MESSAGE_UPDATE: { a: 2 }
+>MESSAGE_UPDATE : { a: 2; }
+>a : 2
+
+    MESSAGE_DELETE: { a: 3 }
+>MESSAGE_DELETE : { a: 3; }
+>a : 3
+}
+
+function assertMessage(event: { a: 1 }) { }
+>assertMessage : (event: {    a: 1;}) => void
+>event : { a: 1; }
+>a : 1
+
+export async function adaptSession(input: GatewayPayload) {
+>adaptSession : (input: GatewayPayload) => Promise<void>
+>input : GatewayPayload
+
+    if (input.t === 'MESSAGE_CREATE') {
+>input.t === 'MESSAGE_CREATE' : boolean
+>input.t : "MESSAGE_CREATE" | "MESSAGE_UPDATE" | "MESSAGE_DELETE" | undefined
+>input : GatewayPayload
+>t : "MESSAGE_CREATE" | "MESSAGE_UPDATE" | "MESSAGE_DELETE" | undefined
+>'MESSAGE_CREATE' : "MESSAGE_CREATE"
+
+        assertMessage(input.d)
+>assertMessage(input.d) : void
+>assertMessage : (event: { a: 1; }) => void
+>input.d : { a: 1; }
+>input : GatewayPayloadStructure<GatewayOpcode.DISPATCH, "MESSAGE_CREATE", { a: 1; }>
+>d : { a: 1; }
+    }
+}
+
--- a/tests/cases/compiler/neverAsDiscriminantType.ts
+++ b/tests/cases/compiler/neverAsDiscriminantType.ts
@ -0,0 +1,66 @@
+// @strict: true, false
+
+type Foo1 = { kind: 'a', a: number } | { kind: 'b' } | { kind: never };
+
+function f1(foo: Foo1) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+
+type Foo2 = { kind?: 'a', a: number } | { kind?: 'b' } | { kind?: never };
+
+function f2(foo: Foo2) {
+    if (foo.kind === 'a') {
+        foo.a;
+    }
+}
+
+// Repro from #50716
+
+export interface GatewayPayloadStructure<O extends GatewayOpcode, T extends keyof GatewayEvents, D> {
+    op: O
+    d: D
+    t?: T
+    s?: number
+}
+
+export type GatewayPayload = {
+    [O in GatewayOpcode]: O extends GatewayOpcode.DISPATCH
+    ? {
+        [T in keyof GatewayEvents]: GatewayPayloadStructure<GatewayOpcode.DISPATCH, T, GatewayEvents[T]>
+    }[keyof GatewayEvents]
+    : GatewayPayloadStructure<O, never, O extends keyof GatewayParams ? GatewayParams[O] : never>
+}[GatewayOpcode]
+
+export interface GatewayParams {
+    [GatewayOpcode.HELLO]: { b: 1 }
+}
+
+export enum GatewayOpcode {
+    DISPATCH = 0,
+    HEARTBEAT = 1,
+    IDENTIFY = 2,
+    PRESENCE_UPDATE = 3,
+    VOICE_STATE_UPDATE = 4,
+    RESUME = 6,
+    RECONNECT = 7,
+    REQUEST_GUILD_MEMBERS = 8,
+    INVALID_SESSION = 9,
+    HELLO = 10,
+    HEARTBEAT_ACK = 11,
+}
+
+export interface GatewayEvents {
+    MESSAGE_CREATE: { a: 1 }
+    MESSAGE_UPDATE: { a: 2 }
+    MESSAGE_DELETE: { a: 3 }
+}
+
+function assertMessage(event: { a: 1 }) { }
+
+export async function adaptSession(input: GatewayPayload) {
+    if (input.t === 'MESSAGE_CREATE') {
+        assertMessage(input.d)
+    }
+}