diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 8f8be4a10ef..69a46ed6cb2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -19686,7 +19686,7 @@ namespace ts {
                     return undefined;
                 }
 
-                return typeAsAwaitable.awaitedTypeOfType = getUnionType(types, /*subtypeReduction*/ true);
+                return typeAsAwaitable.awaitedTypeOfType = getUnionType(types);
             }
 
             const promisedType = getPromisedTypeOfPromise(type);
diff --git a/tests/baselines/reference/awaitUnionPromise.js b/tests/baselines/reference/awaitUnionPromise.js
new file mode 100644
index 00000000000..df4c937fc63
--- /dev/null
+++ b/tests/baselines/reference/awaitUnionPromise.js
@@ -0,0 +1,89 @@
+//// [awaitUnionPromise.ts]
+/// @target: es2015
+// https://github.com/Microsoft/TypeScript/issues/18186
+
+class AsyncEnumeratorDone { };
+
+interface IAsyncEnumerator<T> {
+    next1(): Promise<T | AsyncEnumeratorDone>;
+    next2(): Promise<T> | Promise<AsyncEnumeratorDone>;
+    next3(): Promise<T | {}>;
+    next4(): Promise<T | { x: string }>;
+}
+
+async function main() {
+    const x: IAsyncEnumerator<number> = null;
+    let a = await x.next1();
+    let b = await x.next2();
+    let c = await x.next3();
+    let d = await x.next4();
+}
+
+//// [awaitUnionPromise.js]
+/// @target: es2015
+// https://github.com/Microsoft/TypeScript/issues/18186
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    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) : new P(function (resolve) { resolve(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 = y[op[0] & 2 ? "return" : op[0] ? "throw" : "next"]) && !(t = t.call(y, op[1])).done) return t;
+            if (y = 0, t) op = [0, 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 };
+    }
+};
+var AsyncEnumeratorDone = /** @class */ (function () {
+    function AsyncEnumeratorDone() {
+    }
+    return AsyncEnumeratorDone;
+}());
+;
+function main() {
+    return __awaiter(this, void 0, void 0, function () {
+        var x, a, b, c, d;
+        return __generator(this, function (_a) {
+            switch (_a.label) {
+                case 0:
+                    x = null;
+                    return [4 /*yield*/, x.next1()];
+                case 1:
+                    a = _a.sent();
+                    return [4 /*yield*/, x.next2()];
+                case 2:
+                    b = _a.sent();
+                    return [4 /*yield*/, x.next3()];
+                case 3:
+                    c = _a.sent();
+                    return [4 /*yield*/, x.next4()];
+                case 4:
+                    d = _a.sent();
+                    return [2 /*return*/];
+            }
+        });
+    });
+}
diff --git a/tests/baselines/reference/awaitUnionPromise.symbols b/tests/baselines/reference/awaitUnionPromise.symbols
new file mode 100644
index 00000000000..fb26ba1bfac
--- /dev/null
+++ b/tests/baselines/reference/awaitUnionPromise.symbols
@@ -0,0 +1,67 @@
+=== tests/cases/compiler/awaitUnionPromise.ts ===
+/// @target: es2015
+// https://github.com/Microsoft/TypeScript/issues/18186
+
+class AsyncEnumeratorDone { };
+>AsyncEnumeratorDone : Symbol(AsyncEnumeratorDone, Decl(awaitUnionPromise.ts, 0, 0))
+
+interface IAsyncEnumerator<T> {
+>IAsyncEnumerator : Symbol(IAsyncEnumerator, Decl(awaitUnionPromise.ts, 3, 30))
+>T : Symbol(T, Decl(awaitUnionPromise.ts, 5, 27))
+
+    next1(): Promise<T | AsyncEnumeratorDone>;
+>next1 : Symbol(IAsyncEnumerator.next1, Decl(awaitUnionPromise.ts, 5, 31))
+>Promise : Symbol(Promise, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(awaitUnionPromise.ts, 5, 27))
+>AsyncEnumeratorDone : Symbol(AsyncEnumeratorDone, Decl(awaitUnionPromise.ts, 0, 0))
+
+    next2(): Promise<T> | Promise<AsyncEnumeratorDone>;
+>next2 : Symbol(IAsyncEnumerator.next2, Decl(awaitUnionPromise.ts, 6, 46))
+>Promise : Symbol(Promise, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(awaitUnionPromise.ts, 5, 27))
+>Promise : Symbol(Promise, Decl(lib.d.ts, --, --))
+>AsyncEnumeratorDone : Symbol(AsyncEnumeratorDone, Decl(awaitUnionPromise.ts, 0, 0))
+
+    next3(): Promise<T | {}>;
+>next3 : Symbol(IAsyncEnumerator.next3, Decl(awaitUnionPromise.ts, 7, 55))
+>Promise : Symbol(Promise, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(awaitUnionPromise.ts, 5, 27))
+
+    next4(): Promise<T | { x: string }>;
+>next4 : Symbol(IAsyncEnumerator.next4, Decl(awaitUnionPromise.ts, 8, 29))
+>Promise : Symbol(Promise, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(awaitUnionPromise.ts, 5, 27))
+>x : Symbol(x, Decl(awaitUnionPromise.ts, 9, 26))
+}
+
+async function main() {
+>main : Symbol(main, Decl(awaitUnionPromise.ts, 10, 1))
+
+    const x: IAsyncEnumerator<number> = null;
+>x : Symbol(x, Decl(awaitUnionPromise.ts, 13, 9))
+>IAsyncEnumerator : Symbol(IAsyncEnumerator, Decl(awaitUnionPromise.ts, 3, 30))
+
+    let a = await x.next1();
+>a : Symbol(a, Decl(awaitUnionPromise.ts, 14, 7))
+>x.next1 : Symbol(IAsyncEnumerator.next1, Decl(awaitUnionPromise.ts, 5, 31))
+>x : Symbol(x, Decl(awaitUnionPromise.ts, 13, 9))
+>next1 : Symbol(IAsyncEnumerator.next1, Decl(awaitUnionPromise.ts, 5, 31))
+
+    let b = await x.next2();
+>b : Symbol(b, Decl(awaitUnionPromise.ts, 15, 7))
+>x.next2 : Symbol(IAsyncEnumerator.next2, Decl(awaitUnionPromise.ts, 6, 46))
+>x : Symbol(x, Decl(awaitUnionPromise.ts, 13, 9))
+>next2 : Symbol(IAsyncEnumerator.next2, Decl(awaitUnionPromise.ts, 6, 46))
+
+    let c = await x.next3();
+>c : Symbol(c, Decl(awaitUnionPromise.ts, 16, 7))
+>x.next3 : Symbol(IAsyncEnumerator.next3, Decl(awaitUnionPromise.ts, 7, 55))
+>x : Symbol(x, Decl(awaitUnionPromise.ts, 13, 9))
+>next3 : Symbol(IAsyncEnumerator.next3, Decl(awaitUnionPromise.ts, 7, 55))
+
+    let d = await x.next4();
+>d : Symbol(d, Decl(awaitUnionPromise.ts, 17, 7))
+>x.next4 : Symbol(IAsyncEnumerator.next4, Decl(awaitUnionPromise.ts, 8, 29))
+>x : Symbol(x, Decl(awaitUnionPromise.ts, 13, 9))
+>next4 : Symbol(IAsyncEnumerator.next4, Decl(awaitUnionPromise.ts, 8, 29))
+}
diff --git a/tests/baselines/reference/awaitUnionPromise.types b/tests/baselines/reference/awaitUnionPromise.types
new file mode 100644
index 00000000000..f1d91c038de
--- /dev/null
+++ b/tests/baselines/reference/awaitUnionPromise.types
@@ -0,0 +1,76 @@
+=== tests/cases/compiler/awaitUnionPromise.ts ===
+/// @target: es2015
+// https://github.com/Microsoft/TypeScript/issues/18186
+
+class AsyncEnumeratorDone { };
+>AsyncEnumeratorDone : AsyncEnumeratorDone
+
+interface IAsyncEnumerator<T> {
+>IAsyncEnumerator : IAsyncEnumerator<T>
+>T : T
+
+    next1(): Promise<T | AsyncEnumeratorDone>;
+>next1 : () => Promise<AsyncEnumeratorDone | T>
+>Promise : Promise<T>
+>T : T
+>AsyncEnumeratorDone : AsyncEnumeratorDone
+
+    next2(): Promise<T> | Promise<AsyncEnumeratorDone>;
+>next2 : () => Promise<T> | Promise<AsyncEnumeratorDone>
+>Promise : Promise<T>
+>T : T
+>Promise : Promise<T>
+>AsyncEnumeratorDone : AsyncEnumeratorDone
+
+    next3(): Promise<T | {}>;
+>next3 : () => Promise<{} | T>
+>Promise : Promise<T>
+>T : T
+
+    next4(): Promise<T | { x: string }>;
+>next4 : () => Promise<T | { x: string; }>
+>Promise : Promise<T>
+>T : T
+>x : string
+}
+
+async function main() {
+>main : () => Promise<void>
+
+    const x: IAsyncEnumerator<number> = null;
+>x : IAsyncEnumerator<number>
+>IAsyncEnumerator : IAsyncEnumerator<T>
+>null : null
+
+    let a = await x.next1();
+>a : number | AsyncEnumeratorDone
+>await x.next1() : number | AsyncEnumeratorDone
+>x.next1() : Promise<number | AsyncEnumeratorDone>
+>x.next1 : () => Promise<number | AsyncEnumeratorDone>
+>x : IAsyncEnumerator<number>
+>next1 : () => Promise<number | AsyncEnumeratorDone>
+
+    let b = await x.next2();
+>b : number | AsyncEnumeratorDone
+>await x.next2() : number | AsyncEnumeratorDone
+>x.next2() : Promise<AsyncEnumeratorDone> | Promise<number>
+>x.next2 : () => Promise<AsyncEnumeratorDone> | Promise<number>
+>x : IAsyncEnumerator<number>
+>next2 : () => Promise<AsyncEnumeratorDone> | Promise<number>
+
+    let c = await x.next3();
+>c : number | {}
+>await x.next3() : number | {}
+>x.next3() : Promise<number | {}>
+>x.next3 : () => Promise<number | {}>
+>x : IAsyncEnumerator<number>
+>next3 : () => Promise<number | {}>
+
+    let d = await x.next4();
+>d : number | { x: string; }
+>await x.next4() : number | { x: string; }
+>x.next4() : Promise<number | { x: string; }>
+>x.next4 : () => Promise<number | { x: string; }>
+>x : IAsyncEnumerator<number>
+>next4 : () => Promise<number | { x: string; }>
+}
diff --git a/tests/cases/compiler/awaitUnionPromise.ts b/tests/cases/compiler/awaitUnionPromise.ts
new file mode 100644
index 00000000000..2c8470e97a6
--- /dev/null
+++ b/tests/cases/compiler/awaitUnionPromise.ts
@@ -0,0 +1,19 @@
+/// @target: es2015
+// https://github.com/Microsoft/TypeScript/issues/18186
+
+class AsyncEnumeratorDone { };
+
+interface IAsyncEnumerator<T> {
+    next1(): Promise<T | AsyncEnumeratorDone>;
+    next2(): Promise<T> | Promise<AsyncEnumeratorDone>;
+    next3(): Promise<T | {}>;
+    next4(): Promise<T | { x: string }>;
+}
+
+async function main() {
+    const x: IAsyncEnumerator<number> = null;
+    let a = await x.next1();
+    let b = await x.next2();
+    let c = await x.next3();
+    let d = await x.next4();
+}
\ No newline at end of file