diff --git a/tests/baselines/reference/unionWithIndexSignature.js b/tests/baselines/reference/unionWithIndexSignature.js
new file mode 100644
index 0000000000000..31e0ccb88d837
--- /dev/null
+++ b/tests/baselines/reference/unionWithIndexSignature.js
@@ -0,0 +1,47 @@
+//// [unionWithIndexSignature.ts]
+interface NumList {
+  kind: 'n';
+  [x: number]: number;
+}
+interface StrList {
+  kind: 's';
+  [x: number]: string;
+}
+
+export function foo<T extends NumList | StrList>(arr: T & (NumList | StrList)) {
+  let zz = arr[1];  // Error
+}
+
+// Repro from #38102
+
+export type TypedArray = Int32Array | Uint8Array;
+
+export function isTypedArray(a: {}): a is Int32Array | Uint8Array {
+  return a instanceof Int32Array || a instanceof Uint8Array;
+}
+
+export function flatten<T extends number|TypedArray>(arr: T) {
+  if (isTypedArray(arr)) {
+      arr[1];
+  }
+}
+
+
+//// [unionWithIndexSignature.js]
+"use strict";
+exports.__esModule = true;
+exports.flatten = exports.isTypedArray = exports.foo = void 0;
+function foo(arr) {
+    var zz = arr[1]; // Error
+}
+exports.foo = foo;
+function isTypedArray(a) {
+    return a instanceof Int32Array || a instanceof Uint8Array;
+}
+exports.isTypedArray = isTypedArray;
+function flatten(arr) {
+    if (isTypedArray(arr)) {
+        arr[1];
+    }
+}
+exports.flatten = flatten;
diff --git a/tests/baselines/reference/unionWithIndexSignature.symbols b/tests/baselines/reference/unionWithIndexSignature.symbols
new file mode 100644
index 0000000000000..5f78136b361a6
--- /dev/null
+++ b/tests/baselines/reference/unionWithIndexSignature.symbols
@@ -0,0 +1,72 @@
+=== tests/cases/compiler/unionWithIndexSignature.ts ===
+interface NumList {
+>NumList : Symbol(NumList, Decl(unionWithIndexSignature.ts, 0, 0))
+
+  kind: 'n';
+>kind : Symbol(NumList.kind, Decl(unionWithIndexSignature.ts, 0, 19))
+
+  [x: number]: number;
+>x : Symbol(x, Decl(unionWithIndexSignature.ts, 2, 3))
+}
+interface StrList {
+>StrList : Symbol(StrList, Decl(unionWithIndexSignature.ts, 3, 1))
+
+  kind: 's';
+>kind : Symbol(StrList.kind, Decl(unionWithIndexSignature.ts, 4, 19))
+
+  [x: number]: string;
+>x : Symbol(x, Decl(unionWithIndexSignature.ts, 6, 3))
+}
+
+export function foo<T extends NumList | StrList>(arr: T & (NumList | StrList)) {
+>foo : Symbol(foo, Decl(unionWithIndexSignature.ts, 7, 1))
+>T : Symbol(T, Decl(unionWithIndexSignature.ts, 9, 20))
+>NumList : Symbol(NumList, Decl(unionWithIndexSignature.ts, 0, 0))
+>StrList : Symbol(StrList, Decl(unionWithIndexSignature.ts, 3, 1))
+>arr : Symbol(arr, Decl(unionWithIndexSignature.ts, 9, 49))
+>T : Symbol(T, Decl(unionWithIndexSignature.ts, 9, 20))
+>NumList : Symbol(NumList, Decl(unionWithIndexSignature.ts, 0, 0))
+>StrList : Symbol(StrList, Decl(unionWithIndexSignature.ts, 3, 1))
+
+  let zz = arr[1];  // Error
+>zz : Symbol(zz, Decl(unionWithIndexSignature.ts, 10, 5))
+>arr : Symbol(arr, Decl(unionWithIndexSignature.ts, 9, 49))
+}
+
+// Repro from #38102
+
+export type TypedArray = Int32Array | Uint8Array;
+>TypedArray : Symbol(TypedArray, Decl(unionWithIndexSignature.ts, 11, 1))
+>Int32Array : Symbol(Int32Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Uint8Array : Symbol(Uint8Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+export function isTypedArray(a: {}): a is Int32Array | Uint8Array {
+>isTypedArray : Symbol(isTypedArray, Decl(unionWithIndexSignature.ts, 15, 49))
+>a : Symbol(a, Decl(unionWithIndexSignature.ts, 17, 29))
+>a : Symbol(a, Decl(unionWithIndexSignature.ts, 17, 29))
+>Int32Array : Symbol(Int32Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Uint8Array : Symbol(Uint8Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+  return a instanceof Int32Array || a instanceof Uint8Array;
+>a : Symbol(a, Decl(unionWithIndexSignature.ts, 17, 29))
+>Int32Array : Symbol(Int32Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>a : Symbol(a, Decl(unionWithIndexSignature.ts, 17, 29))
+>Uint8Array : Symbol(Uint8Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+}
+
+export function flatten<T extends number|TypedArray>(arr: T) {
+>flatten : Symbol(flatten, Decl(unionWithIndexSignature.ts, 19, 1))
+>T : Symbol(T, Decl(unionWithIndexSignature.ts, 21, 24))
+>TypedArray : Symbol(TypedArray, Decl(unionWithIndexSignature.ts, 11, 1))
+>arr : Symbol(arr, Decl(unionWithIndexSignature.ts, 21, 53))
+>T : Symbol(T, Decl(unionWithIndexSignature.ts, 21, 24))
+
+  if (isTypedArray(arr)) {
+>isTypedArray : Symbol(isTypedArray, Decl(unionWithIndexSignature.ts, 15, 49))
+>arr : Symbol(arr, Decl(unionWithIndexSignature.ts, 21, 53))
+
+      arr[1];
+>arr : Symbol(arr, Decl(unionWithIndexSignature.ts, 21, 53))
+  }
+}
+
diff --git a/tests/baselines/reference/unionWithIndexSignature.types b/tests/baselines/reference/unionWithIndexSignature.types
new file mode 100644
index 0000000000000..d80e8d01ca3c4
--- /dev/null
+++ b/tests/baselines/reference/unionWithIndexSignature.types
@@ -0,0 +1,62 @@
+=== tests/cases/compiler/unionWithIndexSignature.ts ===
+interface NumList {
+  kind: 'n';
+>kind : "n"
+
+  [x: number]: number;
+>x : number
+}
+interface StrList {
+  kind: 's';
+>kind : "s"
+
+  [x: number]: string;
+>x : number
+}
+
+export function foo<T extends NumList | StrList>(arr: T & (NumList | StrList)) {
+>foo : <T extends NumList | StrList>(arr: T & (NumList | StrList)) => void
+>arr : (T & NumList) | (T & StrList)
+
+  let zz = arr[1];  // Error
+>zz : string | number
+>arr[1] : string | number
+>arr : (T & NumList) | (T & StrList)
+>1 : 1
+}
+
+// Repro from #38102
+
+export type TypedArray = Int32Array | Uint8Array;
+>TypedArray : Int32Array | Uint8Array
+
+export function isTypedArray(a: {}): a is Int32Array | Uint8Array {
+>isTypedArray : (a: {}) => a is Int32Array | Uint8Array
+>a : {}
+
+  return a instanceof Int32Array || a instanceof Uint8Array;
+>a instanceof Int32Array || a instanceof Uint8Array : boolean
+>a instanceof Int32Array : boolean
+>a : {}
+>Int32Array : Int32ArrayConstructor
+>a instanceof Uint8Array : boolean
+>a : {}
+>Uint8Array : Uint8ArrayConstructor
+}
+
+export function flatten<T extends number|TypedArray>(arr: T) {
+>flatten : <T extends number | Int32Array | Uint8Array>(arr: T) => void
+>arr : T
+
+  if (isTypedArray(arr)) {
+>isTypedArray(arr) : boolean
+>isTypedArray : (a: {}) => a is Int32Array | Uint8Array
+>arr : T
+
+      arr[1];
+>arr[1] : number
+>arr : (T & Int32Array) | (T & Uint8Array)
+>1 : 1
+  }
+}
+
diff --git a/tests/cases/compiler/unionWithIndexSignature.ts b/tests/cases/compiler/unionWithIndexSignature.ts
new file mode 100644
index 0000000000000..14b20f3d57612
--- /dev/null
+++ b/tests/cases/compiler/unionWithIndexSignature.ts
@@ -0,0 +1,28 @@
+// @strict: true
+
+interface NumList {
+  kind: 'n';
+  [x: number]: number;
+}
+interface StrList {
+  kind: 's';
+  [x: number]: string;
+}
+
+export function foo<T extends NumList | StrList>(arr: T & (NumList | StrList)) {
+  let zz = arr[1];  // Error
+}
+
+// Repro from #38102
+
+export type TypedArray = Int32Array | Uint8Array;
+
+export function isTypedArray(a: {}): a is Int32Array | Uint8Array {
+  return a instanceof Int32Array || a instanceof Uint8Array;
+}
+
+export function flatten<T extends number|TypedArray>(arr: T) {
+  if (isTypedArray(arr)) {
+      arr[1];
+  }
+}