Repro for multiple declaration warning

src/index.ts

@@ -1,4 +1,33 @@
-/**
- * Some code reproducing a bug.
- */
-export const bug = 123;
+export interface TemplatedTypeBase {
+  /**
+   * Doc here
+   */
+  prop?: string[];
+}
+
+export interface One extends TemplatedTypeBase {}
+
+export interface Two extends TemplatedTypeBase {}
+
+export type Type = One | Two;
+
+// errors
+export function isTemplateInstance(
+  type: Type
+): type is Type & { prop: string[] } {
+  return true;
+}
+
+// also error
+export function isTemplateInstance2(
+  type: Type
+): type is Type & Required<TemplatedTypeBase> {
+  return true;
+}
+
+// this works
+export function isTemplateInstance3(
+  type: Type
+): type is Type & TemplatedTypeBase {
+  return true;
+}

tsconfig.json

@@ -1,6 +1,7 @@
 {
-    "compilerOptions": {
-        "strict": true
-    },
-    "include": ["src"]
+  "compilerOptions": {
+    "strict": true,
+    "lib": ["es2019"]
+  },
+  "include": ["src"]
 }