Add regression test

tests/cases/compiler/observableInferenceCanBeMade.ts

@@ -0,0 +1,22 @@
+// @strict: true
+
+// Repro from #33131
+
+declare function of<T>(a: T): Observable<T>;
+declare function from<O extends ObservableInput<any>>(input: O): Observable<ObservedValueOf<O>>;
+
+type ObservedValueOf<O> = O extends ObservableInput<infer T> ? T : never;
+
+interface Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+}
+type ObservableInput<T> = Subscribable<T> | Subscribable<never>;
+
+
+declare class Observable<T> implements Subscribable<T> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): void;
+}
+
+function asObservable(input: string | ObservableInput<string>): Observable<string> {
+    return typeof input === 'string' ? of(input) : from(input)
+}