fix(no-misused-observables): accept Observable-returning function for union types (#236)

Allow returning Observables from union types of properties like:

```ts
interface Hook {
  onInit?: ((field: string) => void) | ((field: string) => Observable<any>);
}

const hook: Hook = {
  onInit: field => of(field),
};
```

This corner case was skipped when the initial rule was created based on
tseslint's `no-misused-promises`.

Resolves #193

Author: JasonWeinzierl

src/rules/no-misused-observables.ts

@@ -11,6 +11,7 @@ import {
   isJSXExpressionContainer,
   isMethodDefinition,
   isPropertyDefinition,
+  couldBeType as tsutilsEtcCouldBeType,
 } from '../etc';
 import { ruleCreator } from '../utils';
 
@@ -418,6 +419,12 @@ function isVoidReturningFunctionType(
     for (const signature of subType.getCallSignatures()) {
       const returnType = signature.getReturnType();
 
+      // If a certain positional argument accepts both Observable and void returns,
+      // an Observable-returning function is valid.
+      if (tsutilsEtcCouldBeType(returnType, 'Observable')) {
+        return false;
+      }
+
       hasVoidReturn ||= tsutils.isTypeFlagSet(returnType, ts.TypeFlags.Void);
     }
   }

tests/rules/no-misused-observables.test.ts

@@ -233,6 +233,18 @@ ruleTester({ types: true }).run('no-misused-observables', noMisusedObservablesRu
         bar,
       };
     `,
+    stripIndent`
+      // void return property; union type
+      import { Observable, of } from "rxjs";
+
+      interface Hook {
+        onInit?: ((field: string) => void) | ((field: string) => Observable<any>);
+      }
+
+      const hook: Hook = {
+        onInit: field => of(field),
+      };
+    `,
     // #endregion valid; void return property
     // #region valid; void return return value
     {