fix: return toPrimitive function instead of throwing for React 19 compat

src/ObservableObject.ts

@@ -395,11 +395,16 @@ export function flushPending() {
 const proxyHandler: ProxyHandler<any> = {
     get(node: NodeInfo, p: any, receiver: any) {
         if (p === symbolToPrimitive) {
-            throw new Error(
-                process.env.NODE_ENV === 'development'
-                    ? '[legend-state] observable should not be used as a primitive. You may have forgotten to use .get() or .peek() to get the value of the observable.'
-                    : '[legend-state] observable is not a primitive.',
-            );
+            // Return a toPrimitive function instead of throwing so that external code
+            // (e.g. React 19's dev-mode logComponentRender) can safely coerce observables
+            // without crashing. A dev-mode warning is still emitted to help catch
+            // accidental primitive usage in user code.
+            if (process.env.NODE_ENV === 'development') {
+                console.warn(
+                    '[legend-state] observable is being converted to a primitive. You may have forgotten to use .get() or .peek() to get the value of the observable.',
+                );
+            }
+            return (hint: string) => (hint === 'number' ? NaN : '[Observable]');
         }
         if (p === symbolGetNode) {
             return node;

tests/tests.test.ts

@@ -3493,16 +3493,26 @@ describe('_', () => {
     });
 });
 describe('Built-in functions', () => {
-    test('Adding observables should throw', () => {
+    test('Adding observables should warn and return NaN', () => {
         const obs = observable({ x: 0, y: 0 });
 
         const x = obs.x;
         const y = obs.y;
 
-        expect(() => {
-            // @ts-expect-error Testing error
-            x + y;
-        }).toThrowError(/observable is not a primitive/);
+        const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+
+        // @ts-expect-error Testing primitive coercion
+        const result = x + y;
+
+        // Should produce NaN since both observables coerce to NaN for numeric hint
+        expect(result).toBeNaN();
+
+        // Should warn in development mode
+        expect(warnSpy).toHaveBeenCalledWith(
+            expect.stringContaining('observable is being converted to a primitive'),
+        );
+
+        warnSpy.mockRestore();
     });
 });
 describe('setAtPath', () => {