From e583f4c80811202ffbd321b98cc957fd586df1bd Mon Sep 17 00:00:00 2001
From: Jessica Janiuk <jessica.janiuk@gmail.com>
Date: Tue, 31 Mar 2026 11:50:31 -0700
Subject: [PATCH] refactor(core): Add more detail to NG0750 error message

This adds a bit more context to the NG0750 error message to provide details about which module failed to load when executing the dependencyResolverFn. This can help with debugging a failed lazy load in a defer block.

(cherry picked from commit 8218d2e34a48601ca34aababf104fe4f727843e5)
---
 packages/core/src/defer/triggering.ts       | 29 ++++++---
 packages/core/test/acceptance/defer_spec.ts | 68 +++++++++++++++++++++
 2 files changed, 90 insertions(+), 7 deletions(-)

diff --git a/packages/core/src/defer/triggering.ts b/packages/core/src/defer/triggering.ts
index 8a11d2d7d89..2f6869d7f5b 100644
--- a/packages/core/src/defer/triggering.ts
+++ b/packages/core/src/defer/triggering.ts
@@ -222,10 +222,12 @@ export function triggerResourceLoading(
   // Start downloading of defer block dependencies.
   tDetails.loadingPromise = Promise.allSettled(dependenciesFn()).then((results) => {
     let failed = false;
+    let failedReason: Error | null = null;
     const directiveDefs: DirectiveDefList = [];
     const pipeDefs: PipeDefList = [];
 
-    for (const result of results) {
+    for (let i = 0; i < results.length; i++) {
+      const result = results[i];
       if (result.status === 'fulfilled') {
         const dependency = result.value;
         const directiveDef = getComponentDef(dependency) || getDirectiveDef(dependency);
@@ -239,6 +241,8 @@ export function triggerResourceLoading(
         }
       } else {
         failed = true;
+        failedReason =
+          result.reason instanceof Error ? result.reason : new Error(String(result.reason));
         break;
       }
     }
@@ -248,13 +252,24 @@ export function triggerResourceLoading(
 
       if (tDetails.errorTmplIndex === null) {
         const templateLocation = ngDevMode ? getTemplateLocationDetails(lView) : '';
-        const error = new RuntimeError(
-          RuntimeErrorCode.DEFER_LOADING_FAILED,
-          ngDevMode &&
+        let errorMsg: string | false = false;
+
+        if (ngDevMode) {
+          errorMsg =
             'Loading dependencies for `@defer` block failed, ' +
-              `but no \`@error\` block was configured${templateLocation}. ` +
-              'Consider using the `@error` block to render an error state.',
-        );
+            `but no \`@error\` block was configured${templateLocation}. ` +
+            'Consider using the `@error` block to render an error state.';
+
+          const depsFnStr = tDetails.dependencyResolverFn?.toString() ?? 'unknown';
+          const errorReason = failedReason ? failedReason.message : 'Unknown';
+          errorMsg +=
+            `\n\nAngular tried to invoke the following dependency function (compiler-generated):\n` +
+            `\`\`\`\n${depsFnStr}\n\`\`\`\n` +
+            `but it resulted in the following error:\n\n` +
+            `${errorReason}`;
+        }
+
+        const error = new RuntimeError(RuntimeErrorCode.DEFER_LOADING_FAILED, errorMsg);
         handleUncaughtError(lView, error);
       }
     } else {
diff --git a/packages/core/test/acceptance/defer_spec.ts b/packages/core/test/acceptance/defer_spec.ts
index 59dfb90848b..a65c6ad748b 100644
--- a/packages/core/test/acceptance/defer_spec.ts
+++ b/packages/core/test/acceptance/defer_spec.ts
@@ -1034,6 +1034,74 @@ describe('@defer', () => {
       expect(reportedErrors[0].message).toContain(`(used in the 'MyCmp' component template)`);
     });
 
+    it('should include detailed failure info in the error message when no `@error` block is defined', async () => {
+      @Component({
+        selector: 'nested-cmp',
+        template: 'NestedCmp',
+        changeDetection: ChangeDetectionStrategy.Eager,
+      })
+      class NestedCmp {}
+
+      @Component({
+        selector: 'simple-app',
+        imports: [NestedCmp],
+        template: `
+          @defer (when isVisible) {
+            <nested-cmp />
+          } @loading {
+            Loading...
+          } @placeholder {
+            Placeholder
+          }
+        `,
+        changeDetection: ChangeDetectionStrategy.Eager,
+      })
+      class MyCmp {
+        isVisible = false;
+      }
+
+      const failedReason = new Error('Failed to load module X');
+      const deferDepsInterceptor = {
+        intercept() {
+          return () => [new Promise((_, reject) => setTimeout(() => reject(failedReason), 0))];
+        },
+      };
+
+      const reportedErrors: Error[] = [];
+      TestBed.configureTestingModule({
+        rethrowApplicationErrors: false,
+        providers: [
+          {
+            provide: ɵDEFER_BLOCK_DEPENDENCY_INTERCEPTOR,
+            useValue: deferDepsInterceptor,
+          },
+          {
+            provide: ErrorHandler,
+            useClass: class extends ErrorHandler {
+              override handleError(error: Error) {
+                reportedErrors.push(error);
+              }
+            },
+          },
+        ],
+      });
+
+      const fixture = TestBed.createComponent(MyCmp);
+      fixture.detectChanges();
+
+      fixture.componentInstance.isVisible = true;
+      fixture.detectChanges();
+
+      await allPendingDynamicImports();
+      fixture.detectChanges();
+
+      expect(reportedErrors.length).toBe(1);
+      const errorMsg = reportedErrors[0].message;
+      expect(errorMsg).toContain('NG0750');
+      expect(errorMsg).toContain('Angular tried to invoke the following dependency function');
+      expect(errorMsg).toContain('Failed to load module X');
+    });
+
     it('should not render `@error` block if loaded component has errors', async () => {
       @Component({
         selector: 'cmp-with-error',