fix(common): warn if using ngSrcset without a configured image loader (#48804)

Warn the user in the console in case the `ngSrcset` is present and no
loader is configured. In this case, the default loader is used and
it ignores this attribute.

PR Close #48804

goldens/public-api/common/errors.md

@@ -19,6 +19,8 @@ export const enum RuntimeErrorCode {
     // (undocumented)
     NG_FOR_MISSING_DIFFER = -2200,
     // (undocumented)
+    NGSRCSET_WITHOUT_LOADER = 2963,
+    // (undocumented)
     OVERSIZED_IMAGE = 2960,
     // (undocumented)
     PARENT_NG_SWITCH_NOT_FOUND = 2000,

packages/common/src/directives/ng_optimized_image/ng_optimized_image.ts

@@ -385,6 +385,7 @@ export class NgOptimizedImage implements OnInit, OnChanges, OnDestroy {
         assertNoComplexSizes(this);
       }
       assertNotMissingBuiltInLoader(this.ngSrc, this.imageLoader);
+      assertNoNgSrcsetWithoutLoader(this, this.imageLoader);
       if (this.priority) {
         const checker = this.injector.get(PreconnectLinkChecker);
         checker.assertPreconnect(this.getRewrittenSrc(), this.ngSrc);
@@ -953,3 +954,17 @@ function assertNotMissingBuiltInLoader(ngSrc: string, imageLoader: ImageLoader)
     }
   }
 }
+
+/**
+ * Warns if ngSrcset is present and no loader is configured (i.e. the default one is being used).
+ */
+function assertNoNgSrcsetWithoutLoader(dir: NgOptimizedImage, imageLoader: ImageLoader) {
+  if (dir.ngSrcset && imageLoader === noopImageLoader) {
+    console.warn(formatRuntimeError(
+        RuntimeErrorCode.NGSRCSET_WITHOUT_LOADER,
+        `${imgDirectiveDetails(dir.ngSrc)} the \`ngSrcset\` attribute is present but ` +
+            `no image loader is configured (i.e. the default one is being used), ` +
+            `which would result in the same image being used for all configured sizes. ` +
+            `To fix this, provide a loader or remove the \`ngSrcset\` attribute from the image.`));
+  }
+}

packages/common/src/errors.ts

@@ -33,4 +33,5 @@ export const enum RuntimeErrorCode {
   OVERSIZED_IMAGE = 2960,
   TOO_MANY_PRELOADED_IMAGES = 2961,
   MISSING_BUILTIN_LOADER = 2962,
+  NGSRCSET_WITHOUT_LOADER = 2963,
 }

packages/common/test/directives/ng_optimized_image_spec.ts

@@ -1183,6 +1183,24 @@ describe('Image directive', () => {
       expect(consoleWarnSpy.calls.count()).toBe(0);
     });
 
+    it('should warn if there is no image loader but `ngSrcset` is present', () => {
+      setUpModuleNoLoader();
+
+      const template = `<img ngSrc="img.png" ngSrcset="100w, 200w" width="150" height="50">`;
+      const fixture = createTestComponent(template);
+      const consoleWarnSpy = spyOn(console, 'warn');
+      fixture.detectChanges();
+
+      expect(consoleWarnSpy.calls.count()).toBe(1);
+      expect(consoleWarnSpy.calls.argsFor(0)[0])
+          .toBe(
+              'NG02963: The NgOptimizedImage directive (activated on an <img> element ' +
+              'with the `ngSrc="img.png"`) has detected that the `ngSrcset` attribute is ' +
+              'present but no image loader is configured (i.e. the default one is being used), ' +
+              `which would result in the same image being used for all configured sizes. ` +
+              'To fix this, provide a loader or remove the `ngSrcset` attribute from the image.');
+    });
+
     it('should set `src` using the image loader provided via the `IMAGE_LOADER` token to compose src URL',
        () => {
          const imageLoader = (config: ImageLoaderConfig) => `${IMG_BASE_URL}/${config.src}`;

packages/core/test/bundling/image-directive/e2e/oversized-image/oversized-image.ts

@@ -6,13 +6,19 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {NgOptimizedImage} from '@angular/common';
+import {IMAGE_LOADER, ImageLoaderConfig, NgOptimizedImage} from '@angular/common';
 import {Component} from '@angular/core';
 
+const imageLoader = {
+  provide: IMAGE_LOADER,
+  useFactory: () => (config: ImageLoaderConfig) => config.src
+};
+
 @Component({
   selector: 'oversized-image-passing',
   standalone: true,
   imports: [NgOptimizedImage],
+  providers: [imageLoader],
   template: `
       <!-- Image is rendered within threshold range-->
       <div style="width: 500px; height: 500px">
@@ -33,6 +39,7 @@ export class OversizedImageComponentPassing {
   selector: 'oversized-image-failing',
   standalone: true,
   imports: [NgOptimizedImage],
+  providers: [imageLoader],
   template: `
       <!-- Image is rendered too small  -->
       <div style="width: 300px; height: 300px">