fix(migrations): handle templates outside of component in cf migration (#53368)

If a template is passed in as an input, the ng-template will not exist in the same component template. This will leave a template placeholder behind. This fix ensures that template placeholder gets turned into a template outlet.

fixes: #53361

PR Close #53368

packages/core/schematics/ng-generate/control-flow-migration/util.ts

@@ -355,12 +355,30 @@ export function processNgTemplates(template: string): {migrated: string, err: Er
         updateTemplates(template, templates);
       }
     }
+    // template placeholders may still exist if the ng-template name is not
+    // present in the component. This could be because it's passed in from
+    // another component. In that case, we need to replace any remaining
+    // template placeholders with template outlets.
+    template = replaceRemainingPlaceholders(template);
     return {migrated: template, err: undefined};
   } catch (err) {
     return {migrated: template, err: err as Error};
   }
 }
 
+function replaceRemainingPlaceholders(template: string): string {
+  const replaceRegex = new RegExp(`#\\w*\\|`, 'g');
+  const placeholders = [...template.matchAll(replaceRegex)];
+  let migrated = template;
+  for (let ph of placeholders) {
+    const placeholder = ph[0];
+    const name = placeholder.slice(1, placeholder.length - 1);
+    migrated =
+        template.replace(placeholder, `<ng-template [ngTemplateOutlet]="${name}"></ng-template>`);
+  }
+  return migrated;
+}
+
 /**
  * determines if the CommonModule can be safely removed from imports
  */

packages/core/schematics/test/control_flow_migration_spec.ts

@@ -4012,6 +4012,40 @@ describe('control flow migration', () => {
         `}\n`,
       ].join('\n'));
     });
+
+    it('should add an ngTemplateOutlet when the template placeholder does not match a template',
+       async () => {
+         writeFile('/comp.ts', `
+        import {Component} from '@angular/core';
+        import {NgIf} from '@angular/common';
+
+        @Component({
+          templateUrl: './comp.html'
+        })
+        class Comp {
+          show = false;
+        }
+      `);
+
+         writeFile('/comp.html', [
+           `<button *ngIf="active; else defaultTemplate">`,
+           `  Hello!`,
+           `</button>`,
+         ].join('\n'));
+
+         await runMigration();
+         const content = tree.readContent('/comp.html');
+
+         expect(content).toBe([
+           `@if (active) {`,
+           `  <button>`,
+           `    Hello!`,
+           `  </button>`,
+           `} @else {`,
+           `  <ng-template [ngTemplateOutlet]="defaultTemplate"></ng-template>`,
+           `}`,
+         ].join('\n'));
+       });
   });
 
   describe('formatting', () => {