fix(migrations): Fixes control flow migration if then else case (#53006)

With if then else use cases, we now properly account for the length
of the original element's contents when tracking new offsets.

fixes: #52927

PR Close #53006

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

@@ -175,7 +175,9 @@ function buildIfThenElseBlock(
 
   const updatedTmpl = tmplStart + ifThenElseBlock + tmplEnd;
 
-  const pre = originals.start.length - startBlock.length;
+  // We ignore the contents of the element on if then else.
+  // If there's anything there, we need to account for the length in the offset.
+  const pre = originals.start.length + originals.childLength - startBlock.length;
   const post = originals.end.length - postBlock.length;
 
   return {tmpl: updatedTmpl, offsets: {pre, post}};

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

@@ -336,24 +336,27 @@ export function removeImports(
  * retrieves the original block of text in the template for length comparison during migration
  * processing
  */
-export function getOriginals(
-    etm: ElementToMigrate, tmpl: string, offset: number): {start: string, end: string} {
+export function getOriginals(etm: ElementToMigrate, tmpl: string, offset: number):
+    {start: string, end: string, childLength: number} {
   // original opening block
   if (etm.el.children.length > 0) {
+    const childStart = etm.el.children[0].sourceSpan.start.offset - offset;
+    const childEnd = etm.el.children[etm.el.children.length - 1].sourceSpan.end.offset - offset;
     const start = tmpl.slice(
         etm.el.sourceSpan.start.offset - offset,
         etm.el.children[0].sourceSpan.start.offset - offset);
     // original closing block
     const end = tmpl.slice(
         etm.el.children[etm.el.children.length - 1].sourceSpan.end.offset - offset,
         etm.el.sourceSpan.end.offset - offset);
-    return {start, end};
+    const childLength = childEnd - childStart;
+    return {start, end, childLength};
   }
   // self closing or no children
   const start =
       tmpl.slice(etm.el.sourceSpan.start.offset - offset, etm.el.sourceSpan.end.offset - offset);
   // original closing block
-  return {start, end: ''};
+  return {start, end: '', childLength: 0};
 }
 
 function isI18nTemplate(etm: ElementToMigrate, i18nAttr: Attribute|undefined): boolean {

packages/core/schematics/test/control_flow_migration_spec.ts

@@ -828,6 +828,59 @@ describe('control flow migration', () => {
       ].join('\n'));
     });
 
+    it('should migrate a complex if then else case on ng-containers', 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', [
+        `<ng-container>`,
+        `  <ng-container`,
+        `    *ngIf="loading; then loadingTmpl; else showTmpl"`,
+        `  >`,
+        `  </ng-container>`,
+        `<ng-template #showTmpl>`,
+        `  <ng-container`,
+        `    *ngIf="selected; else emptyTmpl"`,
+        `  >`,
+        `    <div></div>`,
+        `  </ng-container>`,
+        `</ng-template>`,
+        `<ng-template #emptyTmpl>`,
+        `  Empty`,
+        `</ng-template>`,
+        `</ng-container>`,
+        `<ng-template #loadingTmpl>`,
+        `  <p>Loading</p>`,
+        `</ng-template>`,
+      ].join('\n'));
+
+      await runMigration();
+      const content = tree.readContent('/comp.html');
+
+      expect(content).toBe([
+        `<ng-container>`,
+        `  @if (loading) {`,
+        `    <p>Loading</p>`,
+        `  } @else {`,
+        `    @if (selected) {`,
+        `      <div></div>`,
+        `    } @else {`,
+        `      Empty`,
+        `    }`,
+        `  }`,
+        `</ng-container>`,
+      ].join('\n'));
+    });
+
     it('should migrate but not remove ng-templates when referenced elsewhere', async () => {
       writeFile('/comp.ts', `
         import {Component} from '@angular/core';