test(compiler-cli): add tests for NullishCoalescingNotNullableCheck (#43232)

Add tests to make sure the nullish coalescing check is generating the
correct diagnostics.

Refs #42966

PR Close #43232

packages/compiler-cli/test/ngtsc/extended_template_diagnostics_spec.ts

@@ -103,5 +103,43 @@ runInEachFileSystem(() => {
              .toMatch(
                  /Error: The '_extendedTemplateDiagnostics' option requires 'strictTemplates' to also be enabled./);
        });
+
+    it(`should produce nullish coalescing not nullable warning`, () => {
+      env.write('test.ts', `
+              import {Component} from '@angular/core';
+              @Component({
+                selector: 'test',
+                template: '{{ bar ?? "foo" }}',
+              })
+              export class TestCmp { 
+                bar: string = "text"; 
+              }
+            `);
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.NULLISH_COALESCING_NOT_NULLABLE));
+      expect(getSourceCodeForDiagnostic(diags[0])).toBe('bar ?? "foo"');
+    });
+
+    it(`should not produce nullish coalescing not nullable warning with strictNullChecks disabled`,
+       () => {
+         env.tsconfig(
+             {_extendedTemplateDiagnostics: true, strictTemplates: true, strictNullChecks: false});
+         env.write('test.ts', `
+              import {Component} from '@angular/core';
+              @Component({
+                selector: 'test',
+                template: '{{ bar ?? "foo" }}',
+              })
+              export class TestCmp { 
+                bar: string = undefined; 
+              }
+            `);
+
+         const diags = env.driveDiagnostics();
+         expect(diags.length).toBe(0);
+       });
   });
 });