fix(ivy): more descriptive errors for nested i18n sections

This commit moves nested i18n section detection to an earlier stage where we convert HTML AST to Ivy AST. This also gives a chance to produce better diagnistic message for nested i18n sections, that also includes a file name and location.

packages/compiler-cli/test/compliance/r3_view_compiler_i18n_spec.ts

@@ -3415,14 +3415,57 @@ describe('i18n support in the template compiler', () => {
   });
 
   describe('errors', () => {
+    const verifyNestedSectionsError = (errorThrown: any, expectedErrorText: string) => {
+      expect(errorThrown.ngParseErrors.length).toBe(1);
+      const msg = errorThrown.ngParseErrors[0].toString();
+      expect(msg).toContain(
+          'Could not mark an element as translatable inside of a translatable section');
+      expect(msg).toContain(expectedErrorText);
+      expect(msg).toMatch(/app\/spec\.ts\@\d+\:\d+/);
+    };
+
     it('should throw on nested i18n sections', () => {
       const files = getAppFilesWithTemplate(`
-        <div i18n><div i18n>Some content</div></div>
+        <div i18n>
+          <div i18n>Some content</div>
+        </div>
+      `);
+      try {
+        compile(files, angularFiles);
+      } catch (error) {
+        verifyNestedSectionsError(error, '[ERROR ->]<div i18n>Some content</div>');
+      }
+    });
+
+    it('should throw on nested i18n sections with tags in between', () => {
+      const files = getAppFilesWithTemplate(`
+        <div i18n>
+          <div>
+            <div i18n>Some content</div>
+          </div>
+        </div>
       `);
-      expect(() => compile(files, angularFiles))
-          .toThrowError(
-              'Could not mark an element as translatable inside of a translatable section');
+      try {
+        compile(files, angularFiles);
+      } catch (error) {
+        verifyNestedSectionsError(error, '[ERROR ->]<div i18n>Some content</div>');
+      }
     });
 
+    it('should throw on nested i18n sections represented with <ng-container>s', () => {
+      const files = getAppFilesWithTemplate(`
+        <ng-container i18n>
+          <div>
+            <ng-container i18n>Some content</ng-container>
+          </div>
+        </ng-container>
+      `);
+      try {
+        compile(files, angularFiles);
+      } catch (error) {
+        verifyNestedSectionsError(
+            error, '[ERROR ->]<ng-container i18n>Some content</ng-container>');
+      }
+    });
   });
 });

packages/compiler/src/render3/r3_template_transform.ts

@@ -80,11 +80,21 @@ class HtmlAstToIvyAst implements html.Visitor {
   errors: ParseError[] = [];
   styles: string[] = [];
   styleUrls: string[] = [];
+  private inI18nBlock: boolean = false;
 
   constructor(private bindingParser: BindingParser) {}
 
   // HTML visitor
   visitElement(element: html.Element): t.Node|null {
+    const isI18nRootElement = isI18nRootNode(element.i18n);
+    if (isI18nRootElement) {
+      if (this.inI18nBlock) {
+        this.reportError(
+            'Could not mark an element as translatable inside of a translatable section',
+            element.sourceSpan);
+      }
+      this.inI18nBlock = true;
+    }
     const preparsedElement = preparseElement(element);
     if (preparsedElement.type === PreparsedElementType.SCRIPT) {
       return null;
@@ -209,7 +219,7 @@ class HtmlAstToIvyAst implements html.Visitor {
       // For <ng-template>s with structural directives on them, avoid passing i18n information to
       // the wrapping template to prevent unnecessary i18n instructions from being generated. The
       // necessary i18n meta information will be extracted from child elements.
-      const i18n = isTemplateElement && isI18nRootNode(element.i18n) ? undefined : element.i18n;
+      const i18n = isTemplateElement && isI18nRootElement ? undefined : element.i18n;
 
       // TODO(pk): test for this case
       parsedElement = new t.Template(
@@ -218,6 +228,9 @@ class HtmlAstToIvyAst implements html.Visitor {
           templateVariables, element.sourceSpan, element.startSourceSpan, element.endSourceSpan,
           i18n);
     }
+    if (isI18nRootElement) {
+      this.inI18nBlock = false;
+    }
     return parsedElement;
   }
 

packages/compiler/src/render3/view/template.ts

@@ -535,10 +535,6 @@ export class TemplateDefinitionBuilder implements t.Visitor<void>, LocalResolver
     const isI18nRootElement: boolean =
         isI18nRootNode(element.i18n) && !isSingleI18nIcu(element.i18n);
 
-    if (isI18nRootElement && this.i18n) {
-      throw new Error(`Could not mark an element as translatable inside of a translatable section`);
-    }
-
     const i18nAttrs: (t.TextAttribute | t.BoundAttribute)[] = [];
     const outputAttrs: t.TextAttribute[] = [];
     let ngProjectAsAttr: t.TextAttribute|undefined;