fix(compiler): type-checking error for duplicate variables in templates (#35674)

It's an error to declare a variable twice on a specific template:

```html
<div *ngFor="let i of items; let i = index">
</div>
```

This commit introduces a template type-checking error which helps to detect
and diagnose this problem.

Fixes #35186

PR Close #35674

packages/compiler-cli/src/ngtsc/diagnostics/src/error_code.ts

@@ -122,6 +122,16 @@ export enum ErrorCode {
    */
   WRITE_TO_READ_ONLY_VARIABLE = 8005,
 
+  /**
+   * A template variable was declared twice. For example:
+   *
+   * ```html
+   * <div *ngFor="let i of items; let i = index">
+   * </div>
+   * ```
+   */
+  DUPLICATE_VARIABLE_DECLARATION = 8006,
+
   /**
    * An injectable already has a `ɵprov` property.
    */

packages/compiler-cli/src/ngtsc/typecheck/src/oob.ts

@@ -50,6 +50,17 @@ export interface OutOfBandDiagnosticRecorder {
 
   illegalAssignmentToTemplateVar(
       templateId: TemplateId, assignment: PropertyWrite, target: TmplAstVariable): void;
+
+  /**
+   * Reports a duplicate declaration of a template variable.
+   *
+   * @param templateId the template type-checking ID of the template which contains the duplicate
+   * declaration.
+   * @param variable the `TmplAstVariable` which duplicates a previously declared variable.
+   * @param firstDecl the first variable declaration which uses the same name as `variable`.
+   */
+  duplicateTemplateVar(
+      templateId: TemplateId, variable: TmplAstVariable, firstDecl: TmplAstVariable): void;
 }
 
 export class OutOfBandDiagnosticRecorderImpl implements OutOfBandDiagnosticRecorder {
@@ -100,4 +111,23 @@ export class OutOfBandDiagnosticRecorderImpl implements OutOfBandDiagnosticRecor
           span: target.valueSpan || target.sourceSpan,
         }));
   }
+
+  duplicateTemplateVar(
+      templateId: TemplateId, variable: TmplAstVariable, firstDecl: TmplAstVariable): void {
+    const mapping = this.resolver.getSourceMapping(templateId);
+    const errorMsg =
+        `Cannot redeclare variable '${variable.name}' as it was previously declared elsewhere for the same template.`;
+
+    // The allocation of the error here is pretty useless for variables declared in microsyntax,
+    // since the sourceSpan refers to the entire microsyntax property, not a span for the specific
+    // variable in question.
+    //
+    // TODO(alxhub): allocate to a tighter span once one is available.
+    this._diagnostics.push(makeTemplateDiagnostic(
+        mapping, variable.sourceSpan, ts.DiagnosticCategory.Error,
+        ngErrorCode(ErrorCode.DUPLICATE_VARIABLE_DECLARATION), errorMsg, {
+          text: `The variable '${firstDecl.name}' was first declared here.`,
+          span: firstDecl.sourceSpan,
+        }));
+  }
 }

packages/compiler-cli/src/ngtsc/typecheck/src/type_check_block.ts

@@ -738,7 +738,17 @@ class Scope {
     // has.
     if (templateOrNodes instanceof TmplAstTemplate) {
       // The template's variable declarations need to be added as `TcbVariableOp`s.
+      const varMap = new Map<string, TmplAstVariable>();
+
       for (const v of templateOrNodes.variables) {
+        // Validate that variables on the `TmplAstTemplate` are only declared once.
+        if (!varMap.has(v.name)) {
+          varMap.set(v.name, v);
+        } else {
+          const firstDecl = varMap.get(v.name) !;
+          tcb.oobRecorder.duplicateTemplateVar(tcb.id, v, firstDecl);
+        }
+
         const opIndex = scope.opQueue.push(new TcbVariableOp(tcb, scope, templateOrNodes, v)) - 1;
         scope.varMap.set(v, opIndex);
       }

packages/compiler-cli/src/ngtsc/typecheck/test/test_utils.ts

@@ -400,4 +400,5 @@ export class NoopOobRecorder implements OutOfBandDiagnosticRecorder {
   missingReferenceTarget(): void {}
   missingPipe(): void {}
   illegalAssignmentToTemplateVar(): void {}
+  duplicateTemplateVar(): void {}
 }