From 17db18aa87c33fa2817ed83f8c01afe9b57b5a50 Mon Sep 17 00:00:00 2001
From: Alan Agius <17563226+alan-agius4@users.noreply.github.com>
Date: Tue, 20 Oct 2026 08:57:18 +0000
Subject: [PATCH] fix(@angular/build): preserve diagnostic locations at offset
 0 and guard empty files

Ensure `convertTypeScriptDiagnosticInfo` checks `info.start !== undefined` so diagnostics and related information starting at file offset `0` retain their line, column, and lineText location properties. Also clamp `info.file.text.length - 1` to `0` when computing `lastLineOfFile` to prevent a TypeScript `Debug Failure` when reporting diagnostics on empty files.
---
 .../tools/angular/compilation/diagnostics.ts  |   4 +-
 .../angular/compilation/diagnostics_spec.ts   | 111 ++++++++++++++++++
 2 files changed, 113 insertions(+), 2 deletions(-)
 create mode 100644 packages/angular/build/src/tools/angular/compilation/diagnostics_spec.ts

diff --git a/packages/angular/build/src/tools/angular/compilation/diagnostics.ts b/packages/angular/build/src/tools/angular/compilation/diagnostics.ts
index bf85b69f707c..41563b90ebbd 100644
--- a/packages/angular/build/src/tools/angular/compilation/diagnostics.ts
+++ b/packages/angular/build/src/tools/angular/compilation/diagnostics.ts
@@ -37,7 +37,7 @@ function convertTypeScriptDiagnosticInfo(
     };
 
     // Calculate the line/column location and extract the full line text that has the diagnostic
-    if (info.start) {
+    if (info.start !== undefined) {
       const { line, character } = typescript.getLineAndCharacterOfPosition(info.file, info.start);
       note.location.line = line + 1;
       note.location.column = character;
@@ -50,7 +50,7 @@ function convertTypeScriptDiagnosticInfo(
       // will error if a nonexistent line is passed).
       const { line: lastLineOfFile } = typescript.getLineAndCharacterOfPosition(
         info.file,
-        info.file.text.length - 1,
+        Math.max(0, info.file.text.length - 1),
       );
       const lineEndPosition =
         line < lastLineOfFile
diff --git a/packages/angular/build/src/tools/angular/compilation/diagnostics_spec.ts b/packages/angular/build/src/tools/angular/compilation/diagnostics_spec.ts
new file mode 100644
index 000000000000..8ea79c20ba43
--- /dev/null
+++ b/packages/angular/build/src/tools/angular/compilation/diagnostics_spec.ts
@@ -0,0 +1,111 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import ts from 'typescript';
+import { convertTypeScriptDiagnostic } from './diagnostics';
+
+describe('convertTypeScriptDiagnostic', () => {
+  it('populates line, column, and lineText when diagnostic or relatedInformation starts at offset 0', () => {
+    const sourceFile = ts.createSourceFile(
+      '/src/app/app.component.ts',
+      'const invalid: number = "string";\nconst secondLine = 2;\n',
+      ts.ScriptTarget.Latest,
+    );
+    const templateFile = ts.createSourceFile(
+      '/src/app/app.component.html',
+      '<unknown-element></unknown-element>\n<div>Valid</div>\n',
+      ts.ScriptTarget.Latest,
+    );
+
+    const diagnostic: ts.Diagnostic = {
+      category: ts.DiagnosticCategory.Error,
+      code: -998001,
+      file: templateFile,
+      start: 0,
+      length: 17,
+      messageText: "'unknown-element' is not a known element.",
+      relatedInformation: [
+        {
+          category: ts.DiagnosticCategory.Message,
+          code: 0,
+          file: sourceFile,
+          start: 0,
+          length: 5,
+          messageText: 'Error occurs in the template of component AppComponent.',
+        },
+      ],
+    };
+
+    const result = convertTypeScriptDiagnostic(ts, diagnostic);
+
+    expect(result.text).toBe("NG8001: 'unknown-element' is not a known element.");
+    expect(result.location).toEqual({
+      file: '/src/app/app.component.html',
+      length: 17,
+      line: 1,
+      column: 0,
+      lineText: '<unknown-element></unknown-element>',
+    });
+
+    expect(result.notes).toEqual([
+      {
+        text: 'Error occurs in the template of component AppComponent.',
+        location: {
+          file: '/src/app/app.component.ts',
+          length: 5,
+          line: 1,
+          column: 0,
+          lineText: 'const invalid: number = "string";',
+        },
+      },
+    ]);
+  });
+
+  it('does not throw a TypeScript Debug Failure when diagnostic is reported on an empty file', () => {
+    const emptyFile = ts.createSourceFile('/src/empty.ts', '', ts.ScriptTarget.Latest);
+
+    const diagnosticAtZero: ts.Diagnostic = {
+      category: ts.DiagnosticCategory.Error,
+      code: 2306,
+      file: emptyFile,
+      start: 0,
+      length: 0,
+      messageText: "File '/src/empty.ts' is not a module.",
+    };
+
+    expect(() => convertTypeScriptDiagnostic(ts, diagnosticAtZero)).not.toThrow();
+    expect(convertTypeScriptDiagnostic(ts, diagnosticAtZero)).toEqual({
+      text: "TS2306: File '/src/empty.ts' is not a module.",
+      location: {
+        file: '/src/empty.ts',
+        length: 0,
+        line: 1,
+        column: 0,
+        lineText: '',
+      },
+    });
+
+    const diagnosticPastEnd: ts.Diagnostic = {
+      category: ts.DiagnosticCategory.Error,
+      code: 1005,
+      file: emptyFile,
+      start: 1,
+      length: 0,
+      messageText: 'Unexpected end of text.',
+    };
+
+    expect(() => convertTypeScriptDiagnostic(ts, diagnosticPastEnd)).not.toThrow();
+    expect(convertTypeScriptDiagnostic(ts, diagnosticPastEnd).location).toEqual({
+      file: '/src/empty.ts',
+      length: 0,
+      line: 1,
+      column: 1,
+      lineText: '',
+    });
+  });
+});