From c45a67e4acadaed89d605faa71a26d8ee261c671 Mon Sep 17 00:00:00 2001
From: Alex Talebi <31685655+SalehTZ@users.noreply.github.com>
Date: Wed, 23 Jul 2025 01:41:01 +0330
Subject: [PATCH] Improve assertion message in `AlignmentDirectional.resolve`
 (#172096)

Replaces the generic assertion in `AlignmentDirectional.resolve` and
`_MixedAlignment.resolve` with
`debugCheckCanResolveTextDirection`.

This change provides a more helpful and descriptive error message when
the `TextDirection` is null, improving the developer experience by
clearly stating why the resolution failed and how to fix it.

Fixes: #171813, Fixes: #171814, also fixes: #171812

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---
 .../flutter/lib/src/painting/alignment.dart   |  5 +--
 .../flutter/test/painting/alignment_test.dart | 32 +++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/packages/flutter/lib/src/painting/alignment.dart b/packages/flutter/lib/src/painting/alignment.dart
index 6f3903406e6..2a1365b6721 100644
--- a/packages/flutter/lib/src/painting/alignment.dart
+++ b/packages/flutter/lib/src/painting/alignment.dart
@@ -11,6 +11,7 @@ import 'dart:ui' as ui show lerpDouble;
 import 'package:flutter/foundation.dart';
 
 import 'basic_types.dart';
+import 'debug.dart';
 
 /// Base class for [Alignment] that allows for text-direction aware
 /// resolution.
@@ -584,7 +585,7 @@ class AlignmentDirectional extends AlignmentGeometry {
 
   @override
   Alignment resolve(TextDirection? direction) {
-    assert(direction != null, 'Cannot resolve $runtimeType without a TextDirection.');
+    assert(debugCheckCanResolveTextDirection(direction, '$AlignmentDirectional'));
     return switch (direction!) {
       TextDirection.rtl => Alignment(-start, y),
       TextDirection.ltr => Alignment(start, y),
@@ -653,7 +654,7 @@ class _MixedAlignment extends AlignmentGeometry {
 
   @override
   Alignment resolve(TextDirection? direction) {
-    assert(direction != null, 'Cannot resolve $runtimeType without a TextDirection.');
+    assert(debugCheckCanResolveTextDirection(direction, '$_MixedAlignment'));
     return switch (direction!) {
       TextDirection.rtl => Alignment(_x - _start, _y),
       TextDirection.ltr => Alignment(_x + _start, _y),
diff --git a/packages/flutter/test/painting/alignment_test.dart b/packages/flutter/test/painting/alignment_test.dart
index 815723b1934..e8c63f6fd3b 100644
--- a/packages/flutter/test/painting/alignment_test.dart
+++ b/packages/flutter/test/painting/alignment_test.dart
@@ -4,6 +4,7 @@
 
 import 'dart:math' as math;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -350,4 +351,35 @@ void main() {
     expect(AlignmentGeometry.bottomCenter, Alignment.bottomCenter);
     expect(AlignmentGeometry.bottomRight, Alignment.bottomRight);
   });
+
+  test('AlignmentDirectional.resolve with null TextDirection asserts', () {
+    const AlignmentDirectional alignmentDirectional = AlignmentDirectional(1.0, 2.0);
+
+    expect(
+      () => alignmentDirectional.resolve(null),
+      throwsA(
+        isFlutterError.having(
+          (FlutterError e) => e.message,
+          'message',
+          allOf(contains('No TextDirection found.'), contains('without a Directionality ancestor')),
+        ),
+      ),
+    );
+  });
+
+  test('AlignmentDirectional.resolve with null TextDirection asserts', () {
+    const Alignment a = Alignment(5.0, 6.0);
+    const AlignmentDirectional b = AlignmentDirectional(15.0, 16.0);
+
+    expect(
+      () => a.add(b).resolve(null),
+      throwsA(
+        isFlutterError.having(
+          (FlutterError e) => e.message,
+          'message',
+          allOf(contains('No TextDirection found.'), contains('without a Directionality ancestor')),
+        ),
+      ),
+    );
+  });
 }