Add InputDecorator.error to allow error message customization (#129275)

## Description

This PR is a fork of https://github.com/flutter/flutter/pull/118610 with some changes (mainly applying @justinmc comments).

~~This can be used by KishanBusa8 to update https://github.com/flutter/flutter/pull/118610 or can become a non WIP PR if KishanBusa8 does not respond or can not work on the update.~~

## Related Issue

fixes https://github.com/flutter/flutter/issues/11068

## Tests

Adds 3 tests.

packages/flutter/lib/src/material/input_decorator.dart

@@ -302,6 +302,7 @@ class _HelperError extends StatefulWidget {
     this.helperText,
     this.helperStyle,
     this.helperMaxLines,
+    this.error,
     this.errorText,
     this.errorStyle,
     this.errorMaxLines,
@@ -311,6 +312,7 @@ class _HelperError extends StatefulWidget {
   final String? helperText;
   final TextStyle? helperStyle;
   final int? helperMaxLines;
+  final Widget? error;
   final String? errorText;
   final TextStyle? errorStyle;
   final int? errorMaxLines;
@@ -328,6 +330,8 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
   Widget? _helper;
   Widget? _error;
 
+  bool get _hasError => widget.errorText != null || widget.error != null;
+
   @override
   void initState() {
     super.initState();
@@ -335,7 +339,7 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
       duration: _kTransitionDuration,
       vsync: this,
     );
-    if (widget.errorText != null) {
+    if (_hasError) {
       _error = _buildError();
       _controller.value = 1.0;
     } else if (widget.helperText != null) {
@@ -360,16 +364,19 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
   void didUpdateWidget(_HelperError old) {
     super.didUpdateWidget(old);
 
+    final Widget? newError = widget.error;
     final String? newErrorText = widget.errorText;
     final String? newHelperText = widget.helperText;
+    final Widget? oldError = old.error;
     final String? oldErrorText = old.errorText;
     final String? oldHelperText = old.helperText;
 
+    final bool errorStateChanged = (newError != null) != (oldError != null);
     final bool errorTextStateChanged = (newErrorText != null) != (oldErrorText != null);
     final bool helperTextStateChanged = newErrorText == null && (newHelperText != null) != (oldHelperText != null);
 
-    if (errorTextStateChanged || helperTextStateChanged) {
-      if (newErrorText != null) {
+    if (errorStateChanged || errorTextStateChanged || helperTextStateChanged) {
+      if (newError != null || newErrorText != null) {
         _error = _buildError();
         _controller.forward();
       } else if (newHelperText != null) {
@@ -399,7 +406,7 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
   }
 
   Widget _buildError() {
-    assert(widget.errorText != null);
+    assert(widget.error != null || widget.errorText != null);
     return Semantics(
       container: true,
       child: FadeTransition(
@@ -409,7 +416,7 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
             begin: const Offset(0.0, -0.25),
             end: Offset.zero,
           ).evaluate(_controller.view),
-          child: Text(
+          child: widget.error ?? Text(
             widget.errorText!,
             style: widget.errorStyle,
             textAlign: widget.textAlign,
@@ -435,7 +442,7 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
 
     if (_controller.isCompleted) {
       _helper = null;
-      if (widget.errorText != null) {
+      if (_hasError) {
         return _error = _buildError();
       } else {
         _error = null;
@@ -443,7 +450,7 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
       }
     }
 
-    if (_helper == null && widget.errorText != null) {
+    if (_helper == null && _hasError) {
       return _buildError();
     }
 
@@ -451,7 +458,7 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
       return _buildHelper();
     }
 
-    if (widget.errorText != null) {
+    if (_hasError) {
       return Stack(
         children: <Widget>[
           FadeTransition(
@@ -2365,6 +2372,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
       helperText: decoration.helperText,
       helperStyle: _getHelperStyle(themeData, defaults),
       helperMaxLines: decoration.helperMaxLines,
+      error: decoration.error,
       errorText: decoration.errorText,
       errorStyle: _getErrorStyle(themeData, defaults),
       errorMaxLines: decoration.errorMaxLines,
@@ -2562,6 +2570,7 @@ class InputDecoration {
     this.hintStyle,
     this.hintTextDirection,
     this.hintMaxLines,
+    this.error,
     this.errorText,
     this.errorStyle,
     this.errorMaxLines,
@@ -2601,7 +2610,8 @@ class InputDecoration {
     this.constraints,
   }) : assert(!(label != null && labelText != null), 'Declaring both label and labelText is not supported.'),
        assert(!(prefix != null && prefixText != null), 'Declaring both prefix and prefixText is not supported.'),
-       assert(!(suffix != null && suffixText != null), 'Declaring both suffix and suffixText is not supported.');
+       assert(!(suffix != null && suffixText != null), 'Declaring both suffix and suffixText is not supported.'),
+       assert(!(error != null && errorText != null), 'Declaring both error and errorText is not supported.');
 
   /// Defines an [InputDecorator] that is the same size as the input field.
   ///
@@ -2630,6 +2640,7 @@ class InputDecoration {
        helperStyle = null,
        helperMaxLines = null,
        hintMaxLines = null,
+       error = null,
        errorText = null,
        errorStyle = null,
        errorMaxLines = null,
@@ -2842,6 +2853,13 @@ class InputDecoration {
   /// used to handle the overflow when it is limited to single line.
   final int? hintMaxLines;
 
+  /// Optional widget that appears below the [InputDecorator.child] and the border.
+  ///
+  /// If non-null, the border's color animates to red and the [helperText] is not shown.
+  ///
+  /// Only one of [error] and [errorText] can be specified.
+  final Widget? error;
+
   /// Text that appears below the [InputDecorator.child] and the border.
   ///
   /// If non-null, the border's color animates to red and the [helperText] is
@@ -2849,6 +2867,10 @@ class InputDecoration {
   ///
   /// In a [TextFormField], this is overridden by the value returned from
   /// [TextFormField.validator], if that is not null.
+  ///
+  /// If a more elaborate error is required, consider using [error] instead.
+  ///
+  /// Only one of [error] and [errorText] can be specified.
   final String? errorText;
 
   /// {@template flutter.material.inputDecoration.errorStyle}
@@ -3485,6 +3507,7 @@ class InputDecoration {
     TextStyle? hintStyle,
     TextDirection? hintTextDirection,
     int? hintMaxLines,
+    Widget? error,
     String? errorText,
     TextStyle? errorStyle,
     int? errorMaxLines,
@@ -3537,6 +3560,7 @@ class InputDecoration {
       hintStyle: hintStyle ?? this.hintStyle,
       hintTextDirection: hintTextDirection ?? this.hintTextDirection,
       hintMaxLines: hintMaxLines ?? this.hintMaxLines,
+      error: error ?? this.error,
       errorText: errorText ?? this.errorText,
       errorStyle: errorStyle ?? this.errorStyle,
       errorMaxLines: errorMaxLines ?? this.errorMaxLines,
@@ -3639,6 +3663,7 @@ class InputDecoration {
         && other.hintStyle == hintStyle
         && other.hintTextDirection == hintTextDirection
         && other.hintMaxLines == hintMaxLines
+        && other.error == error
         && other.errorText == errorText
         && other.errorStyle == errorStyle
         && other.errorMaxLines == errorMaxLines
@@ -3694,6 +3719,7 @@ class InputDecoration {
       hintStyle,
       hintTextDirection,
       hintMaxLines,
+      error,
       errorText,
       errorStyle,
       errorMaxLines,
@@ -3747,6 +3773,7 @@ class InputDecoration {
       if (helperMaxLines != null) 'helperMaxLines: "$helperMaxLines"',
       if (hintText != null) 'hintText: "$hintText"',
       if (hintMaxLines != null) 'hintMaxLines: "$hintMaxLines"',
+      if (error != null) 'error: "$error"',
       if (errorText != null) 'errorText: "$errorText"',
       if (errorStyle != null) 'errorStyle: "$errorStyle"',
       if (errorMaxLines != null) 'errorMaxLines: "$errorMaxLines"',

packages/flutter/test/material/input_decorator_test.dart

@@ -1643,6 +1643,47 @@ void main() {
     expect(tester.getBottomLeft(find.text(kHelper1)), const Offset(12.0, 76.0));
   });
 
+  testWidgets('InputDecorator shows error text', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      buildInputDecorator(
+        useMaterial3: useMaterial3,
+        decoration: const InputDecoration(
+          errorText: 'errorText',
+        ),
+      ),
+    );
+
+    expect(find.text('errorText'), findsOneWidget);
+  });
+
+  testWidgets('InputDecorator shows error widget', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      buildInputDecorator(
+        useMaterial3: useMaterial3,
+        decoration: const InputDecoration(
+          error: Text('error', style: TextStyle(fontSize: 20.0)),
+        ),
+      ),
+    );
+
+    expect(find.text('error'), findsOneWidget);
+  });
+
+  testWidgets('InputDecorator throws when error text and error widget are provided', (WidgetTester tester) async {
+    expect(
+      () {
+        buildInputDecorator(
+          useMaterial3: useMaterial3,
+          decoration: InputDecoration(
+            errorText: 'errorText',
+            error: const Text('error', style: TextStyle(fontSize: 20.0)),
+          ),
+        );
+      },
+      throwsAssertionError,
+    );
+  });
+
   testWidgets('InputDecorator prefix/suffix texts', (WidgetTester tester) async {
     await tester.pumpWidget(
       buildInputDecorator(
@@ -5217,7 +5258,6 @@ void main() {
         useMaterial3: useMaterial3,
         // isFocused: false (default)
         decoration: const InputDecoration(
-          // errorText: false (default)
           enabled: false,
           errorBorder: errorBorder,
           focusedBorder: focusedBorder,