a11y improvements for textfield (#40390)

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

diff --git a/packages/flutter/lib/src/material/input_decorator.dart b/packages/flutter/lib/src/material/input_decorator.dart
index d51f53ac..35e8b0a 100644
--- a/packages/flutter/lib/src/material/input_decorator.dart
+++ b/packages/flutter/lib/src/material/input_decorator.dart
@@ -827,14 +827,18 @@
       visitor(prefix);
     if (prefixIcon != null)
       visitor(prefixIcon);
-    if (isFocused && hint != null) {
-      // Bypass opacity to always read hint when focused. This prevents the
-      // label from changing when text is entered.
-      final RenderProxyBox typedHint = hint;
-      visitor(typedHint.child);
-    } else if (!isFocused && label != null) {
+
+    if (label != null) {
       visitor(label);
     }
+    if (hint != null) {
+      if (isFocused) {
+        visitor(hint);
+      } else if (label == null) {
+        visitor(hint);
+      }
+    }
+
     if (input != null)
       visitor(input);
     if (suffixIcon != null)
@@ -2060,6 +2064,7 @@
       opacity: (isEmpty && !_hasInlineLabel) ? 1.0 : 0.0,
       duration: _kTransitionDuration,
       curve: _kTransitionCurve,
+      alwaysIncludeSemantics: true,
       child: Text(
         decoration.hintText,
         style: hintStyle,

packages/flutter/lib/src/widgets/implicit_animations.dart

diff --git a/packages/flutter/lib/src/widgets/implicit_animations.dart b/packages/flutter/lib/src/widgets/implicit_animations.dart
index 40b8364..98c4d07 100644
--- a/packages/flutter/lib/src/widgets/implicit_animations.dart
+++ b/packages/flutter/lib/src/widgets/implicit_animations.dart
@@ -1274,6 +1274,7 @@
     @required this.opacity,
     Curve curve = Curves.linear,
     @required Duration duration,
+    this.alwaysIncludeSemantics = false,
   }) : assert(opacity != null && opacity >= 0.0 && opacity <= 1.0),
        super(key: key, curve: curve, duration: duration);
 
@@ -1290,6 +1291,16 @@
   /// The opacity must not be null.
   final double opacity;
 
+  /// Whether the semantic information of the children is always included.
+  ///
+  /// Defaults to false.
+  ///
+  /// When true, regardless of the opacity settings the child semantic
+  /// information is exposed as if the widget were fully visible. This is
+  /// useful in cases where labels may be hidden during animations that
+  /// would otherwise contribute relevant semantics.
+  final bool alwaysIncludeSemantics;
+
   @override
   _AnimatedOpacityState createState() => _AnimatedOpacityState();
 
@@ -1319,6 +1330,7 @@
     return FadeTransition(
       opacity: _opacityAnimation,
       child: widget.child,
+      alwaysIncludeSemantics: widget.alwaysIncludeSemantics,
     );
   }
 }

packages/flutter/test/material/text_field_test.dart

diff --git a/packages/flutter/test/material/text_field_test.dart b/packages/flutter/test/material/text_field_test.dart
index 9d49ce6..1c458d9 100644
--- a/packages/flutter/test/material/text_field_test.dart
+++ b/packages/flutter/test/material/text_field_test.dart
@@ -4212,6 +4212,100 @@
     expect(tester.getBottomLeft(find.byKey(keyB)).dy, rowBottomY);
   });
 
+  testWidgets('TextField semantics include label when unfocused and label/hint when focused', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    final TextEditingController controller = TextEditingController(text: 'value');
+    final Key key = UniqueKey();
+
+    await tester.pumpWidget(
+      overlay(
+        child: TextField(
+          key: key,
+          controller: controller,
+          decoration: const InputDecoration(
+            hintText: 'hint',
+            labelText: 'label',
+          ),
+        ),
+      ),
+    );
+
+    final SemanticsNode node = tester.getSemantics(find.byKey(key));
+
+    expect(node.label, 'label');
+    expect(node.value, 'value');
+
+    // Focus text field.
+    await tester.tap(find.byKey(key));
+    await tester.pump();
+
+    expect(node.label, 'label\nhint');
+    expect(node.value, 'value');
+    semantics.dispose();
+  });
+
+  testWidgets('TextField semantics always include label when no hint is given', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    final TextEditingController controller = TextEditingController(text: 'value');
+    final Key key = UniqueKey();
+
+    await tester.pumpWidget(
+      overlay(
+        child: TextField(
+          key: key,
+          controller: controller,
+          decoration: const InputDecoration(
+            labelText: 'label',
+          ),
+        ),
+      ),
+    );
+
+    final SemanticsNode node = tester.getSemantics(find.byKey(key));
+
+    expect(node.label, 'label');
+    expect(node.value, 'value');
+
+    // Focus text field.
+    await tester.tap(find.byKey(key));
+    await tester.pump();
+
+    expect(node.label, 'label');
+    expect(node.value, 'value');
+    semantics.dispose();
+  });
+
+  testWidgets('TextField semantics always include hint when no label is given', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    final TextEditingController controller = TextEditingController(text: 'value');
+    final Key key = UniqueKey();
+
+    await tester.pumpWidget(
+      overlay(
+        child: TextField(
+          key: key,
+          controller: controller,
+          decoration: const InputDecoration(
+            hintText: 'hint',
+          ),
+        ),
+      ),
+    );
+
+    final SemanticsNode node = tester.getSemantics(find.byKey(key));
+
+    expect(node.label, 'hint');
+    expect(node.value, 'value');
+
+    // Focus text field.
+    await tester.tap(find.byKey(key));
+    await tester.pump();
+
+    expect(node.label, 'hint');
+    expect(node.value, 'value');
+    semantics.dispose();
+  });
+
   testWidgets('TextField semantics', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);
     final TextEditingController controller = TextEditingController();
@@ -4769,7 +4863,7 @@
     expect(semantics, hasSemantics(TestSemantics.root(
       children: <TestSemantics>[
         TestSemantics.rootChild(
-          label: 'hint',
+          label: 'label\nhint',
           id: 1,
           textDirection: TextDirection.ltr,
           textSelection: const TextSelection(baseOffset: 0, extentOffset: 0),