Text field style merge (#24449)

* Merge TextField style with global style so that style changes don't override unspecified fields

* Test that a style param is merged with the theme

* Test a few more style properties

* Analysis fix

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

@@ -575,7 +575,7 @@ class _TextFieldState extends State<TextField> with AutomaticKeepAliveClientMixi
     );
 
     final ThemeData themeData = Theme.of(context);
-    final TextStyle style = widget.style ?? themeData.textTheme.subhead;
+    final TextStyle style = themeData.textTheme.subhead.merge(widget.style);
     final Brightness keyboardAppearance = widget.keyboardAppearance ?? themeData.primaryColorBrightness;
     final TextEditingController controller = _effectiveController;
     final FocusNode focusNode = _effectiveFocusNode;

packages/flutter/test/material/text_field_test.dart

@@ -3420,6 +3420,56 @@ void main() {
     expect(tapCount, 0);
   });
 
+  testWidgets('TextField style is merged with theme', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/23994
+
+    final ThemeData themeData = ThemeData(
+      textTheme: TextTheme(
+        subhead: TextStyle(
+          color: Colors.blue[500],
+        ),
+      ),
+    );
+
+    Widget buildFrame(TextStyle style) {
+      return MaterialApp(
+        theme: themeData,
+        home: Material(
+          child: Center(
+            child: TextField(
+              style: style,
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Empty TextStyle is overridden by theme
+    await tester.pumpWidget(buildFrame(const TextStyle()));
+    EditableText editableText = tester.widget(find.byType(EditableText));
+    expect(editableText.style.color, themeData.textTheme.subhead.color);
+    expect(editableText.style.background, themeData.textTheme.subhead.background);
+    expect(editableText.style.shadows, themeData.textTheme.subhead.shadows);
+    expect(editableText.style.decoration, themeData.textTheme.subhead.decoration);
+    expect(editableText.style.locale, themeData.textTheme.subhead.locale);
+    expect(editableText.style.wordSpacing, themeData.textTheme.subhead.wordSpacing);
+
+    // Properties set on TextStyle override theme
+    const Color setColor = Colors.red;
+    await tester.pumpWidget(buildFrame(const TextStyle(color: setColor)));
+    editableText = tester.widget(find.byType(EditableText));
+    expect(editableText.style.color, setColor);
+
+    // inherit: false causes nothing to be merged in from theme
+    await tester.pumpWidget(buildFrame(const TextStyle(
+      fontSize: 24.0,
+      textBaseline: TextBaseline.alphabetic,
+      inherit: false,
+    )));
+    editableText = tester.widget(find.byType(EditableText));
+    expect(editableText.style.color, isNull);
+  });
+
   testWidgets('style enforces required fields', (WidgetTester tester) async {
     Widget buildFrame(TextStyle style) {
       return MaterialApp(