Fix Tooltip implementation of PopupMenuButton (#42613)

* Fix tooltip not showing when PopupMenuButton.child was non-null

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

@@ -19,6 +19,7 @@ import 'material.dart';
 import 'material_localizations.dart';
 import 'popup_menu_theme.dart';
 import 'theme.dart';
+import 'tooltip.dart';
 
 // Examples can assume:
 // enum Commands { heroAndScholar, hurricaneCame }
@@ -939,7 +940,8 @@ class PopupMenuButton<T> extends StatefulWidget {
        assert(offset != null),
        assert(enabled != null),
        assert(captureInheritedThemes != null),
-       assert(!(child != null && icon != null)), // fails if passed both parameters
+       assert(!(child != null && icon != null),
+           'You can only pass [child] or [icon], not both.'),
        super(key: key);
 
   /// Called when the button is pressed to create the items to show in the menu.
@@ -1080,16 +1082,21 @@ class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterialLocalizations(context));
-    return widget.child != null
-      ? InkWell(
+
+    if (widget.child != null)
+      return Tooltip(
+        message: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
+        child: InkWell(
           onTap: widget.enabled ? showButtonMenu : null,
           child: widget.child,
-        )
-      : IconButton(
-          icon: widget.icon ?? _getIcon(Theme.of(context).platform),
-          padding: widget.padding,
-          tooltip: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
-          onPressed: widget.enabled ? showButtonMenu : null,
-        );
+        ),
+      );
+
+    return IconButton(
+      icon: widget.icon ?? _getIcon(Theme.of(context).platform),
+      padding: widget.padding,
+      tooltip: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
+      onPressed: widget.enabled ? showButtonMenu : null,
+    );
   }
 }

packages/flutter/test/material/popup_menu_test.dart

@@ -851,6 +851,129 @@ void main() {
     expect(tester.getTopLeft(find.text('Item 0')).dx, 72);
     expect(tester.getTopLeft(find.text('Item 1')).dx, 72);
   });
+
+  test("PopupMenuButton's child and icon properties cannot be simultaneously defined", () {
+    expect(() {
+      PopupMenuButton<int>(
+        itemBuilder: (BuildContext context) => <PopupMenuItem<int>>[],
+        child: Container(),
+        icon: const Icon(Icons.error),
+      );
+    }, throwsAssertionError);
+  });
+
+  testWidgets('PopupMenuButton default tooltip', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: Column(
+            children: <Widget>[
+              // Default Tooltip should be present when [PopupMenuButton.icon]
+              // and [PopupMenuButton.child] are undefined.
+              PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: Text('Tap me please!'),
+                    ),
+                  ];
+                },
+              ),
+              // Default Tooltip should be present when
+              // [PopupMenuButton.child] is defined.
+              PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: Text('Tap me please!'),
+                    ),
+                  ];
+                },
+                child: const Text('Test text'),
+              ),
+              // Default Tooltip should be present when
+              // [PopupMenuButton.icon] is defined.
+              PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: Text('Tap me please!'),
+                    ),
+                  ];
+                },
+                icon: const Icon(Icons.check),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    // The default tooltip is defined as [MaterialLocalizations.showMenuTooltip]
+    // and it is used when no tooltip is provided.
+    expect(find.byType(Tooltip), findsNWidgets(3));
+    expect(find.byTooltip(const DefaultMaterialLocalizations().showMenuTooltip), findsNWidgets(3));
+  });
+
+  testWidgets('PopupMenuButton custom tooltip', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: Column(
+            children: <Widget>[
+              // Tooltip should work when [PopupMenuButton.icon]
+              // and [PopupMenuButton.child] are undefined.
+              PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: Text('Tap me please!'),
+                    ),
+                  ];
+                },
+                tooltip: 'Test tooltip',
+              ),
+              // Tooltip should work when
+              // [PopupMenuButton.child] is defined.
+              PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: Text('Tap me please!'),
+                    ),
+                  ];
+                },
+                tooltip: 'Test tooltip',
+                child: const Text('Test text'),
+              ),
+              // Tooltip should work when
+              // [PopupMenuButton.icon] is defined.
+              PopupMenuButton<int>(
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(
+                      value: 1,
+                      child: Text('Tap me please!'),
+                    ),
+                  ];
+                },
+                tooltip: 'Test tooltip',
+                icon: const Icon(Icons.check),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(Tooltip), findsNWidgets(3));
+    expect(find.byTooltip('Test tooltip',), findsNWidgets(3));
+  });
 }
 
 class TestApp extends StatefulWidget {