[flutter] throw more specific error messages if a lerp'd type does not conform to Tween API (#74684)

packages/flutter/lib/src/animation/tween.dart

@@ -256,6 +256,51 @@ class Tween<T extends dynamic> extends Animatable<T> {
   T lerp(double t) {
     assert(begin != null);
     assert(end != null);
+    assert(() {
+      // Assertions that attempt to catch common cases of tweening types
+      // that do not conform to the Tween requirements.
+      dynamic? result;
+      try {
+        result = begin + (end - begin) * t;
+        result as T;
+        return true;
+      } on NoSuchMethodError {
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Cannot lerp between "$begin" and "$end".'),
+          ErrorDescription(
+            'The type ${begin.runtimeType} might not fully implement `+`, `-`, and/or `*`. '
+            'See "Types with special considerations" at https://api.flutter.dev/flutter/animation/Tween-class.html '
+            'for more information.',
+          ),
+          if (begin is Color || end is Color)
+            ErrorHint('To lerp colors, consider ColorTween instead.')
+          else if (begin is Rect || end is Rect)
+            ErrorHint('To lerp rects, consider RectTween instead.')
+          else
+            ErrorHint(
+              'There may be a dedicated "${begin.runtimeType}Tween" for this type, '
+              'or you may need to create one.'
+            ),
+        ]);
+      } on TypeError {
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Cannot lerp between "$begin" and "$end".'),
+          ErrorDescription(
+            'The type ${begin.runtimeType} returned a ${result.runtimeType} after '
+            'multiplication with a double value. '
+            'See "Types with special considerations" at https://api.flutter.dev/flutter/animation/Tween-class.html '
+            'for more information.',
+          ),
+          if (begin is int || end is int)
+            ErrorHint('To lerp int values, consider IntTween or StepTween instead.')
+          else
+            ErrorHint(
+              'There may be a dedicated "${begin.runtimeType}Tween" for this type, '
+              'or you may need to create one.'
+            ),
+        ]);
+      }
+    }());
     return begin + (end - begin) * t as T;
   }
 

packages/flutter/test/animation/tween_test.dart

@@ -7,7 +7,105 @@ import 'package:flutter/animation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:vector_math/vector_math_64.dart';
 
+const String kApiDocsLink = 'See "Types with special considerations" at https://api.flutter.dev/flutter/animation/Tween-class.html for more information.';
+
 void main() {
+  test('throws flutter error when tweening types that do not fully satisfy tween requirements - Object', () {
+    final Tween<Object> objectTween = Tween<Object>(
+      begin: Object(),
+      end: Object(),
+    );
+
+    FlutterError? error;
+    try {
+      objectTween.transform(0.1);
+    } on FlutterError catch (err) {
+      error = err;
+    }
+
+    if (error == null) {
+      fail('Expected Tween.transform to throw a FlutterError');
+    }
+
+    expect(error.diagnostics.map((DiagnosticsNode node) => node.toString()), <String>[
+      'Cannot lerp between "Instance of \'Object\'" and "Instance of \'Object\'".',
+      'The type Object might not fully implement `+`, `-`, and/or `*`. $kApiDocsLink',
+      'There may be a dedicated "ObjectTween" for this type, or you may need to create one.'
+    ]);
+  });
+
+  test('throws flutter error when tweening types that do not fully satisfy tween requirements - Color', () {
+    final Tween<Color> colorTween = Tween<Color>(
+      begin: const Color(0xFF000000),
+      end: const Color(0xFFFFFFFF),
+    );
+
+    FlutterError? error;
+    try {
+      colorTween.transform(0.1);
+    } on FlutterError catch (err) {
+      error = err;
+    }
+
+    if (error == null) {
+      fail('Expected Tween.transform to throw a FlutterError');
+    }
+
+    expect(error.diagnostics.map((DiagnosticsNode node) => node.toString()), <String>[
+      'Cannot lerp between "Color(0xff000000)" and "Color(0xffffffff)".',
+      'The type Color might not fully implement `+`, `-`, and/or `*`. $kApiDocsLink',
+      'To lerp colors, consider ColorTween instead.',
+    ]);
+  });
+
+  test('throws flutter error when tweening types that do not fully satisfy tween requirements - Rect', () {
+    final Tween<Rect> rectTween = Tween<Rect>(
+      begin: const Rect.fromLTWH(0, 0, 10, 10),
+      end: const Rect.fromLTWH(2, 2, 2, 2)
+    );
+
+    FlutterError? error;
+    try {
+      rectTween.transform(0.1);
+    } on FlutterError catch (err) {
+      error = err;
+    }
+
+    if (error == null) {
+      fail('Expected Tween.transform to throw a FlutterError');
+    }
+
+    expect(error.diagnostics.map((DiagnosticsNode node) => node.toString()), <String>[
+      'Cannot lerp between "Rect.fromLTRB(0.0, 0.0, 10.0, 10.0)" and "Rect.fromLTRB(2.0, 2.0, 4.0, 4.0)".',
+      'The type Rect might not fully implement `+`, `-`, and/or `*`. $kApiDocsLink',
+      'To lerp rects, consider RectTween instead.',
+    ]);
+  });
+
+  test('throws flutter error when tweening types that do not fully satisfy tween requirements - int', () {
+    final Tween<int> colorTween = Tween<int>(
+      begin: 0,
+      end: 1,
+    );
+
+    FlutterError? error;
+    try {
+      colorTween.transform(0.1);
+    } on FlutterError catch (err) {
+      error = err;
+    }
+
+    if (error == null) {
+      fail('Expected Tween.transform to throw a FlutterError');
+    }
+
+    expect(error.diagnostics.map((DiagnosticsNode node) => node.toString()), <String>[
+      'Cannot lerp between "0" and "1".',
+      'The type int returned a double after multiplication with a double value. $kApiDocsLink',
+      'To lerp int values, consider IntTween or StepTween instead.',
+    ]);
+  });
+
   test('Can chain tweens', () {
     final Tween<double> tween = Tween<double>(begin: 0.30, end: 0.50);
     expect(tween, hasOneLineDescription);