Issue 42797. Understand in legacy libraries that a function returning Never (in Null Safety library) never returns.

R=brianwilkerson@google.com

Bug: #42797
Change-Id: I14e50c231935a4025d33762843f5a54a1ee2154c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155500
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -91,7 +91,7 @@ typedef WorkToWaitAfterComputingResult = Future<void> Function(String path);
 /// TODO(scheglov) Clean up the list of implicitly analyzed files.
 class AnalysisDriver implements AnalysisDriverGeneric {
   /// The version of data format, should be incremented on every format change.
-  static const int DATA_VERSION = 106;
+  static const int DATA_VERSION = 107;
 
   /// The length of the list returned by [_computeDeclaredVariablesSignature].
   static const int _declaredVariablesSignatureLength = 4;

pkg/analyzer/lib/src/dart/resolver/exit_detector.dart

@@ -6,6 +6,7 @@ import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/src/dart/element/type.dart';
 
 /// Instances of the class `ExitDetector` determine whether the visited AST node
 /// is guaranteed to terminate by executing a `return` statement, `throw`
@@ -423,7 +424,7 @@ class ExitDetector extends GeneralizingAstVisitor<bool> {
       }
     }
     Element element = node.methodName.staticElement;
-    if (element != null && element.hasAlwaysThrows) {
+    if (_elementExits(element)) {
       return true;
     }
     return _nodeExits(node.argumentList);
@@ -660,4 +661,14 @@ class ExitDetector extends GeneralizingAstVisitor<bool> {
   static bool exits(AstNode node) {
     return ExitDetector()._nodeExits(node);
   }
+
+  static bool _elementExits(Element element) {
+    if (element is ExecutableElement) {
+      var declaration = element.declaration;
+      return declaration.hasAlwaysThrows ||
+          identical(declaration.returnType, NeverTypeImpl.instance);
+    }
+
+    return false;
+  }
 }

pkg/analyzer/test/src/diagnostics/missing_return_test.dart

@@ -7,10 +7,12 @@ import 'package:analyzer/src/test_utilities/package_mixin.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../dart/resolution/driver_resolution.dart';
+import '../dart/resolution/with_null_safety_mixin.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(MissingReturnTest);
+    defineReflectiveTests(MissingReturnWithNullSafetyTest);
   });
 }
 
@@ -213,3 +215,23 @@ class B extends A {
     ]);
   }
 }
+
+@reflectiveTest
+class MissingReturnWithNullSafetyTest extends DriverResolutionTest
+    with WithNullSafetyMixin {
+  test_returnNever() async {
+    newFile('/test/lib/a.dart', content: r'''
+Never foo() {
+  throw 0;
+}
+''');
+    await assertNoErrorsInCode(r'''
+// @dart = 2.8
+import 'a.dart';
+
+int f() {
+  foo();
+}
+''');
+  }
+}