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

Initial: https://dart-review.googlesource.com/c/sdk/+/155500
Reverted: https://dart-review.googlesource.com/c/sdk/+/155540

Bug: #42797

Change-Id: I5f0f1df594ac678718f347ee80b1764f971e42e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155541
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

pkg/analysis_server/benchmark/benchmarks.dart

@@ -224,8 +224,6 @@ class RunCommand extends Command {
         benchmarks.firstWhere((b) => b.id == benchmarkId, orElse: () {
       print("Benchmark '$benchmarkId' not found.");
       exit(1);
-      // Never reached.
-      return null;
     });
 
     var actualIterations = repeatCount;

pkg/analysis_server/test/stress/replay/replay.dart

@@ -129,7 +129,6 @@ class Driver {
       server.printStatistics();
     }
     exit(0);
-    return null;
   }
 
   /// Create and return a parser that can be used to parse the command-line

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 = 108;
 
   /// 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();
+}
+''');
+  }
+}

pkg/front_end/tool/_fasta/command_line.dart

@@ -502,7 +502,6 @@ Future<T> runProtectedFromAbort<T>(Future<T> Function() action,
     // treat this as a crash which is signalled by exiting with 255.
     exit(255);
   }
-  return failingValue;
 }
 
 abstract class ValueSpecification {

pkg/kernel/lib/src/tool/command_line_util.dart

@@ -40,6 +40,5 @@ class CommandLineHelper {
       print(e);
       exit(1);
     }
-    return null;
   }
 }

pkg/test_runner/lib/src/expectation_set.dart

@@ -49,8 +49,6 @@ class ExpectationSet {
     } on SyntaxError catch (error) {
       stderr.writeln(error.toString());
       exit(1);
-
-      throw "unreachable";
     }
   }
 

pkg/test_runner/lib/src/process_queue.dart

@@ -1186,7 +1186,6 @@ class BatchRunnerProcess {
       // it will always fail. So rather than re-trying a 1000+ times, we
       // exit.
       io.exit(1);
-      return true;
     });
   }
 

pkg/test_runner/lib/src/test_suite.dart

@@ -749,7 +749,6 @@ class StandardTestSuite extends TestSuite {
     // Unreachable.
     print("Cannot create URL for path $file. Not in build or dart directory.");
     exit(1);
-    return null;
   }
 
   String _uriForBrowserTest(String pathComponent) {