Migration tool throws exception with "nested extensions"

[DreadBoy]

I'm trying to migrate my project to null-safety with migration tool but it throws exception in one of my files. I assume it has problems with "nested extensions", I've included repro case below. When I run migration with dart migrate --skip-import-check I get this error:

# fvm dart migrate --skip-import-check
Migrating [app]

See https://dart.dev/go/null-safety-migration for a migration guide.

Note: more than one project found; migrating the top-level project.

Analyzing project...
Warning: package has unmigrated dependencies.

Continuing due to the presence of `--skip-import-check`.  To see a complete
list of the unmigrated dependencies, re-run without the `--skip-import-check`
flag.

No analysis issues found.

Generating migration suggestions...
Aborting migration due to an exception.  This most likely is due to a
bug in the migration tool.  Please consider filing a bug report at:

https://github.com/dart-lang/sdk/issues/new
Please include the SDK version (2.12.2) in your bug report.

To attempt to perform migration anyway, you may re-run with
--ignore-exceptions.

Exception details:

Bad state: Unrelated types: class Object and abstract class Iterable<E> at offset 335 in [app]\lib\utils\test.dart (myClass.valueOrNull?.mapIndexed((e, i) => e))

#0      DecoratedClassHierarchy.getDecoratedSupertype (package:nnbd_migration/src/decorated_class_hierarchy.dart:67:10)
#1      DecoratedClassHierarchy.asInstanceOf (package:nnbd_migration/src/decorated_class_hierarchy.dart:38:18)
#2      EdgeBuilder.getOrComputeElementType (package:nnbd_migration/src/edge_builder.dart:270:18)
#3      EdgeBuilder.visitMethodInvocation (package:nnbd_migration/src/edge_builder.dart:1284:20)
#4      MethodInvocationImpl.accept (package:analyzer/src/dart/ast/ast.dart:7186:49)
#5      EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#6      EdgeBuilder._handleAssignment (package:nnbd_migration/src/edge_builder.dart:2329:20)
#7      EdgeBuilder.visitVariableDeclarationList (package:nnbd_migration/src/edge_builder.dart:1924:11)
#8      VariableDeclarationListImpl.accept (package:analyzer/src/dart/ast/ast.dart:10655:15)
#9      EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#10     EdgeBuilder.visitNode (package:nnbd_migration/src/edge_builder.dart:1340:9)
#11     GeneralizingAstVisitor.visitStatement (package:analyzer/dart/ast/visitor.dart:517:39)
#12     GeneralizingAstVisitor.visitVariableDeclarationStatement (package:analyzer/dart/ast/visitor.dart:589:7)
#13     VariableDeclarationStatementImpl.accept (package:analyzer/src/dart/ast/ast.dart:10705:15)
#14     EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#15     EdgeBuilder.visitNode (package:nnbd_migration/src/edge_builder.dart:1340:9)
#16     GeneralizingAstVisitor.visitStatement (package:analyzer/dart/ast/visitor.dart:517:39)
#17     GeneralizingAstVisitor.visitBlock (package:analyzer/dart/ast/visitor.dart:165:31)
#18     BlockImpl.accept (package:analyzer/src/dart/ast/ast.dart:1083:49)
#19     EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#20     EdgeBuilder.visitNode (package:nnbd_migration/src/edge_builder.dart:1340:9)
#21     GeneralizingAstVisitor.visitFunctionBody (package:analyzer/dart/ast/visitor.dart:324:45)
#22     GeneralizingAstVisitor.visitBlockFunctionBody (package:analyzer/dart/ast/visitor.dart:168:55)
#23     BlockFunctionBodyImpl.accept (package:analyzer/src/dart/ast/ast.dart:1038:49)
#24     EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#25     EdgeBuilder.visitFunctionExpression.<anonymous closure> (package:nnbd_migration/src/edge_builder.dart:937:25)
#26     ScopedSet.doScoped (package:nnbd_migration/src/utilities/scoped_set.dart:44:13)
#27     EdgeBuilder.visitFunctionExpression (package:nnbd_migration/src/edge_builder.dart:935:28)
#28     FunctionExpressionImpl.accept (package:analyzer/src/dart/ast/ast.dart:5004:49)
#29     EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#30     EdgeBuilder.visitFunctionDeclaration (package:nnbd_migration/src/edge_builder.dart:886:9)
#31     FunctionDeclarationImpl.accept (package:analyzer/src/dart/ast/ast.dart:4866:49)
#32     EdgeBuilder._dispatch (package:nnbd_migration/src/edge_builder.dart:2207:24)
#33     EdgeBuilder.visitNode (package:nnbd_migration/src/edge_builder.dart:1340:9)
#34     GeneralizingAstVisitor.visitCompilationUnit (package:analyzer/dart/ast/visitor.dart:202:51)
#35     CompletenessTracker.visitCompilationUnit.<anonymous closure> (package:nnbd_migration/src/utilities/completeness_tracker.dart:52:24)
#36     PermissiveModeVisitor.reportExceptionsIfPermissive (package:nnbd_migration/src/utilities/permissive_mode.dart:26:24)
#37     CompletenessTracker.visitCompilationUnit (package:nnbd_migration/src/utilities/completeness_tracker.dart:43:5)
#38     CompilationUnitImpl.accept (package:analyzer/src/dart/ast/ast.dart:2134:49)
#39     NullabilityMigrationImpl.processInput (package:nnbd_migration/src/nullability_migration_impl.dart:244:12)
#40     NonNullableFix.processUnit (package:nnbd_migration/src/front_end/non_nullable_fix.dart:164:15)
#41     _FixCodeProcessor.runLaterPhases.<anonymous closure> (package:nnbd_migration/migration_cli.dart:1154:19)
#42     _FixCodeProcessor.runLaterPhases.<anonymous closure> (package:nnbd_migration/migration_cli.dart:1152:28)
#43     _FixCodeProcessor.processResources (package:nnbd_migration/migration_cli.dart:1081:30)
<asynchronous suspension>
#44     _FixCodeProcessor.runLaterPhases (package:nnbd_migration/migration_cli.dart:1152:5)
<asynchronous suspension>
#45     MigrationCliRunner.run (package:nnbd_migration/migration_cli.dart:749:24)
<asynchronous suspension>
#46     MigrateCommand.run (package:nnbd_migration/migration_cli.dart:185:7)
<asynchronous suspension>
#47     CommandRunner.runCommand (package:args/command_runner.dart:196:13)
<asynchronous suspension>
#48     DartdevRunner.runCommand (package:dartdev/dartdev.dart:199:18)
<asynchronous suspension>
#49     runDartdev (package:dartdev/dartdev.dart:53:16)
<asynchronous suspension>
#50     main (file:///C:/b/s/w/ir/cache/builder/src/third_party/dart/pkg/dartdev/bin/dartdev.dart:11:3)
<asynchronous suspension>

File that trips the migration tool:

class MyClass<T> {}

extension IterableX<E> on Iterable<E> {
  Iterable<T> mapIndexed<T>(T Function(E e, int i) f) {
    var i = 0;
    return this.map((e) => f(e, i++));
  }
}

extension MyClassX<T> on MyClass<T> {
  T get valueOrNull => null;
}

void main() {
  final myClass = MyClass<List<String>>();
  final list = myClass.valueOrNull?.mapIndexed((e, i) => e);
}

My system:

# dart --version
Dart SDK version: 2.12.2 (stable) (Wed Mar 17 10:30:20 2021 +0100) on "windows_x64"

# fvm flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel unknown, 2.0.4, on Microsoft Windows [Version 10.0.19042.964], locale en-GB)
[!] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    X Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/windows#android-setup for more details.
[√] Chrome - develop for the web
[√] Android Studio (version 4.1.0)
[√] IntelliJ IDEA Ultimate Edition (version 2021.1)
[√] Connected device (3 available)

! Doctor found issues in 1 category.

[stereotype441]

Possibly reelated to #45206

[stereotype441]

As of 1c7fe71, the null safety migration tool has been removed from active development and retired. No further work on the tool is planned.

If you still need help, or you believe this issue has been closed in error, please feel free to reopen.