[CFE] Calculating least upper bound crashes the compiler

[sgrekhov]

The following tests crashes the compiler

// SharedOptions=--enable-experiment=inline-class

import '../../Utils/static_type_helper.dart';

extension type ET1(Object? o) {}
extension type ET2(Object o) {}

extension type ET3(Object? o) implements ET1 {}
extension type ET4(Object o) implements ET2 {}

main() {
  var v1 = 2 > 1 ? ET1(Object()) : ET2(Object());
  v1.expectStaticType<Exactly<Object?>>();

  var v2 = 2 > 1 ? ET1(null) : ET2("String");
  v2.expectStaticType<Exactly<Object?>>();

  var v3 = 2 > 1 ? ET1(Object()) : ET3(Object());
  v3.expectStaticType<Exactly<Object?>>();

  var v4 = 2 > 1 ? ET1(Object()) : ET4(Object());
  v4.expectStaticType<Exactly<Object?>>();

  var v5 = 2 > 1 ? ET2(Object()) : ET4(Object());
  v5.expectStaticType<Exactly<Object>>();
}

Output is

/home/sgrekhov/Google/dart-sdk/sdk/out/ReleaseX64/dart-sdk/bin/dart --enable-experiment=inline-class --enable-asserts /home/sgrekhov/Google/sgrekhov/co19/LanguageFeatures/Extension-types/upper_bound_A01_t01.dart
Crash when compiling file:///home/sgrekhov/Google/sgrekhov/co19/LanguageFeatures/Extension-types/upper_bound_A01_t01.dart at character offset 1112:
Unsupported operation: moretop(ExtensionType(ET1), ExtensionType(ET3))

#0      StandardBounds.moretop (package:kernel/src/standard_bounds.dart:124:5)
#1      StandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:kernel/src/standard_bounds.dart:659:42)
#2      TypeSchemaStandardBounds.getNullabilityAwareStandardUpperBoundInternal (package:front_end/src/fasta/type_inference/standard_bounds.dart:59:18)
#3      StandardBounds._getNullabilityAwareStandardUpperBound (package:kernel/src/standard_bounds.dart:642:12)
#4      StandardBounds.getStandardUpperBound (package:kernel/src/standard_bounds.dart:629:14)
#5      InferenceVisitorImpl.visitConditionalExpression (package:front_end/src/fasta/type_inference/inference_visitor.dart:1009:51)
#6      ConditionalExpression.accept1 (package:kernel/ast.dart:6760:9)
#7      InferenceVisitorImpl._inferExpression (package:front_end/src/fasta/type_inference/inference_visitor.dart:269:27)
#8      InferenceVisitorImpl.inferExpression (package:front_end/src/fasta/type_inference/inference_visitor.dart:301:40)
#9      InferenceVisitorImpl.visitVariableDeclaration (package:front_end/src/fasta/type_inference/inference_visitor.dart:8647:11)
#10     VariableDeclaration.accept (package:kernel/ast.dart:10532:43)
#11     InferenceVisitorImpl.inferStatement (package:front_end/src/fasta/type_inference/inference_visitor.dart:252:26)
#12     InferenceVisitorImpl._visitStatements (package:front_end/src/fasta/type_inference/inference_visitor.dart:856:50)
#13     InferenceVisitorImpl.visitBlock (package:front_end/src/fasta/type_inference/inference_visitor.dart:877:31)
#14     Block.accept (package:kernel/ast.dart:8996:43)
#15     InferenceVisitorImpl.inferStatement (package:front_end/src/fasta/type_inference/inference_visitor.dart:252:26)
#16     TypeInferrerImpl.inferFunctionBody (package:front_end/src/fasta/type_inference/type_inferrer.dart:198:17)
#17     BodyBuilder.finishFunction (package:front_end/src/fasta/kernel/body_builder.dart:1263:43)
#18     DietListener.buildFunctionBody (package:front_end/src/fasta/source/diet_listener.dart:1217:19)
#19     DietListener.endTopLevelMethod (package:front_end/src/fasta/source/diet_listener.dart:387:5)
#20     Parser.parseTopLevelMethod (package:_fe_analyzer_shared/src/parser/parser_impl.dart:3755:14)
#21     Parser.parseTopLevelMemberImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:3499:14)
#22     Parser.parseTopLevelDeclarationImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:621:14)
#23     Parser.parseUnit (package:_fe_analyzer_shared/src/parser/parser_impl.dart:411:15)
#24     SourceLoader.buildBody (package:front_end/src/fasta/source/source_loader.dart:1248:12)
<asynchronous suspension>
#25     SourceLoader.buildBodies (package:front_end/src/fasta/source/source_loader.dart:670:7)
<asynchronous suspension>
#26     KernelTarget.buildComponent.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:596:7)
<asynchronous suspension>
#27     withCrashReporting (package:front_end/src/fasta/crash.dart:133:12)
<asynchronous suspension>
#28     KernelTarget.buildComponent (package:front_end/src/fasta/kernel/kernel_target.dart:577:12)
<asynchronous suspension>
#29     _buildInternal (package:front_end/src/kernel_generator_impl.dart:210:19)
<asynchronous suspension>
#30     withCrashReporting (package:front_end/src/fasta/crash.dart:133:12)
<asynchronous suspension>
#31     generateKernel.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:49:12)
<asynchronous suspension>
#32     CompilerContext.clear (package:front_end/src/fasta/compiler_context.dart:139:3)
<asynchronous suspension>
#33     generateKernel (package:front_end/src/kernel_generator_impl.dart:48:10)
<asynchronous suspension>
#34     kernelForModule (package:front_end/src/api_prototype/kernel_generator.dart:100:11)
<asynchronous suspension>
#35     SingleShotCompilerWrapper.compileInternal (file:///home/sgrekhov/Google/dart-sdk/sdk/pkg/vm/bin/kernel_service.dart:414:11)
<asynchronous suspension>
#36     Compiler.compile.<anonymous closure> (file:///home/sgrekhov/Google/dart-sdk/sdk/pkg/vm/bin/kernel_service.dart:220:45)
<asynchronous suspension>
#37     _processLoadRequest (file:///home/sgrekhov/Google/dart-sdk/sdk/pkg/vm/bin/kernel_service.dart:897:37)
<asynchronous suspension>


Process finished with exit code 252

Tested on the edge (Aug 21, 2023) SDK version on Linux x64

[lrhn]

An extension type is never a top type, so the bug is earlier in the algorithm, or in the implementation of TOP.
An extension type shouldn't become an argument to MORE_TOP.