[ddc, expression_compiler_worker] Null check failure in expression compilation

[annagrin]

Discovered by dwds tests and happens in Dart SDK version: 2.18.0-238.0.dev (dev) (Wed Jun 29 13:41:53 2022 -0700) on "linux_x64":

Repro

Run VSCode in webdev/fixtures/_webDevSoundSmoke, set a breakpoint on the //Breakpoint ... line and evaluate expression true
You might need to add the following configuration to launch.json:

{
    "name": "Dart web",
    "program": "web",
    "request": "launch",
    "type": "dart"
}

Details from the failing test run in dwds CI

Command

file:///opt/hostedtoolcache/dart-sdk/bin/snapshots/dartdevc.dart.snapshot --experimental-expression-compiler --libraries-file file:///opt/hostedtoolcache/dart-sdk/lib/libraries.json --dart-sdk-summary file:///opt/hostedtoolcache/dart-sdk/lib/_internal/ddc_outline_sound.dill --asset-server-address localhost --asset-server-port 37469 --module-format amd --verbose --sound-null-safety

Dart code being compiled

https://github.com/dart-lang/webdev/tree/master/fixtures/_webdevSoundSmoke

Expression true evaluated at the following breakpoint

https://github.com/dart-lang/webdev/blob/1a70a1aa4ef254d9fea4876635d03f13899e8fc5/fixtures/_webdevSoundSmoke/web/main.dart#L21

Logs

[{"event":"daemon.log","params":{"log":"[FINEST] ExpressionCompilerService: Compiling \"true\" at org-dartlang-app:///web/main.dart:23"}}]
    0:00:01.091215: Compiling expression to JavaScript in module web/main in 353ms.
    0:00:01.107122: Collecting libraries for web/main in 15ms.
    0:00:01.108097: Collected libraries for web/main in 0ms.
    0:00:01.112180: Started building UriTranslator in 4ms.
    0:00:01.121226: Read libraries file in 8ms.
    0:00:01.121476: Read packages file in 0ms.
    0:00:01.122141: Read packages file in 0ms.
    0:00:01.123305: About to initializeFromComponent in 1ms.
    0:00:01.124365: Ran initializeFromComponent in 1ms.
    0:00:01.124947: Appended libraries in 0ms.
    0:00:01.[125](https://github.com/dart-lang/webdev/runs/7137057153?check_suite_focus=true#step:7:168)151: Indexed 34 libraries (0 bytes) in 0.181ms, that is,
           0.000 bytes/ms, and
           0.005 ms/libraries.
    0:00:01.133175: Built outlines for 0 compilation units (0 bytes) in 7.207ms, that is,
           0.000 bytes/ms, and
        Infinity ms/compilation unit.
    0:00:01.149370: Resolved parts in 15ms.
    0:00:01.149410: Applied patches in 0ms.
    0:00:01.150359: Computed library scopes in 0ms.
    0:00:01.150490: Resolved 0 types in 0ms.
    0:00:01.150513: Computed variances of 0 type variables in 0ms.
    0:00:01.150533: Computed default types for 0 type variables in 0ms.
    0:00:01.150562: Checked class hierarchy in 0ms.
    0:00:01.150584: Resolved 0 type-variable bounds in 0ms.
    0:00:01.150603: Built component in 0ms.
    0:00:01.150623: Installed Object as implicit superclass in 0ms.
    0:00:01.150750: Linked component in 0ms.
    0:00:01.155513: Computed core types in 4ms.
    0:00:01.155565: Built class hierarchy in 0ms.
    0:00:01.155597: Checked supertypes in 0ms.
    0:00:01.155615: Installed synthetic constructors in 0ms.
    0:00:01.155627: Resolved 0 constructors in 0ms.
    0:00:01.155644: Built class hierarchy members in 0ms.
    0:00:01.164764: Computed class hierarchy in 9ms.
    0:00:01.164809: Computed show and hide elements in 0ms.
    0:00:01.164858: Performed top level inference in 0ms.
    0:00:01.164872: Checked 0 overrides in 0ms.
    0:00:01.164894: Checked for restricted members inheritance in enums. in 0ms.
    0:00:01.164906: Finished initializing formals in 0ms.
    0:00:01.164917: Computed 0 combined member signatures in 0ms.
    0:00:01.164925: Updated 0 classes in kernel hierarchy in 0ms.
    0:00:01.164938: Added noSuchMethod forwarders in 0ms.
    0:00:01.164951: Checked mixin declaration applications in 0ms.
    0:00:01.164967: Build outline expressions in 0ms.
    0:00:01.164981: Checked type arguments of supers against the bounds in 0ms.
    0:00:01.164994: Checked redirecting factories in 0ms.
    0:00:01.165009: Cloned default values of formals in 0ms.
    0:00:01.165459: Building component in 0ms.
    0:00:01.165521: Built bodies for 0 compilation units (0 bytes) in 0.048ms, that is,
           0.000 bytes/ms, and
        Infinity ms/compilation unit.
    0:00:01.165958: Checked mixin application super-accesses in 0ms.
    0:00:01.165977: Cloned default values of formals in 0ms.
    0:00:01.165987: Finished deferred load tearoffs 0 in 0ms.
    0:00:01.165996: Finished forwarders for 0 procedures in 0ms.
    0:00:01.166010: Finished 0 native methods in 0ms.
    0:00:01.166019: Finished 0 patch methods in 0ms.
    0:00:01.166033: Finished constructors in 0ms.
    0:00:01.166245: Evaluated constants in 0ms.
    0:00:01.166387: Added constant coverage in 0ms.
    0:00:01.173096: Computed delta for expression in 6ms.
    0:00:01.194619: Emitted module for expression in 21ms.
    0:00:01.196427: Compiling expression 
    true in 1ms.
    0:00:01.201581: Compiled expression to JavaScript in 5ms.
    [{"event":"daemon.log","params":{"log":"[FINEST] ExpressionCompilerService: Failed to compile \"true\": Internal error: Null check operator used on a null value:
#0      DartScopeBuilder.visitVariableDeclaration (package:dev_compiler/src/kernel/expression_compiler.dart:172:29)\n
#1      VariableDeclaration.accept (package:kernel/ast.dart:10795:43)\n
#2      Let.visitChildren (package:kernel/ast.dart:8728:14)\n
#3      DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#4      TreeVisitor.defaultExpression (package:kernel/visitor.dart:196:43)\n
#5      TreeVisitor.visitLet (package:kernel/visitor.dart:319:27)\n
#6      Let.accept (package:kernel/ast.dart:8721:44)\n
#7      visitList (package:kernel/ast.dart:14200:14)\n
#8      Arguments.visitChildren (package:kernel/ast.dart:5346:5)\n
#9      DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#10     TreeVisitor.visitArguments (package:kernel/visitor.dart:429:39)\n
#11     Arguments.accept (package:kernel/ast.dart:5338:38)\n
#12     InstanceInvocation.visitChildren (package:kernel/ast.dart:5730:15)\n
#13     DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#14     TreeVisitor.defaultExpression (package:kernel/visitor.dart:196:43)\n
#15     TreeVisitor.visitInstanceInvocation (package:kernel/visitor.dart:241:57)\n
#16     InstanceInvocation.accept (package:kernel/ast.dart:5719:44)\n
#17     ExpressionStatement.visitChildren (package:kernel/ast.dart:9167:16)\n
#18     DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#19     TreeVisitor.defaultStatement (package:kernel/visitor.dart:339:41)\n
#20     TreeVisitor.visitExpressionStatement (package:kernel/visitor.dart:342:7)\n
#21     ExpressionStatement.accept (package:kernel/ast.dart:9159:43)\n
#22     visitList (package:kernel/ast.dart:14200:14)\n
#23     Block.visitChildren (package:kernel/ast.dart:9222:5)\n
#24     DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#25     TreeVisitor.defaultStatement (package:kernel/visitor.dart:339:41)\n
#26     TreeVisitor.visitBlock (package:kernel/visitor.dart:344:31)\n
#27     DartScopeBuilder.visitBlock (package:dev_compiler/src/kernel/expression_compiler.dart:181:13)\n
#28     Block.accept (package:kernel/ast.dart:9215:43)\n
#29     FunctionNode.visitChildren (package:kernel/ast.dart:3887:11)\n
#30     DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#31     TreeVisitor.visitFunctionNode (package:kernel/visitor.dart:428:45)\n
#32     DartScopeBuilder.visitFunctionNode (package:dev_compiler/src/kernel/expression_compiler.dart:160:13)\n
#33     FunctionNode.accept (package:kernel/ast.dart:3874:38)\n
#34     Procedure.visitChildren (package:kernel/ast.dart:3192:14)\n
#35     DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#36     TreeVisitor.defaultMember (package:kernel/visitor.dart:387:35)\n
#37     DartScopeBuilder.defaultMember (package:dev_compiler/src/kernel/expression_compiler.dart:150:13)\n
#38     TreeVisitor.visitProcedure (package:kernel/visitor.dart:391:39)\n
#39     Procedure.accept (package:kernel/ast.dart:3179:40)\n
#40     visitList (package:kernel/ast.dart:14200:14)\n
#41     Library.visitChildren (package:kernel/ast.dart:574:5)\n
#42     DartScopeBuilder.defaultTreeNode (package:dev_compiler/src/kernel/expression_compiler.dart:118:10)\n
#43     TreeVisitor.visitLibrary (package:kernel/visitor.dart:422:35)\n
#44     DartScopeBuilder.visitLibrary (package:dev_compiler/src/kernel/expression_compiler.dart:132:29)\n
#45     Library.accept (package:kernel/ast.dart:561:38)\n
#46     DartScopeBuilder.findScope (package:dev_compiler/src/kernel/expression_compiler.dart:106:13)\n
#47     ExpressionCompiler._findScopeAt (package:dev_compiler/src/kernel/expression_compiler.dart:404:34)\n
#48     ExpressionCompiler.compileExpressionToJs (package:dev_compiler/src/kernel/expression_compiler.dart:311:23)\n
#49     ExpressionCompilerWorker._compileExpression (package:dev_compiler/src/kernel/expression_compiler_worker.dart:427:54)\n<asynchronous suspension>\n
#50     ExpressionCompilerWorker.run (package:dev_compiler/src/kernel/expression_compiler_worker.dart:274:26)\n<asynchronous suspension>\n
#51     ExpressionCompilerWorker.createAndStart (package:dev_compiler/src/kernel/expression_compiler_worker.dart:[128](https://github.com/dart-lang/webdev/runs/7137057153?check_suite_focus=true#step:7:171):9)\n<asynchronous suspension>\n
#52     internalMain (package:dev_compiler/ddc.dart:36:5)\n<asynchronous suspension>\n"}}]