internal error when f.Keys is used to define a const field Keys of a datatype

[erniecohen]

Dafny version

4.2.0

Code to produce this issue

datatype T = T(
    f:map<nat,nat>
) {
    const Keys := f.Keys. // no problem if another name is used for the constant
}

Command to run and resulting output

vscode

What happened?

Dafny encountered an internal error. Please report it at https://github.com/dafny-lang/dafny/issues.
System.InvalidCastException: Unable to cast object of type 'Microsoft.Dafny.InferredTypeProxy' to type 'Microsoft.Dafny.SetType'.
at Microsoft.Dafny.Translator.GetReadonlyField(Field f)
at Microsoft.Dafny.Translator.AddClassMembers(TopLevelDeclWithMembers c, Boolean includeAllMethods, Boolean includeInformationAboutType)
at Microsoft.Dafny.Translator.AddRevealableTypeDecl(RevealableTypeDecl d)
at Microsoft.Dafny.Translator.DoTranslation(Program p, ModuleDefinition forModule)
at Microsoft.Dafny.Translator.Translate(Program p, ErrorReporter reporter, TranslatorFlags flags)+MoveNext()
at System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) at Microsoft.Dafny.LanguageServer.Language.DafnyProgramVerifier.<>c__DisplayClass4_0.<GetVerificationTasksAsync>b__1() at System.Threading.Tasks.Task1.InnerInvoke()
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
at Microsoft.Dafny.LanguageServer.Language.DafnyProgramVerifier.GetVerificationTasksAsync(CompilationAfterResolution compilation, CancellationToken cancellationToken)
at Microsoft.Dafny.LanguageServer.Workspace.CompilationManager.PrepareVerificationTasksAsync(CompilationAfterResolution loaded, CancellationToken cancellationToken)
at Microsoft.Dafny.LanguageServer.Workspace.CompilationManager.TranslateAsync()

What type of operating system are you experiencing the problem on?

Mac