datatype NatOutcome =
| NatSuccess(value: nat)
| NatFailure(error: string)
{
    predicate method IsFailure() {
        this.NatFailure?
    }
        function method PropagateFailure(): NatOutcome requires IsFailure() {
        this
    }
        function method Extract(): nat requires !IsFailure() {
        this.value
    }
}

C:/Users/kggruett/Documents/git/PR_Review/dafny/Test/exceptions/NatOutcomeDt.dfy(12,20): Error: internal error, shallow-type assignment was done incorrectly, this:ref != DatatypeType

Unhandled Exception: cce+UnreachableException: Exception of type 'cce+UnreachableException' was thrown.
   at Microsoft.Boogie.IdentifierExpr.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\AbsyExpr.cs:line 1026
   at Microsoft.Boogie.NAryExpr.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\AbsyExpr.cs:line 2570
   at Microsoft.Boogie.NAryExpr.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\AbsyExpr.cs:line 2570
   at Microsoft.Boogie.AssumeCmd.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\AbsyCmd.cs:line 3228
   at Microsoft.Boogie.Block.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\AbsyCmd.cs:line 1093
   at Microsoft.Boogie.Implementation.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\Absy.cs:line 3792
   at Microsoft.Boogie.Program.Typecheck(TypecheckingContext tc) in C:\Users\kggruett\Documents\git\boogie\Source\Core\Absy.cs:line 476
   at Microsoft.Boogie.Program.Typecheck(IErrorSink errorSink) in C:\Users\kggruett\Documents\git\boogie\Source\Core\Absy.cs:line 466
   at Microsoft.Boogie.Program.Typecheck() in C:\Users\kggruett\Documents\git\boogie\Source\Core\Absy.cs:line 461
   at Microsoft.Boogie.ExecutionEngine.ResolveAndTypecheck(Program program, String bplFileName, LinearTypeChecker& linearTypeChecker, CivlTypeChecker& civlTypeChecker) in C:\Users\kggruett\Documents\git\boogie\Source\ExecutionEngine\ExecutionEngine.cs:line 738
   at Microsoft.Dafny.DafnyDriver.BoogiePipelineWithRerun(Program program, String bplFileName, PipelineStatistics& stats, String programId) in C:\Users\kggruett\Documents\git\dafny\Source\DafnyDriver\DafnyDriver.cs:line 385
   at Microsoft.Dafny.DafnyDriver.BoogieOnce(String baseFile, String moduleName, Program boogieProgram, String programId, PipelineStatistics& stats, PipelineOutcome& oc) in C:\Users\kggruett\Documents\git\dafny\Source\DafnyDriver\DafnyDriver.cs:line 275
   at Microsoft.Dafny.DafnyDriver.Boogie(String baseName, IEnumerable`1 boogiePrograms, String programId, Dictionary`2& statss, PipelineOutcome& oc) in C:\Users\kggruett\Documents\git\dafny\Source\DafnyDriver\DafnyDriver.cs:line 296
   at Microsoft.Dafny.DafnyDriver.ProcessFiles(IList`1 dafnyFiles, ReadOnlyCollection`1 otherFileNames, ErrorReporter reporter, Boolean lookForSnapshots, String programId) in C:\Users\kggruett\Documents\git\dafny\Source\DafnyDriver\DafnyDriver.cs:line 213
   at Microsoft.Dafny.DafnyDriver.ThreadMain(String[] args) in C:\Users\kggruett\Documents\git\dafny\Source\DafnyDriver\DafnyDriver.cs:line 56
   at Microsoft.Dafny.DafnyDriver.<>c__DisplayClass1_0.<Main>b__0() in C:\Users\kggruett\Documents\git\dafny\Source\DafnyDriver\DafnyDriver.cs:line 33
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()