Compiler error FS0193: error : not a nominal type when using an enum from a managed C++ project

The F# compiler gives error "FS0193: error : not a nominal type" in a project when it performs operations with an enum provided by a managed C++ project reference.

Specifically, there is a Managed C++ project called ProjectedFSLib.Managed.API that contains a enum called [HResult](https://github.com/jhorv/ProjFS-Managed-API/blob/fs-compiler-error-FS0193/ProjectedFSLib.Managed.API/HResult.h).

In an F# project that references this Managed C++ project, as soon as any F# expressions operate on that enum, the F# compilation fails with "FS0193: error : not a nominal type".

**Repro steps**

Provide the steps required to reproduce the problem:

1. Clone this repository:
https://github.com/jhorv/ProjFS-Managed-API/tree/fs-compiler-error-FS0193

This repo is a fork of Microsoft's Projected Filesystem managed API project. My changes are simply to reimplement their sample, the "SimpleProviderManaged" C# project, in F# to a) play with F# and b) experiment with the Microsoft Projected FS API.

2. Open in Visual Studio 2019
3. Restore Nuget packages and compile. The F# compiler will fail with FS0193 errors.

**Expected behavior**

The code to compile successfully.

**Actual behavior**

The compilation fails with the following stack trace:

3>optimize error FS0193: error : not a nominal type
3>parameter error FS0193: error : not a nominal type
3> Unhandled Exception: System.InvalidOperationException: not a nominal type
3> at FSharp.Compiler.AbstractIL.IL.ILType.get_TypeSpec()
3> at FSharp.Compiler.Tastops.underlyingTypeOfEnumTy$cont@4773(TcGlobals g, TypeDefMetadata matchValue, Unit unitVar)
3> at FSharp.Compiler.Tastops.underlyingTypeOfEnumTy(TcGlobals g, TType ty)
3> at FSharp.Compiler.Tastops.checkTypes@4814(TcGlobals g, TType a, TType b)
3> at FSharp.Compiler.Tastops.decideStaticOptimizationConstraint(TcGlobals g, StaticOptimization c)
3> at FSharp.Compiler.Tastops.DecideStaticOptimizations(TcGlobals g, FSharpList`1 cs)
3> at FSharp.Compiler.Tastops.mkStaticOptimizationExpr(TcGlobals g, FSharpList`1 cs, Expr e1, Expr e2, range m)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.remapExpr(TcGlobals g, ValCopyFlag compgen, Remap tmenv, Expr expr)
3> at FSharp.Compiler.Tastops.MakeApplicationAndBetaReduceAux(TcGlobals g, Expr f, TType fty, FSharpList`1 tyargsl, FSharpList`1 argsl, range m)
3> at FSharp.Compiler.Optimizer.OptimizeApplication(cenv cenv, IncrementalOptimizationEnv env, Expr f0, TType f0ty, FSharpList`1 tyargs, FSharpList`1 args, range m)
3> at FSharp.Compiler.Optimizer.OptimizeSwitch(cenv cenv, IncrementalOptimizationEnv env, Expr e, FSharpList`1 cases, FSharpOption`1 dflt, range m)
3> at FSharp.Compiler.Optimizer.OptimizeLinearExpr(cenv cenv, IncrementalOptimizationEnv env, Expr expr, FSharpFunc`2 contf)
3> at FSharp.Compiler.Optimizer.OptimizeLinearExpr(cenv cenv, IncrementalOptimizationEnv env, Expr expr, FSharpFunc`2 contf)
3> at FSharp.Compiler.Optimizer.OptimizeLambdas(FSharpOption`1 vspec, cenv cenv, IncrementalOptimizationEnv env, ValReprInfo topValInfo, Expr e, TType ety)
3> at FSharp.Compiler.Optimizer.OptimizeBinding(cenv cenv, Boolean isRec, IncrementalOptimizationEnv env, Binding _arg4)
3> at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ReraiseIfWatsonable(Exception exn)
3> at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ErrorLogger.ErrorRecovery(ErrorLogger x, Exception exn, range m)
3> at FSharp.Compiler.Optimizer.OptimizeBinding(cenv cenv, Boolean isRec, IncrementalOptimizationEnv env, Binding _arg4)
3> at FSharp.Compiler.Optimizer.OptimizeModuleBinding(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, ModuleOrNamespaceBinding x)
3> at FSharp.Compiler.Optimizer.OptimizeModuleBindings@3375-1.Invoke(ModuleOrNamespaceBinding x)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFoldToFreshConsTail[TResult,TState,T](FSharpList`1 cons, FSharpFunc`3 f, TState acc, FSharpList`1 xs)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFold[TState,T,TResult](FSharpFunc`2 f, TState acc, FSharpList`1 xs)
3> at Microsoft.FSharp.Collections.ListModule.MapFold[T,TState,TResult](FSharpFunc`2 mapping, TState state, FSharpList`1 list)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDef(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, ModuleOrNamespaceExpr x)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDefs@3390-1.Invoke(ModuleOrNamespaceExpr x)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFold[TState,T,TResult](FSharpFunc`2 f, TState acc, FSharpList`1 xs)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDefs(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, FSharpList`1 defs)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDef(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, ModuleOrNamespaceExpr x)
3> at FSharp.Compiler.Optimizer.OptimizeModuleBinding(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, ModuleOrNamespaceBinding x)
3> at FSharp.Compiler.Optimizer.OptimizeModuleBindings@3375-1.Invoke(ModuleOrNamespaceBinding x)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFold[TState,T,TResult](FSharpFunc`2 f, TState acc, FSharpList`1 xs)
3> at Microsoft.FSharp.Collections.ListModule.MapFold[T,TState,TResult](FSharpFunc`2 mapping, TState state, FSharpList`1 list)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDef(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, ModuleOrNamespaceExpr x)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDefs@3390-1.Invoke(ModuleOrNamespaceExpr x)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFold[TState,T,TResult](FSharpFunc`2 f, TState acc, FSharpList`1 xs)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDefs(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, FSharpList`1 defs)
3> at FSharp.Compiler.Optimizer.OptimizeModuleDef(cenv cenv, IncrementalOptimizationEnv env, FSharpList`1 bindInfosColl, ModuleOrNamespaceExpr x)
3> at FSharp.Compiler.Optimizer.OptimizeModuleExpr(cenv cenv, IncrementalOptimizationEnv env, ModuleOrNamespaceExprWithSig x)
3> at FSharp.Compiler.Optimizer.OptimizeImplFileInternal(cenv cenv, IncrementalOptimizationEnv env, Boolean isIncrementalFragment, SignatureHidingInfo hidden, TypedImplFile _arg5)
3> at FSharp.Compiler.Optimizer.OptimizeImplFile(OptimizationSettings settings, CcuThunk ccu, TcGlobals tcGlobals, FSharpFunc`2 tcVal, ImportMap importMap, IncrementalOptimizationEnv optEnv, Boolean isIncrementalFragment, Boolean emitTaicalls, SignatureHidingInfo hidden, TypedImplFile mimpls)
3> at FSharp.Compiler.CompileOptions.ApplyAllOptimizations@1703-1.Invoke(TypedImplFile implFile)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFoldToFreshConsTail[TResult,TState,T](FSharpList`1 cons, FSharpFunc`3 f, TState acc, FSharpList`1 xs)
3> at Microsoft.FSharp.Primitives.Basics.List.mapFold[TState,T,TResult](FSharpFunc`2 f, TState acc, FSharpList`1 xs)
3> at FSharp.Compiler.CompileOptions.ApplyAllOptimizations(TcConfig tcConfig, TcGlobals tcGlobals, FSharpFunc`2 tcVal, String outfile, ImportMap importMap, Boolean isIncrementalFragment, IncrementalOptimizationEnv optEnv, CcuThunk ccu, FSharpList`1 implFiles)
3> at FSharp.Compiler.Driver.main2a[a,b,c,d,e,f,g](Args`1 _arg1)
3> at FSharp.Compiler.Driver.typecheckAndCompile(CompilationThreadToken ctok, String[] argv, Resolver legacyReferenceResolver, Boolean bannerAlreadyPrinted, ReduceMemoryFlag reduceMemoryUsage, CopyFSharpCoreFlag defaultCopyFSharpCore, Exiter exiter, ErrorLoggerProvider loggerProvider, FSharpOption`1 tcImportsCapture, FSharpOption`1 dynamicAssemblyCreator)
3> at FSharp.Compiler.CommandLineMain.Driver.main(String[] argv)
3> at FSharp.Compiler.CommandLineMain.main(String[] argv)
3> at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ReraiseIfWatsonable(Exception exn)
3> at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ErrorLogger.ErrorRecovery(ErrorLogger x, Exception exn, range m)
3> at FSharp.Compiler.CommandLineMain.main(String[] argv)
3> The command exited with code -532462766.

**Known workarounds**

None

**Related information**

Provide any related information (optional):

I encountered this same problem on 2 different computers, though both were running:

* latest Windows 10 non-insider version: Version 1903, OS build 18362.295
* the F# project is .NET Framework 4.6.1, same as the C# project I'm re-implementing
* Visual Studio 2019 16.2.3