GetUnionCases fails on an option of provided type

[kerams]

Description

The following snippet causes an error message box to appear in the VS instance that consumes the type provider.

let typ = ProvidedTypeDefinition ("Blah", Some typeof<obj>)
let optionTyp = ProvidedTypeBuilder.MakeGenericType (typedefof<_ option>, [ typ ])
let cases = FSharp.Reflection.FSharpType.GetUnionCases optionTyp

Expected behavior

Expecting the function call so succeed and return 2 cases.

Actual behavior

A design time error message with The operation 'GetNestedType' on item 'FSharpOption``1' should not be called on provided type, member or parameter of type 'ProviderImplementation.ProvidedTypes.TypeSymbol' and a stack trace.

Is there any workaround for this?

[7sharp9]

I believe this is because GetUnionCases is expecting the providedtype typ to have a member called Tags present which is retrieved via reflection.

It's possible to get around it via some fakery if you really need to: https://github.com/jet/falanx/blob/fe3bfbce4a9ec40aecfd7db6704aaab91283c3a5/src/Falanx.Machinery/ProvidedTypesExtensions.fs#L16-L70

[github-actions]

🤖 This is an automated response from Repo Assist.

I've investigated this bug and created a fix: PR #458 — Fix GetNestedType on generic provided type symbols.

Root cause: ProvidedTypeBuilder.MakeGenericType(typedefof<_ option>, [someProvidedType]) produces an internal TypeSymbol(OtherGeneric(...)). Its GetNestedType override unconditionally threw NotSupportedException, even though the underlying generic type definition (FSharpOption(T)) is a real .NET type that fully supports GetNestedType. FSharpType.GetUnionCases calls GetNestedType("Tags", ...) internally, which triggered the error.

Fix: A one-line delegate to the underlying generic type definition's GetNestedType, mirroring the pattern already used by GetNestedTypes at the same site. Same fix applied to ProvidedTypeSymbol (for completeness).

All 103 existing tests pass and a regression test has been added.

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