[Repo Assist] Fix FSharpOption not recognized as union type in F# reflection#4529
Draft
github-actions[bot] wants to merge 1 commit intomainfrom
Draft
Conversation
FSharpType.IsUnion/GetUnionCases now correctly treats option<'T> as a union type with two cases (None=tag 0, Some=tag 1), matching .NET behaviour. FSharpValue.GetUnionFields and MakeUnion are updated to handle the erased runtime representation of options (None = undefined in JS/TS, None = None in Python; Some wraps via the Some helper). Changes: - src/fable-library-ts/Reflection.ts: add cases to option_type, handle option in getUnionFields and makeUnion - src/fable-library-py/fable_library/reflection.py: same fixes for Python using SomeWrapper awareness - tests/Js/Main/ReflectionTests.fs: add regression test - tests/Python/TestReflection.fs: add regression test Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
github-actions
Bot
added
automation
This was referenced Apr 13, 2026
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
🤖 This PR was created by Repo Assist, an automated AI assistant.
Closes #4082
Root Cause
The
option_type()function infable-library-ts/Reflection.tsandfable-library-py/fable_library/reflection.pycreated aTypeInfowithout acasesproperty. As a result:FSharpType.IsUnion(typeof<option<int>>)returnedfalseFSharpType.GetUnionCases(typeof<option<int>>)threw an exceptionFSharpValue.GetUnionFieldsandMakeUnionalso failed for option typesFix
option_type()to include acaseslazy property exposingNone(tag 0) andSome(tag 1) union cases.getUnionFields/get_union_fields: Fable erases the option tag at runtime (None = undefined/null, Some(x) = x or wrapper), so we detect the case from the value itself.makeUnion/make_unionto correctly constructNoneandSome(x)values.Tests Added
tests/Js/Main/ReflectionTests.fs: FSharp.Reflection: Option is a union typetests/Python/TestReflection.fs: test FSharp.Reflection: Option is a union typeBoth tests verify
IsUnion,GetUnionCases,GetUnionFields, andMakeUnionforoption<int>.Trade-offs
src/Fable.AST/orsrc/Fable.Core/(no breaking changes).