[dbrattli]

Summary

Follow-up to #4725, targeting its branch.

#4725 speeds up union case field reads by emitting cast(Case, x).field instead of the .fields-rebuild property. To make Pyright accept those casts it disables reportUnnecessaryCast globally, because some sites (single-case unions, already-narrowed values) make the cast redundant from Pyright's point of view.

This PR moves that "cast" into a tiny library helper so generated code stays cast-free and the global rule can stay on.

Changes

• fable_library/union.py — add narrow[T](case: type[T], value: object) -> T, a typed identity function (documented, exported in __all__). It plays the same role as typing.cast but is scoped to unions.
• Fable2Python.Transforms.fs — union field reads now emit narrow(Case, x).field instead of cast(Case, x).field.
• pyrightconfig.json — revert reportUnnecessaryCast back to true; narrow is not typing.cast, so the rule no longer fires on these reads (and still guards the rest of the generated code).

Generated output before → after:

# before (#4725)
return cast(Tree_Node, t).left_
# after (this PR)
return narrow(Tree_Node, t).left_

Why not cast(T, value) inside narrow?

narrow's body is return value # pyright: ignore[reportReturnType] rather than return cast(T, value). typing.cast is a plain Python function (return val), so routing through it would make two calls per field read instead of one — a needless regression on the exact hot path this optimization targets (~8M reads in the #4725 benchmark). The ignore is targeted, and reportUnnecessaryTypeIgnoreComment (enabled) confirms it's genuinely necessary.

Verification

• Pyright on union.py: 0 errors/warnings; the ignore is not flagged as unnecessary.
• Call-site check: narrow(Tree_Leaf, x).value_ resolves to the field's real type (int).
• Quicktest compiles and emits narrow(...) for both user unions and library unions (Result/Choice); the only remaining cast in output is a legitimately-necessary one that reportUnnecessaryCast: true accepts.

🤖 Generated with Claude Code

[MangelMaxime]

I tried to find a way to not disable reportUnnecessaryCast and could not come up with a trick for that. Great finding