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A local variable holding a union of record types is now narrowed when its literal discriminant key is refined, e.g. by `if event[:type] == "CLICK"`. Previously only the pure call `event[:type]` was narrowed; `event` itself stayed the full union. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
`case event[:type] when "CLICK"` now narrows `event` the same way `if`/`unless` already did. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Narrowing went through `refine_types`, which drops every pure call on the variable as if it were reassigned, so `x[:name]` narrowed by an earlier guard reverted to its declared type. Add `TypeEnv#narrow_types`, which keeps them. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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Summary
Narrowing
hash[:type]viaif/unless/case-whennow also narrowshashitself, whenhashis a union of record types.Details
Given
hash: a | b, wherea/bare record types with a shared literal discriminant key, checking that key's value narrowshashto the member whose key type matches:if hash[:type] == "A"— narrowshashtoain the truthy branch,bin the falsy branchcase hash[:type] when "A"— narrowshashtoain thatwhenclause, and to the bottom type in an exhaustiveelse?type: "A") are treated as"A" | nilThis lets you access member-specific fields (e.g.
hash[:position]) right after the branch, without an extra cast.Not supported
Narrowing is skipped entirely, not partially applied, when the discriminant key's value type is:
Unionof literalsFor this PR scope, I want to keep the specifications as simple as possible.