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Evaluate the subject of to_existing_atom/2 once - #16043
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josevalim merged 2 commits intoNov 1, 2026
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When the allowed list is a literal, String.to_existing_atom/2 and List.to_existing_atom/2 are compiled into a case expression whose fallback clause calls __to_existing_atom__/2 to raise. The translated first argument was spliced into both the case subject and the fallback call, so any argument that is not a variable was evaluated twice when no atom matched, and the error could report a value the case never saw. Bind the subject to a fresh variable first, as the lists:member/2 rewrite already does, so it is evaluated exactly once. Assisted-by: Kiro:claude-fable-5.1
Assisted-by: Claude Code:claude-opus-5-5
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When the allowed list is a literal, String.to_existing_atom/2 and List.to_existing_atom/2 are compiled into a case expression whose fallback clause calls to_existing_atom/2 to raise. The translated first argument was spliced into both the case subject and the fallback call, so any argument that is not a variable was evaluated twice when no atom matched, and the error could report a value the case never saw.
Bind the subject to a fresh variable first, as the lists:member/2 rewrite already does, so it is evaluated exactly once.
Assisted-by: Kiro:claude-fable-5.1