AIGEN: Fix #988: Forbid underscore-prefixed variable values #1076
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Underscore-prefixed variables in overlapping patterns cause segfaults and incorrect results due to a binding collision in match lowering. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
The ? operator desugars to a match expression that binds the inner value to a temporary variable. This variable was named `__x` which starts with underscore. Rename to `!x` to follow the compiler temp variable convention and avoid triggering the underscore-prefixed variable restriction. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
When pattern matching on fields with underscore prefix (e.g., _value_name), use explicit binding syntax `_value_name => value_name` to bind the value to a non-underscore variable name. This allows the value to be used while avoiding the underscore-prefixed variable restriction. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Don't bind underscore-prefixed variables (_x, _y, etc.) in pattern matching. They are still allowed in patterns but act as wildcards, so reading their value triggers the existing "You cannot use a variable that starts with _" error. This fixes issue #988 where overlapping patterns with _x-style variables caused binding collisions in the lowering phase, resulting in segfaults or incorrect results. Changes: - check_locals_pattern: Skip binding for is_wildcard_name variables - bind_pattern_name: Use startsWith("_") for duplicate checking - Enable the repro test from tests/todo/ in tests/syntax/invalid/ Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Defense in depth: when lowering match expressions, treat any binding whose name starts with _ as an underscore binding (not just exact "_"). This prevents creating bindings for _x-style variables during match lowering, complementing the fix in skipNaming.sk. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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bennostein
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Feb 12, 2026
| SkipAst.Pat_type( | ||
| SkipAst.Tid_object(SkipAst.Tclass((range, "Success"))), | ||
| Some(Positional(Array[(range, SkipAst.Pat_var((range, "__x")))])), | ||
| Some(Positional(Array[(range, SkipAst.Pat_var((range, "!x")))])), |
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Is this now using "!x" as an identifier? surprising that we allow that! Or is this conversion pass already downstream of where that gets checked?
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Summary
_no longer bind in patterns (they act as wildcards)_-prefixed variable value produces a clear error message_field => var) if you need the valueTest plan
tests/syntax/invalid/under_ident_3make STAGE=1Fixes #988
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