name: shadow-internal-symbols internalName: ShadowInternalSymbols timeline:

• date: 2024-11-18 release: 2.92.0 cls: [2206, 2295] message: Introduced as a parser error for the symbols __sub, __mul, __div, __lessThan.
• date: 2026-01-30 release: 2.95.0 cls: [4788] message: Expanded the deprecation to also forbid shadowing the builtin symbols null, true and false.

Shadowing symbol names used internally by the parser is deprecated. As an example, 5 - 3 internally expands to __sub 5 3 in the parser. __sub is a symbol name in global scope, which could be shadowed by let bindings like any other. Shadowing internal symbols is deprecated because it can lead to confusing unintended semantics in code. Using this to override operators with custom implementations is not supported and will lead to unintended semantics. (For example, overriding __lessThan will be ignored within builtin functions that perform comparison operations like sorting.)

Note that the check happens at usage site, so let __sub = null; in body remains allowed for as long as body does not contain a subtraction operation. Similarly, let __sub = null; in __sub remains allowed here because the explicit __sub usage is obvious and there is less potential for confusion.

Affected symbol names:

• __sub
• __mul
• __div
• __lessThan
• true
• false
• null