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Chris Tate
4780e7ef22
`s[i]` with an index the frontend cannot prove in bounds reads the character or undefined: temps for the string and the index, a range and integer test, and a string-or-undefined union. A lexer that tests `src[pos] === " "` or `c < "a"` on `const c = src[pos]` compared that union with a union-wrapped literal, or boxed it for a dynamic comparison, on every character. Tests of such a read against a literal of one UTF-16 code unit now compare `i === Math.floor(i) ? s.charCodeAt(i) : NaN` instead. Undefined, like NaN, equals no literal and fails every ordered comparison, and charCodeAt reads NaN out of range. A read of plain operands is evaluated in place; others keep the read's temps. An immutable local initialized from such a read, and used only in those tests, in tests for undefined and in a switch over one-unit cases, holds the code unit. Tests for undefined become NaN tests. Copies of the local, such as a switch's discriminant temp, follow it. The code-unit reads are charCodeAt reads, so a loop version can cover them.