Issue #10924: Fix LeftCurly empty blocks after supplementary Unicode - #21506

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Link: https://github.com/checkstyle/checkstyle/actions/runs/34152070045

Github, generate report for LeftCurly/all-examples-in-one

@arturict , please always run diff testing on wild real projects, even update looks simple. Surprises are very frequent.

understood, i will trigger the report before approving from now on, also on the small ones.

Report for LeftCurly/all-examples-in-one:
https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/diff/40240556cd058f18d6e65d05b077595b5dbafe9a_2026140108/index.html

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/diff/40240556cd058f18d6e65d05b077595b5dbafe9a_2026140108/checkstyle/index.html#A1

Why this violation is disappearing?

all three are an empty {} on a line with an emoji before the brace, e.g. if (a == "12🤣") {} else {...} at 31:25.

columnNo counts code points, but master does braceLine.charAt(columnNo + 1) on the String. after a surrogate pair that index still lands on the { itself, so the {} skip in verifyBrace is missed and nl reports it. same line without the emoji (if (a == "12") {}) gives nothing on master. so these were false positives, and reading the line as code points is what removes them.

@ishu7w , please respond to my question

@romani Sorry for the delayed response. The disappearing violations are false positives for empty {} blocks with a supplementary Unicode character, such as an emoji, earlier on the same line.

The AST column counts Unicode code points, while String.charAt() indexes UTF-16 code units. The emoji occupies two UTF-16 units, so using columnNo + 1 with charAt() reads the wrong character. In these examples, it reads { instead of }, causing the check to miss the empty-block exception and incorrectly report a violation.

The fix uses code-point indexing consistently, so the empty block is recognized correctly and the false positive disappears. The equivalent code without the emoji already produces no violation.