RFR: 8354908: javac mishandles supplementary character in character literal

[Naoto Sato]

On Wed, 30 Apr 2025 13:05:04 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

> Some Unicode characters consist of two surrogates, i.e. two `char`s. And, such Unicode characters cannot be part of a char literal, as there's no way to represent them as a character literal. But, javac currently accepts code with such characters, and only puts the char, the high surrogate, into the literal, ignoring the second one.
> 
> For example, the JDK 24 behavior is:
> 
> $ cat /tmp/T.java 
> public class T {
>     public static void main(String... args) {
>        char c = '😊';
>        System.err.println(Integer.toHexString((int) c));
>        System.err.println(Character.isHighSurrogate(c));
>     }
> }
> $ java /tmp/T.java
> d83d
> true
> 
> 
> But, in JDK 11, such literals have been rejected:
> 
> $ java /tmp/T.java
> /tmp/T.java:3: error: unclosed character literal
>        char c = '😊';
>                 ^
> /tmp/T.java:3: error: illegal character: '\ude0a'
>        char c = '😊';
>                   ^
> /tmp/T.java:3: error: unclosed character literal
>        char c = '😊';
>                    ^
> 3 errors
> error: compilation failed
> 
> 
> The proposal in this PR is to explicitly check for this case when scanning character literal, and produce explicit error when a multi-surrogate character is used. javac will produce an error like:
> 
> $ java /tmp/T.java
> /tmp/T.java:3: error: character literal contains more than one UTF-16 code point
>        char c = '😊';
>                 ^
> 1 error
> error: compilation failed

src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties line 698:

> 696: 
> 697: compiler.err.illegal.char.literal.multiple.surrogates=\
> 698:     character literal contains more than one UTF-16 code point

The error message is kind of vague. How about using "surrogate code point"?
https://www.unicode.org/glossary/#surrogate_code_point

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PR Review Comment: https://git.openjdk.org/jdk/pull/24964#discussion_r2074321207